CONSTRUCTION of a THERMOCOUPLE THERMOMETER by Jim Roberts, Professor of Physics and Material Science the University of North Texas

SUPPORT MATERIALS

EXploring science collect and analyze data using the casio fx-9750 and ea-200 CONSTRUCTION OF A THERMOCOUPLE THERMOMETER By Jim Roberts, Professor of Physics and Material Science The University of North Texas

OBJECTIVE: This experiment is designed to show you how to construct a thermocouple or a device made of two (couple) of dissimilar metals that can produce a voltage when heat is applied, to collect voltage data, to plot the results on a fx-9750G graphing calculator and make a thermometer using the results.

INTRODUCTION There are two types of effects that arise when two dissimilar metals are brought in contact with each other and the temperature is changed at the junction. One effect produces an electrical potential (Seebeck effect) when heat is applied and the other effect is to cool the junction when a current is passed through the junction in the proper direction (Peltier effect). These two effects can be very useful. Since the voltage at the junction depends upon the temperature of the end points, we may generate a voltage by heating one junction while holding the other constant in temperature, a source of electromotive force. The other effect is to make a cooling device, a refrigerator, by passing a current through the junction in the proper direction. In figure 1 is shown a thermocouple. This is the structure of a commercial thermocouple that is capable of sensing temperature changes at the junction. A very practical usage of the thermocouple is to control the safety of gas delivering systems. This is done by allowing the voltage produced to control the valve that delivers gas to the burners. If the burner goes out, there is no heat to the thermocouple and the voltage drops to release the valve and close off the gas flow, thus preventing a potential explosion. All gas systems are now required, by law, to have such safety valves. The device shown in figure 1 can be produced by the use of two electrically dissimilar metals such as copper and iron or copper and constantan. When the device has been constructed, it can be calibrated to read voltage and convert this into a temperature scale. If the amplifier gain is high enough the voltage can be read using an EA-200 to collect the data points. When this has been completed, the data are transferred into the graphing calculator for processing and testing for linear behavior over the temperature range of interest.

Figure 1. A thermocouple mounted into a finger and the thermocouple wires and junction exposed so it can be seen what is in the sensing probe.

PROCEDURE Construct your thermal junction by twisting a copper and an iron or constantan wire together at the ends to form a closed loop. Cut the copper wire in two pieces at the center and clean the surface to make good electrical contact with the iron. The voltmeter mode of the EA-200 can now be used to measure any potential difference at the terminals.

Figure 2. Schematic of the thermocouple set up for making a thermometer using the voltage generated by a thermocouple. The signal from the thermocouple was amplified to raise the voltage output and to match impedance to the EA-200 Data Collector/Analyzer.

Place the thermocouple in a small test tube to isolate it from water. If the temperature of air is to be measured, the isolation test tube is not needed. The tube is to isolate the junction from electrical interaction with the water. Place the thermocouple in the tube in about 250 ml of water in a beaker and place the beaker on a hot plate. Connect the voltage probe to the terminals of the thermocouple to warm it over a range of temperatures. You can also change the temperature by using a hair dryer to blow hot air over the temperature probe and the thermocouple junction. Put the temperature probe in channel 2 with the voltage probe in channel 1. You are ready to collect temperature and voltage data so the thermocouple voltage can be calibrated to become temperature.

6 4 2 0 10.7 16.8 24.5 33.4 43.1 53.4 63.9 75.3 86.1 -2 -4 VOLTAGE -6 -8 -10 TEMPERATURE

Figure 3. (Left) An Excel plot of voltage versus centigrade temperature for the thermocouple with a reference bath of ice and water to reference to 0 degrees centigrade. (Right) A picture of the fx-9750G calculator display window; the bold line is produced by equation 1 below.

Usually the voltage produced by the thermocouple junction is linear over a reasonable range of temperatures. When the data has been transferred to the fx-9750G graphing calculator it is tested for linear behavior. When you finish the plot the statistics for a linear least squares routine can be analyzed to determine how well the data fit a linear response by the r2 value. If the data do not fit a linear response, the graphing calculator function for X2 is used. The

number r2 should be very close to 1. The values change from +1…..-1. If the number is +1 the data has a perfectly linear response and the data are well correlated to a linear fit. If the value is -1 the data and a linear response are dis-correlated and the data have the greatest departure from a linear response. The data in figure 1 were fit by equation 1 as given below: T = -7.0592V + 33.524 (1)

2.4 2.3 2.2 2.1 2 1.9

VOLTAGE 1.8 1.7 1.6 1.5 14.8 19.1 23 28 33.6 39.5 46 52.7 59 64.9

TEMPERATURE (C)

Figure 4. (Left) An Excel plot of the amplified voltage versus centigrade temperature for the thermocouple with a reference bath of ice and water to reference to 0 degrees centigrade. (Right) A picture of the fx-9750G calculator display for the same experiment. Note the curvature is more pronounced and the data fit a quadratic model. This effect is due to the nonlinear property of the amplifier. The pictures were taken with a QV-7000-SX digital camera.

T = -104.849V2+331.175V -196.23 (2)

The thermocouple can be used to measure temperature by making a voltage measurement and converting to centigrade temperature by using equation 1. The voltage depends on the gain of the amplifier so each unit must be calibrated for the amplifier used in the measurement. The thermocouples used were obtained from a hardware store and produce up to 30 millivolts when heated with a blue gas flame. They serve as safety devices in conventional hot water heaters.

SUMMARY

The temperature sensor needs to be calibrated against a reference. This may be ice and

water at standard pressure or by use of a reference voltage against which the instrument is

calibrated. Both procedures were tested in this experiment. The type thermocouple chosen

must be one that will produce sufficient voltage to activate the meter used to measure the

output voltage. Of an amplifier is used, any nonlinear response must be considered for the

instrument to be accurate.

QUESTIONS

1. Can you save money by taking the energy from the Sun and converting it into electricity by using a thermocouple? Discuss the costs involved in providing such energy, if you answered yes to the question.

2. In the thermocouple part of this experiment you learned about converting heat to electricity. Discuss how this may be done efficiently by using the Sun's energy. Recall that focusing the rays of the Sun will multiply the heat energy falling on a given area.

3. The apparatus shown in figure 1 of this exercise is a pyrometer or a device for determining temperature. Discuss how you think this thing works.

4. One of the properties of nature is that if one process works, the reverse is true. That is, the generator of electricity produces electricity when a magnet is moved in a coil of wire. (The generator rule.) The inverse of this is that a current through a wire will cause a magnet to move. (The motor rule.) Since heating the thermocouple junction produces a voltage, might a current through the junction cool it? Look up the Seebeck Effect and the Peltier Effect on the internet and discuss these in the light of the “two faces of scientific processes”.

5. How many thermocouples of the composition studied above will need to be placed in series to light a 120 volt light bulb?

6. Since the energy from the Sun can be used to heat the thermocouple to produce electricity in the day time, discuss how we can store this electrical energy to be used when the Sun is not shining.

7. The reference junction of the thermocouple system needs to be kept at a fixed temperature to provide a reference for the second junction. Describe how the fact that the temperature of the soil at the surface of the Earth relative to a few feet below the surface is several degrees higher can be used to provide a temperature change that can drive the thermocouple system.

8. Based on what you learned about the voltage potential, how much voltage can be produced by the temperature difference found at a depth of one meter relative to the surface temperature. Determine the temperature difference by using a temperature probe and the EA-200 Data Collector/Analyzer.

STUDYING VOLTAGE GENERATION BY THE FORCE OF GRAVITY By Jim Roberts, Professor of Physics and Material Science The University of North Texas

OBJECTIVE: This activity is designed to see how we can generate electricity using gravity, to collect data with a Data Collector/Analyzer and display the data on a fx-9750G graphing calculator and to quantify the results.

INTRODUCTION It has been long known that the force of gravity can be used to work for us. The harnessing of gravity from falling water to first power machines and then to produce electricity has proven to useful in satisfying some of our energy needs. This experiment is designed to show how a falling object can drive an electric generator/motor to produce a voltage output for a mechanical force input. It can be shown that the acceleration of the mass shown in figure 1 produces a change in the output voltage generated as the object speeds up in its fall. By studying the amount of voltage generated a determination can be made of the potential energy available in waterfalls that can be converted to electrical energy. Moreover, we can develop a device to measure the speed and acceleration by using the principle of the conversion of notion to electrical energy by using the electric generator.

PROCEDURE

Set up the motor assembly as shown in figure 1. There are a number of small d. c. electric motors for sale today at modest prices. Any motor/generator will do. Remember that a generator takes a mechanical energy input and produces an electrical energy output while a motor takes an electrical energy input and produces a mechanical energy output. The process is an electro-mechanical transducer that can be operated either way, mechanical force in for an electrical output or an electrical force in for a mechanical output. Oersted discovered the “motor rule” in 1820 and Faraday discovered the generator rule in 1830. The processes are reciprocals of each other. Figure 1 shows schematics of the generator set up. The shaft of the generator (motor) shown on the left of figure 1 is turned as the mass M is allowed to fall in the earth’s gravitational field. The voltage output is proportional to the speed of turning of the coil of wire, shown on the right of figure 1, as the mass speeds up in its fall. In figure 2 is shown pictures of the setup for this experiment. The overall setup is shown on the left and the small motor with pulley and string is shown on the right side of the figure.

Figure 1 (Left) A sketch of a motor with a pulley and string to drive it using gravity and the mass M. (Right) A sketch showing the principle that a rotating coil in the field of magnets can produce electricity when motion is produced.

A picture of the apparatus for making the measurements is shown in figure 2. On the left is shown the overall setup that consists of the d. c. generator and the Data Collector for collecting time and voltage input. On the right is shown the motor with pulley and string drive to activate it in the gravitational field of the earth. Program the EA-100 or EA-200 for 10 mSec and 20 readings. The mass M will fall with a velocity given by:

V(t) = gt. (1)

The distance of fall is given by:

2 S = ½ gt + vot + So . (2)

If the mass is started from rest and the distance of fall begins at zero only one term remains to study the free fall of the mass. The equation reduces to:

S = ½ gt2 (3)

We can let the data collection begin for the cycle as the mass M is released and begins its travel downward. The distance of fall is given by:

S = ½(9.8m/sec2)(0.2sec)2 ≈ 20 cm. (4)

You can experiment with the number of data points and the time for each as needed by your specific circumstances. These depend upon the space that you have available to conduct the experiment. Transfer the data points from the Data Collector to the graphing calculator for analysis. The results will appear as shown in figure 3. Make your analysis of the results using a fx- 9750G or equivalent calculator. You may do a more detailed analysis by using a spreadsheet such as Excel and determine the ratio between the rate of fall and the voltage generated. The device can act as an accelerometer for moving objects.

Figure 2. (Left) An overview of the data collection and transfer system for taking data from the motor as the mass falls and turns the d. c. motor. (Right) A close-up view of the d. c. motor, with the capstan on the shaft to turn as the string unwinds and changes the speed of the motor. Any suitable mass is OK to serve as a driving force as it falls in the gravitational field.

Figure 3. Left. A display of the data taken with the EA-100 Data Collector/Analyzer. The data were analyzed by using the internal routines of the calculator. Right. An Excel plot of the voltage versus time for a d. c. motor made to spin using a falling mass and a pulley attached to the motor shaft. Note that as the object falls it picks up speed and the voltage generated increases accordingly.

The data curve displayed on the graphing calculator is quadratic in form. Analysis of the plot is achieved with the functions in the calculator. Once the profile and equation of the curve are known an algorithm can be written to determine the acceleration of the mass or any other mass for the system. This experiment has demonstrated the versatility of the EA-100 Data Collector/Analyzer and the fx-9750G to collect and analyze data. There are a number of applications that can be made of this result. The Atwood machine can be set up with the rotating pulley attached to an electric generator and a study made of the motion of the masses in the system.

QUESTIONS

1. If the system described in figure 1 is set up on the moon and the same experiment done, what will be the amount of voltage generated for the same distance of travel of the mass M?

2. A bicycle rider decided that if he/she could pace their rate of travel in making a journey through the mountains, they would minimize the strain that they were under by using such a device attached to their bicycle wheel to study its motion. Describe how this approach may be used to improve the biking strategy.

3. If the generator is loaded with a resistance and the string released to allow the mass M to spin the generator, what will this do to the acceleration of the falling mass? Remember Lenz’s law in answering this question.

4. If a light bulb is attached to the output terminals of the generator and the mass is dropped, what will happen to the glow of the light as the mass moves in the field of gravity?

5. Would different masses for M change the voltage output of the generator?

Data Analysis Using a Graphing Calculator and a Simple Harmonic Oscillator

By Jim Roberts, Professor of Physics and Material Science The University of North Texas

OBJECTIVE: This experiment is designed to use the fx-9750G graphing calculator to analyze data taken on an oscillating mass and spring assembly and to test linear properties of matter.

INTRODUCTION

In this experiment we will use a graphing calculator to plot data taken for an oscillating mass attached to a spring supported in the gravitational field of the earth. The characteristic property of the stretched spring and the subsequent oscillations set up will be studied using the properties of the graphing calculator. Specifically, we will determine the value of g through the use of this experiment.

Newton proposed the universal law of gravity that holds throughout the universe. It is stated as follows:

F = GmM/r2. (1)

Where G is a universal constant, m is the mass of an object in the gravitational field of the earth, M is the mass of the earth and r is the distance between the centers of mass of the two objects m and M. This equation held for over 300 years until Einstein showed that it is a specialized case of relativity. The important thing to remember is that it works and gives us the correct answer. When we are given certain values and are able to measure other values, it is possible to make select measurements. In this experiment we intend to measure the value of g for the earth by using an oscillating mass attached to a spring whose force constant is determined.

1 The graphing calculator is a very useful tool to plot data and analyze it. We need to set up an experiment and calibrate the spring that will serve as our force to drive the oscillator. This is done by choosing a set of mass increments that allows us to collect data on the amount of stretch of the spring X and the mass attached to the spring to stretch it.

The masses can be copper pennies that can be counted as they are loaded into a tray attached to the end of the spring. Washers can be used but consider the fact there is not uniform mass distribution over a box of washers. You can, however, weigh each washer and take into consideration that there is a spread in mass for the washers in each box that may be 10% or more from one washer to another. Usually, copper pennies are reasonably close to the same values, unless corrosion is on the coin. You can assume the spring stretch is linear and test the mass distribution of the washers in a given set.

Experimental Setup

Set up the oscillator as shown in figure 1 below. This apparatus is simply a spring with a mass attached to the end so that the assembly can be attached to a rigid mount and set in oscillation. The oscillator frequency is determined by the mass m, the mass of the earth and the force constant K of the spring.

6 MASS (GMS) 4

0 25 30 35 40 STRETCH (CM)

Figure 1. (Left) A diagram of a simple harmonic oscillator constructed using a spring and a mass attached to a vertical mount in the gravitational field g for “weighing” the earth. (Middle) A graph of mass vs displacement for a simple spring to find the force constant of the system. (Right) A graph of a rubber band stretched with masses. Note that the spring shows linear behavior and the rubber band does not over all of its range of stretch.

Figure 2. (Left)A calculator display of a graph of mass vs displacement for a simple spring to find the force constant of the system. (Right) A graph of a rubber band stretched with masses. Note that the spring shows linear behavior and the rubber band does not over all of its range of stretch. Pictures taken with the QV-7000SX digital camera.

Add units of mass to the spring and measure accurately the amount of stretch for the given mass. Enter the mass and displacement in table 1.

When the table has been completed, input the data for mass and displacement in the first two columns of your graphing calculator. The data are then analyzed by using the linear least square regression of the calculator. The value of K can be obtained from the slope of the mass vs displacement graph.

The results from table 1 can now be used as part of the next phase of the experiment to determine the gravity of the earth by assuming some simple values. G is well known and we can precisely measure the period of oscillation of the mass attached to the end of the spring. When we solve our equations we can determine the value of earth’s gravity to a reasonable degree of accuracy.

Set up a table for calibrating the spring to obtain its spring constant as follows.

3 Table 1. A summary of mass versus displacement data.

DETERMINATION OF THE FORCE CONSTANT K MASS DISPLACEMENT OF MASS

Discussion of Linear Systems

The stretch of a spring is linear over a reasonable range of stretch. Many materials behave in this way. There are some materials that are not linear. We can use the graphing calculator to display the linear and nonlinear properties of matter simply by using stretch and weights to measure the property of the material.

Equations of Motion for the Oscillator

The simple harmonic oscillator can be solved mathematically in the following way. The energy of stretch must equal the energy released when the spring recoils. This is a powerful concept. When the spring is stretched to a length X and released, the motion of the mass is simple harmonic motion. The period of oscillation for the motion is given by solutions of the equation of motion:

This equation can be solved to provide an equation for the frequency of oscillation for the mass for a spring of constant K and a mass m. The universal law of gravity given by Newton can be used to provide a relationship between the period of oscillation, mass m and the constant K.

4 Equation (1) can be rewritten by using the assumption that F = mg for the mass hanging on the end of the spring. Combining this result with equation (1) gives the result:

F = GmM/r2 = mg = -kX (3)

When all of the results are combined the frequency of oscillation of the mass on the end of the spring can be found from the equation:

f = 2Π(m/k)2 (4)

When you have set up the apparatus, choose units of mass and allow the oscillator to execute 10 oscillations for which you obtain accurate time and from the oscillation time calculate the frequency. Use equation (4) to calculate the theoretical oscillation frequency. Enter these values in table 2. When you have tabulated all of the experimental data, use your graphing calculator to calculate the difference and percent differences.

Table 2. A summary of the experimental results compared to calculated values for a oscillating mass and spring assembly.

EXPERIMENTAL THEORETICAL FREQUENCY FREQUENCY DIFFERENCE % DIFFERENCE

Set up the experiment again with a rubber band supporting the masses. Calibrate the displacement constant. Plot the displacement versus mass to see if it is linear. The first experiment is one done by Robert Hooke that showed that the displacement (stretch) versus mass (weight) is linear. Many systems obey Hooke’s law behavior. An interesting application of this law is Boyle’s Law for gasses under pressure.

5 Table 3. A summary of the experimental results compared to calculated values for a oscillating mass and rubber band assembly.

EXPERIMENTAL THEORETICAL FREQUENCY FREQUENCY DIFFERENCE % DIFFERENCE

Questions

1. An oscillator is driven by the force of gravity. Can you devise an experiment in which the value of gravity can be obtained from your data?

2. If you conduct this same set of experiments on the Moon, what do you expect to find out about the period of oscillation of the mass?

3. A group of “smart kids” decided that they could use the oscillator to see how fast they were climbing a high sloping roadway that went up into the mountains. Can you critique this experiment and see what may be wrong with it?

4. A pilot decided that he could use the oscillator to determine what his altitude is when he is flying at 40,000 feet. This requires that he is able to measure the period of oscillations at ground level and then at 40,000 above the ground. Use your graphing calculator to calculate the value of g at ground level and then at 40,000 feet. Do you think the pilot was realistic in his/her desire to measure altitude in with this oscillator?

5. What observations did you make about the stretched, oscillating rubber band as compared to the coiled spring?

6. What are some sources of error that may change our results?

7. Explain what is meant by a “linear coefficient of expansion”.

8. Compare the values of r2 (Chi square fits, not the radii) values that you got from the calculator for each phase of the experiment for the rubber band and for the spring. Elaborate on what may cause the difference.

9. Do you think that the amplitude of oscillation will affect the frequency of oscillation?

10. The energy stored in the spring when it is stretched must be converted to kinetic energy as the mass is released to oscillate. You should detect a small fixed error in the equation for change in K. E. = change in P. E. Observe the mass very carefully and determine if it simply moves up and down or if it “twists” as is oscillates. Do you think the energy difference between the experimental energy and the theoretical energy can be accounted for by rotational energy?

11. A “smart” physicist was also a fisherman. He/she enjoyed applying physics to all that was done. During a fishing expedition he/she cast a fishing line in the water with a cork on it to float the apparatus. The cork “bobbled” as it went into the water. The physicist decided to time the oscillations to see if a fish was nibbling on the bait to make the cork bob or if the initial oscillation was just the buoyant force of the water causing the oscillation. Is it reasonable to think one could find the difference in a fish tugging at the bait and a natural oscillation of the cork? Explain.

7 DETERMINATION OF THE HEAT OF FUSION OF ICE By Jim Roberts, Professor of Physics and Material Science The University of North Texas

OBJECTIVE: This experiment is designed to show you how much thermal energy is needed to melt a specified quantity of ice and to plot the results on a fX-9750G graphing calculator.

INTRODUCTION When a material changes phase a specific amount of energy is needed to change through each transition, solid to liquid, liquid to gas. The three phases of matter requires specific amounts of thermal energy to break the bonds and change the order of each molecular system as the system is cycled over the three phases, beginning with the solid phase and changing to liquid and then steam (gas). Water is one of the most studied molecular systems known as the water is cycled over the three phases of matter. Indeed, the age of the steam engine was characterized by harvesting the energy recovered from water as it was cycled from steam at 101° C to water at 99° C or below to operate steam engines to do the work needed . One of the most efficient methods of heating buildings was by using steam and utilizing the energy exchange as the steam cooled from above 100° C to below 100° C. The steam was transported into each room via a radiation strategically placed in the room. The steam was cooled by exchanging heat with the air in the room with the result that the air in the room was heated with the energy extracted from the steam. This experiment is designed to determine the amount of heat energy needed to melt a known amount of water ice, the heat of fusion. A specific amount of ice is placed in an insulated bath of water of specific mass. An immersion neater with known Wattage is used to heat the mixture of water and ice for specified time. The specific amounts of water and ice depends upon the Wattage of the

heater. This can all be calculated, but some preliminary results allow the experiment to be duplicated readily.

1 PROCEDURE Place a 1000 ml beaker inside a thermal jacket to control the heat flow. The shielding used

in this experiment was ordinary newspaper. Dry newspaper is a very good insulator of heat.

Put 5000 ml of water in the insulated beaker. Put 150 mg of ice in a small beaker, 500 ml

volume, and place the beaker in the larger container so that no water can enter this beaker.

Two temperatures are to be measured, that of the outer beaker as the heater raises the

temperature and that of the inner beaker containing the ice to be melted. It is important

that the temperature in two containers be independently measured. This can be achieved by shielding the temperature probe, all except the tip, and the top of the 150 ml is closed with insulating material. When all of the components are assembled the experiment is ready to be set up and data taken.

In Figure 1 the nature of the energy versus temperature profile expected for the experiment is shown. On the left is shown the profile of changing from solid to liquid to vapor. The energy needed to change one gram of ice into water is 80 cal/gm. The energy change in changing water form water to steam is 540 cal/gm. Measurement of this energy is more difficult than measuring the phase change from solid to liquid. Figure 2 (left) show schematically the thermal bath and calorimeter used in this experiment. A specific amount of ice is placed in the container with water. The volume of the ice is determined by measuring the amount of water produced when it melts.

2 Figure 1 (left) A schematic profile of the change in energy for a water system as it changes from ice to water to steam. (Right) An experimental result for the system described in this work, the heat of fusion for water is 80cal/gm. The upper curve shows the change in temperature of the heating bath and the lower curve shows the temperature of the ice and the resultant water as the ice melts.

The results of the experiment are shown in figure 1. (Right.) The lower curve is the curve for

temperature change as the ice is melted in the inner container. The upper curve is the temperature change in the outer container holding water to heat the inner container with

the ice to be melted in it.

Figure 2 (left)The experimental setup with the heat chamber, calorimeter, holding the water and ice to be melted. (Right) A picture of the experimental setup. The ice is contained in an inner container to isolate the heater from the thermometer. The entire bath is contained in a thermally insulated container.

The total amount of energy put in the calorimiter bath can be obtained from the power of

the heater and the time of the experiment. The heater used was 300 Watts. Thus, with

the time known for the heat exchange the total energy is given by:

Energy Input = Wattage (Joules/sec)Xtime(sec) = Joules (1)

E(J) = 300WX600sec = 180,000 Joules (2)

3 This energy serves to heat the total amount of water and melt the ice contained in the

calorimeter. The standard balance of “Heat lost = Heat gained” is used in

determining the results.

Figure 3 (left) A plot of the energy versus temperature for 160 gms of ice changing from ice at 0º C to water at 70º C. (right) A plot of the energy versus temperature for 160 gms of ice rising in temperature of –10º C changing from ice at 0º C to water at 70º C. The plateau at 0º C can clearly be seen in the figure to the right. During this transition the heat energy continues to flow into the ice to melt it but the temperature does not rise. Pictures taken with the QV-7000SX digital camera.

Immersion heaters are readily available at 200 Watts and 300 Watts at most hardware stores. Any unit can be used as long as the wattage is knows for the device. A more precise measurement can be made by using a set up that measures the voltage and current for each device to determine the Watts used to heat the system.

The amount of heat energy provided is given by equation 2. This heat energy will serve to heat the 4000 gms of water, melt the ice and raise the temperature of the entire system to the final value. Be careful to take into consideration all of the heat losses and heat gains in the system to obtain the best value of the heat of fusion of the ice.

4 QUESTIONS

1. Use the graphing calculator to solve the following problem. Ten grams of ice is

cooled to a temperature of -10 degrees in a deep freeze and then allowed to rise in

temperature until the water from the melted ice rises to 20° C. Calculate the energy

needed to change the ice from -10° C, melt the ice and then raise the temperature

of the ensuing water to 20° C.

2. How much energy is released when 10 gms of steam are cooled from 101° C (vapor)

to 99° C (liquid)?

3. If the energy released in problem 2 is used to move a piston in a cylinder of and

engine with the following geometry, how much weight can the assembly lift in the

field of gravity over a distance of 1 meter?

4. If the volume V in problem 3 is one m3 and filled with steam at 101° C, what will be

the volume when the steam cools to water at 99° C? Recall that the gas laws use

absolute temperature rather than °C.

5. Assume that the change in energy versus temperature curve in figure 1 is linear,

what value can you obtain for the Joule Mechanical Equivalent of Heat?

6. An ice storm “hits” an area of the country and deposits a sheet of ice one cm thick over the area. If the Sun’s energy rate is equivalent to 1400 Watts/m2, (This is equivalent to two hairdryers running at 750 Watts each.) how long will it take to melt a sheet of ice one meter square and one cm thick? Use what you have learned about the amount of heat energy needed to change the phase of ice from ice to water. 5

ELECTROCHEMISTRY: FARADAY'S LAWS AND ELECTROLYSIS By Jim Roberts, Professor of Physics and Material Sciences The University of North Texas

OBJECTIVE: This activity is designed to show how a simple experiment with electrical current can be used to count atoms. The data are displayed and analyzed using a graphing calculator such as the fx-9750G. The current is measured using an EA-100 Data Collector/Analyzer.

INTRODUCTION

All chemical reactions can be understood from the point of understanding the way in which electrons are shared among atoms to form molecules. A study of such electron exchanges is basic chemistry. The entire field of chemistry may be simplified as an exchange or sharing of electrons among the atoms to form compounds. It is important that we understand that this leads to the law of definite and multiple proportions. From this concept we can relate the number of atoms formed to the number of electrons produced in a current flow. The first part of the experiment we use the process in chemistry of oxidation-reduction. In a companion reaction, one of the partners is reduced and the other is oxidized. This exchange of electrons may lead to a pairing of atoms or chemical fragments to form a complete molecule or the process may be used to separate components into ions. Some oxidation- reduction reactions occur spontaneously, others have to be aided. The process can be accelerated or retarded by the use of electrical charges (currents). The potential difference across the ions will determine whether the components will be able to produce a reaction. In an electrolytic cell a difference of potential is impressed across the electrodes in the cell. These electrodes may supply additional electrons to the electrolyte or they may act as an agent to remove ions from the solution. The electrolyte contains both positive and negative charged elements. The positive charged component will migrate toward the negative electrode and the negative ion will migrate toward the positive electrode. This flow of ions gives rise to a current in the solution. Those ions that move toward the positive electrode (anode) are anions and those that move toward the negative electrode (cathode) are cations. The ions may be collected from the solution to form complete atoms as in the process of neutralizing the copper ions to produce pure copper, which is collected on one of the two electrodes in the solution. This 1 process of removing metallic components from solution is a very important industry. The two processes of oxidation and reduction associate with the electrodes in that at the positive electrode we say we have "oxidation" in the charge exchange process and at the cathode we have "reduction" in the charge process. There are a number of examples of electrolysis in nature. In fact, the process of "corrosion" in which materials are destroyed or weakened by charge and metal exchange, runs maintenance costs into billions of dollars each year. One process, which is useful in industry, is the reclamation of sodium (Na) from its compound with chlorine (Cl). This process of separation is achieved by heating the NaCl to a molten state and then placing electrodes in an apparatus similar to that shown in figure 1. When a current is passed through the electrolyte the Cl- ion is attracted to the positive electrode, forms a gas and "bubbles" away unless it is trapped. The Na+ ion is attracted to the negative electrode, becomes sodium metal and is deposited onto the electrode.

The electrolyte of NaCl may be replaced with NaCl and H2O and the reaction is slightly + - - + + modified. The electrolyte consists of Na , Cl , OH and (H3O) ions. The H ion will displace the Na+ ion at the negative electrode because it has a stronger affinity for the electrode than does the sodium ion. The sodium gains a "partner" by choosing the OH- ion to produce

NaOH. The by products of the process are H2, NaOH and Cl2. The H2 and Cl2 will escape as

a gas unless trapped. This is one way of producing these components industrially. In fact, the process of reclaiming metals or reducing corrosion by electrolysis is a very important industry.

Figure 1. A solution of NaCl in water with electrodes to remove the ions from solution. Note that NaCl is a solid and Na is a solid but Cl is a gas at room temperature. Thus, when the ions are changed to elements, one will “bubble” away and one will become attached to the other electrode.

2 It should be clear that if the components can be reclaimed from solution, the process has application in electroplating. Electroplating can be achieved by choosing the correct ion in solution for plating out of solution and then pass a current through the solution to remove the metal ions. The ions need not be metallic for their removal from solution. In fact an experiment that has promise for future application can be conducted by separating the hydrogen and oxygen ions in water to form gases. The hydrogen gas can then be burned in the presence of the oxygen and the waste product is water. One can see the potential of such a procedure for fueling internal combustion engines.

Figure 2. Basic apparatus for conducting experiments in electrochemistry to reclaim metal ions of copper from a copper sulfate solution.

Michael Faraday in the Nineteenth century formulated a set of laws to deal with the processes of separating ions in solution from one another through the use of electricity. These laws are referred to as the Faraday Laws of Electrolysis and have broad application.

The laws may be stated as: 1. The total amount of charge Q passed through an electrolytic cell will liberate a given amount of mass m. 2. The amount of mass m liberated is proportional to the gram equivalent of the material liberated. The valence of each ion must be considered to establish the ratio between the gram equivalent and the mass of a given atom. A monovalent ion requires one electron per atom, a bivalent ion two electrons per atom, etc. Faraday combined these laws into a form that can be used: m = QA/(Fn) = ItA/(Fn) 3 Where, A is the gram equivalent of the atom, Q the total charge (determined by Q=It, with I the current and t the time.), n the valence number and F a Faraday or 96,500 Coulomb/mole. The laws can now be used to determine the properties of valence and the value of Avogadro's number. You might guess that if the electrical current can be used to separate charges is it possible to use the reverse action of the electrolytic cell. This reverse process relies on the spontaneous oxidation-reduction process to drive the charges around an external loop to produce a battery. This cell is a battery or a fuel, cell which relies upon the chemical reaction in the electrolyte to produce potential energy. In a storage battery, the external charging source serves to store energy in the electrolyte for future delivery to the external circuits. By utilizing the circuit given in figure 1, we can conduct our experiment in electroplating and in determining the value of Avogadro's number.

PROCEDURE:

The experiment that we will perform is one that can yield Avogadro's number from simple results and using Faraday's laws of electrolysis. 1. Set up the apparatus as shown in figure 3. *2. Set the current to pass through the solution at a value of 0.1 Amperes by setting the voltage control on the power supply shown in figure 3. 3. Start the timer to make a measurement of elapsed time and close the circuit to start the current flowing in the circuit. 4. We need the total charge Q that is transferred through the loop. This is given by Q = It. (Note that I is in Coulombs/second so you have to convert your time to seconds to get the proper units.) 5. Once you have allowed the equipment to run for about 30 minutes, you will have enough data to complete the data table given below. *You may need to adjust the current setting to a different value that depends upon the strength of the copper sulfate solution. If one electrode is turning black you are using too much current for the strength solution used.

Figure 3. The setup for measuring the current passed through the electrolyte solution. The schematic diagram is shown on the left and the actual setup is shown on the right. The current is determined by measuring the voltage dropped across a standard value resistor. If a one-Ohm resistor is chosen, the meter will convert voltage directly into current. The copper sulfate solution is shown as blue in the beaker containing the electrodes to be measured. Data for current and time can be taken either with the EA-100 or the EA-200 Data Collector/analyzer.

Summary Table for the Experimental Data

I t Q V M1 M2 ∆M No Electrod e E1 E2

EXPERIMENTAL DATA

E1 and E2 are the different electrodes in the solution. Record the mass changes for each electrode to get two values for Avogadro’s number. One electrode will lose mass to the solution and the other will gain mass in the same amount. One electrode returns copper to the solution as ions and the other captures ions to form copper metal in the process.

The data table provides you with data from which you can calculate Avogadro's number. Notice that one electrode has its mass reduced while the other electrode has its mass increased. The two measurements in change in mass ΔM should give you the same answer. One electrode has been "sacrificed" to the solution and one has metal plated onto it.

5 This process of sacrificing one material to save another is used in the pipeline industry to retard corrosion, which costs billions of dollars each year. The electrochemical processes of the soil are retarded by means of this anode, which is often magnesium in composition. Prevention of corrosion of the space capsules becomes important when they are to be introduced into some planetary atmospheres. How much does your answer differ from that of the experts? You can calculate this by the equation: Percent error = [(standard reading-your reading)]X100% (standard reading)

% error = [(6.0221367X1023 - your reading)/6.0221367X1023]X100

When a current is passed through the solution, the electrons will go into solution at the negative electrode and "neutralize" the positive ions, whereas, the positive electrode will offer neutralization at its surface to neutralize the negatively charged ions in solution. This part of the experiment is now complete. You have counted atoms and should have a reasonably good number for your efforts.

6 QUESTIONS

1. Why are the two volumes of gasses liberated at two platinum electrodes placed in water and a current passed through the electrolyte produced in the ratio 2:1?

2. Why do you think aluminum cans are recycled but ordinary tin cans are not?

3. Much of the earth is covered by water that contains hydrogen and oxygen. The experiment that you have conducted shows that we can reclaim hydrogen from water and burn it in the presence of oxygen by using two electrodes to separate the hydrogen from the oxygen in the water. Why don't we collect the hydrogen from the ocean and use it for automotive fuel?

4. What is the waste product of burning hydrogen in the presence of oxygen?

5. The human body burns carbon to produce carbon dioxide as a waste gas from the lungs. The plants use this waste as their fuel and produce oxygen as their waste product. What do you think will happen to our atmosphere in the next 100 years if we keep cutting the trees and covering the grass with blacktop?

6. The byproduct (waste) of plants is oxygen. How can we improve our oxygen content in the atmosphere using this knowledge about plant life?

7. In producing pure metals from their oxides heat is used to melt the metal and drive out the impurities. Why can’t this process be used for aluminum oxides?

8. If the one-Ohm resistor R in figure 3 is replaced with a 10-Ohm resistor, what will change in the results?

9. If we assume that Avogadro’s number is correct, can you describe how we can calibrate the value of a standard resistor?

10. If we want to calibrate a voltmeter, how can we use the results of question 9 to do so by using Faraday’s law and Avogadro’s number?

7 Heat Exchange in Cooling by Evaporation

By Jim Roberts, Professor of Physics and Material Science University of North Texas

OBJECTIVE: This activity is designed to show how the basic laws of heat exchange can be used to show that objects wrapped in a wet paper towel cool by evaporation. Data collectors and graphing calculators are used to show how the data can be collected and then displayed for analysis.

Introduction

The basic law of heat exchange is summarized in the figure below. This is an expression of the exchange of energy law that pervades all of nature. Heat energy may flow into one object from another with the result that one object cools and the other is heated up. All of the energy available is conserved for the “system”. One of the ways to solve problems scientifically is to view the processes in nature as comprising a system subject to fundamental laws.

Figure 1. A picture of the burrow of a tarantula spider. The thin web over the holed is designed to reduce the flow of infrared energy entering it. This figure shown the application of laws of radiation and cooling or heating processes associated with the exchange of heat energy. The web will both shield out certain wavelengths and admit other wavelengths. Visible waves are hardly stopped by the web, allowing light into the hole. The soil surrounding the hole collects heat during the daytime and radiates it out at night and the area is “cooled by radiation”.

Interesting effects occur on the earth through the variable exchange of energy between land mass and water masses. This simple experiment can be used to show how the thermal energy of the sun can warm land and water bodies at different rates and then when the sun goes down the bodies cool at different rates causing the winds to blow in patterns that vary from the evening to morning times. Moreover, the nature of cooling of the earth can be better understood when we speak of cooling of the earth from night to day. The flow of heat energy out of the system leads to cooling at night and then warming during the day as new solar energy flows into the earth. When the rains come and wet the soil the evaporation of the water will cool the area. In some parts of the world evaporation coolers are used to cool houses. Air is allowed to slow across an absorber with water vapor in it. As the water evaporates in the device, the flowing air is cooled and the houses can be cooled in the process. Some ancient castles in the Mideast were cooled by allowing water to flow around the castle wall in troughs that overflowed to provide a mist through which the air blew to cool the castle.

Two different type containers are used in this experiment to study heat energy flowing out of and into reservoirs of water. The heat energy is exchanged between the volume of water and the air for a given time. The cooling rate for each container is monitored using the EA-200 to collect the temperature over time for about one half hour at intervals of 30 seconds.

Figure 2. A schematic diagram showing the basic law for heat exchange. One part of the system loses energy and the other absorbs the energy. This process is fundamental in the behavior of a “system”.

Procedure

Put about 60 ml (60 cm3) in two containers. One of the containers is wrapped in a cloth wet with water. The second container is wrapped in dry cloth of the same padding. Both containers are made identical in every way except for one cloth being wet with water. This procedure allows only one variable to change at a time. This brings up a basic rule in science, “fix all variables but one and allow only it to change”. Everything else must be kept the same.

Figure 3. A picture of the apparatus needed to study the heat energy exchange in this experiment. These results of the study allow us to understand how heat exchange and cooling by evaporation can be measured. The beaker contains 60 cm3 of water.

Set up the temperature probe and the EA-200 with a folded paper towel soaked in the liquid chosen for evaporation. The experiment is now ready to be conducted.

Set the EA 200 Data Collector up to read a total of 200 readings with one reading at each1 second interval. This should provide sufficient data to study the heat exchange associated with the experiment. The data plot shown in figure 4 demonstrates how the process of evaporation of a liquid can be used to cool an area.

The apparatus is set up as shown in figure 4 to demonstrate how different liquids will change the rate of cooling of the area. Two liquids were chosen, water and ethanol. The temperature probe was placed between two layers of paper towel and the liquids were allowed to evaporate from the towel. The data show a more rapid rate of cooling for alcohol than for water.

Different liquids can be chosen with varying intervals of time to show how the process of heat exchange takes place.

Figure 4. A close up view of the set up to measure the thermal energy exchange between the temperature probe and the air surrounding them. The liquid in the towel is water for the curve shown.

23.5 23 22.5 E 22 21.5 21 20.5 20 TEMPERATUR 19.5 19 18.5 0 10 20 30 40 50 60 70 80 90 100 TIME

Figure 5. A plot of the temperature change versus time for two liquids, ethanol ▲and water ■. Note the more rapid rate of cooling for the ethanol as compared to water.

22.6 22.55 22.5 22.45 22.4 22.35 22.3 TEMPERATURE C 22.25 22.2 10 30 50 70 90 110 130 150 170 190 210 230 245 ELAPSED TIME

Figure 6. A picture of the graphing calculator with data for 60 ml of water cooled by wrapping it in a paper towel soaked with water to show cooling by evaporation. The graph shows an exponential decrease in temperature with time. Data were gathered using an EA-200 Data Collector/Analyzer. The curve to the right is an excel plot of the same data with number of points reduced to 25 instead of using all of the points collected by the EA-200.

Figure 6 shows the effect of cooling for a larger volume of material than the air surrounding the temperature probe. This shows that with the proper amount of liquid to evaporate, a reasonably large volume of material can be cooled.

QUESTIONS

1. If the rate of evaporation changes, how will this affect the temperature of the water in the container?

2. If the water is replaced with different alcohols, how do you think the cooling will respond?

3. Think of several liquids to use in the experiment and make a scientific guess the time needed to produce a specific amount of cooling.

4. The rate of evaporation is determined by the surface area exposed to the environment. Can you design an experiment to show that the rate of cooling follows a similar trend and that the rate of cooling is proportional to the area of the surface exposed to the environment?

5. Compare the coefficients in the quadratic fit of the data with the fx-9750G graphing calculator and the analysis made using the excel fits.

6. Automobiles have radiators on them to cool the engine so that it will not overheat. Discuss how the radiator increases the surface area to release more heat from the engine.

7. Dip the temperature probe into several different liquids and use the EA-200 to collect time and temperature for about five minutes. Does the rate of cooling depend upon the different liquids as it did in question 3 above?

HOW GOOD IS THAT BATTERY By Jim Roberts, Professor of Physics and Material Science The University of North Texas

OBJECTIVE: This experiment is designed to show how to determine the life time of a battery, to plot the results on a fX-9750G graphing calculator, to determine an equation for the change of voltage over time under a specific load and from the results predict failure times for batteries.

INTRODUCTION All sources of energy have finite lifetimes before they become too exhausted to function properly. Batteries are in extensive usage to day in many different devices. If the voltage and current demands for an electrical device are known, the length of time that the battery can operate the device efficiency can be determined. Many companies produce batteries with differing quality of manufacture. This activity allows us to test a specific battery to determine its ability to deliver the energy claimed. It becomes useful to test each battery product to determine what potential life exists in a given battery. These units can be tested by using a simple circuit for a fixed load and measure the rate of drain from the battery.

PROCEDURE Set up the circuit shown in figure 1 so the time and voltage can be measured for a given interval of time. This set up enables us to determine the change in voltage with time or V(t) and see how long it takes for the battery to have its voltage reduced by a specified amount. This might be the value of half of its original value. We can refer to this value as the “half- life” of the battery. If the battery cannot operate the device at a value below 80%, for example, the usage of the battery can be predicted for the time and load at which the battery is technically “dead” to operate that device.

Figure 1 (Left) A schematic diagram of the circuit needed to test the lifetime of a battery. (Right) A picture of the setup to measure the decay of voltage versus time for select batteries. Either the EA-100 or the EA-200 Data/Collector Analyzer can be used to obtain the voltage over time. The picture was taken using a QV-7000SX Casio digital camera.

A plot of voltage versus time will tell the story of how the ability of the battery to deliver a certain amount of voltage to the load. The formula that describes the curve of voltage versus time can be used to predict when the battery will be “dead” or when it has reached a terminal value such that it can no longer provide the voltage needed to operate a certain device.

2 1.55 y = 0.0002x - 0.0097x + 1.5188 R2 = 0.9919 1.5

1.45

VOLTS 1.4

1.35

1.3 1 3 5 7 9 11 13 15 17 19 21 23 25 27 TIME (MIN)

Figure 2 (Left) An Excel plot of the decay curve for a battery. The data from the calculator was reduced by taking only every fourth data entry. (Right) A graphing calculator display of the time dependent voltage curve for a loaded battery. The load was a 10Ω resistor. Note that the average of the voltage follows a power law shape. The equation for this battery decay is 0.0002X2 - 0.0097X + 1.500. X is the time in seconds. The intercept may change from 1.5 V if the calibration is not correct and if the battery’s initial voltage is different. The trend in decay is the same.

2 The voltage of the battery and the resistance of the load will determine how long the experiment needs to run. The source of voltage chosen in this activity was an AA battery which delivers 1.5 volts when it is at its maximum value. Two loads were chosen, one of 10Ω and another of 2.5Ω. This will allow a range of operation to be obtained. The equation that relates the time of failure of the battery tested with a 10Ω load is given by the equation below: V(t) = 0.0002X2 - 0.0097X + 1.500 (1)

A fX-9750G graphing calculator was used to display the data and to obtain the equation that best fits the data curve. An Excel analysis was used to finalize the equations and to compare the results with the analysis obtained with the graphing calculator.

3 QUESTIONS

1. Discuss how this experiment can be used to set up a quality control study to test

lifetimes of batteries produced by a specific manufacturer.

2. How much time will it take for the battery tested in figure 2 to be reduced to one half

of its original voltage when loaded with a ten Ohm resistor?

3. A portable CD player consumes 0.01125 A of current at 3.0 volts to make it operate

properly. If the device fails to operate when the batteries have been reduced in

voltage to 0.8 of the original value, how long will the device operate? Assume the

power dissipated is described by equation 1 and scale the equation from 1.5 volts to 3

volts.

4. You have been hired to test the quality of batteries produced by several

manufacturers of batteries. Describe how you can test the batteries from each source

to show what capabilities each battery has.

5. Some “prankish” students decided to connect a piece of wire that had a resistance of

1Ω across a battery like the one tested in tested in this experiment. Assuming the

batteries are identical, calculate with your graphing calculation and using equation 1

the time it will take for the battery to be reduced to ½ of its original voltage.

6. How much power is produced by the wire when the battery is fully active at 1.500

volts?

7. In question 6, how much power does the battery produce across the resistor when the

voltage has been reduced to ½ its original value of 1.5 volts?

4 Construction of a Chemical Battery Using Lemons and Dissimilar Electrodes

By Jim Roberts, Professor of Physics and Material Science The University of North Texas

OBJECTIVE: The objective of this activity is to show that an electrical potential can be produced by the use of electrodes and lemons. The voltage can be measured by using the EZ-200 Data Collector/Analyzer.

INTRODUCTION

The construction of a battery requires the basic components, two electrodes (dissimilar metal) and an electrolyte. Such cells are the result of electrochemical potentials produced when chemical reactions occur between the members of the cell. By choosing a suitable electrolyte and different electrodes, select potential differences can be produced.

In this activity various electrodes are selected and placed into a lemon to determine what the electrochemical potential will be. A series of lemon cells are arranged in series to produce a battery that can be used to energize electrical devices or to light a light emitting diode. The potential differences are measured by using a data collector. The voltage produced by each cell is measured and the data set of voltage versus number of cells is input into a graphing calculator to show the linear dependence of voltage versus number of cells in a linear array. PROCEDURE

The cell is constructed by using alternate layers of lemon slices and two dissimilar metals such as shown in figure 1.

Figure 1. (Left) A sketch of a single stack of electrodes and an electrolyte (lemon) used to produce an electrical potential. (Right) A stack of “cells” used to produce a lemon battery. The stack of cells should produce six times the potential of a single cell. Since silver is expensive, the electrodes for this experiment were zinc plated iron and copper.

There is some ambiguity in describing the difference between a cell and a battery. Cells come in increments of 1.5 volts for carbon and zinc electrodes with ammonia paste as the electrolyte. A stack of these in series of six produces a 9-volt battery and four of these in series will produce six volts.

Figure 2. (Left) A picture of simple lemon battery constructed by using six lemons and two metal electrodes, copper and zinc plated iron and the EA 200 Data collector used to gather the data and the fx-9750G used to graph the data. (Right) A picture of the graphing calculator screen plot showing how the voltage changes each lemon is added in series with the others. The pictures were taken with the QV-7000 SX digital camera.

The data were taken with the EA-200 set for one second intervals and the probe was moved from each cell to the next until all cell voltages had been tested. The data were then adjusted to retain four data points for each voltage setting. A linear Least Square regression was made of the data using the graphing calculator. The data for the electrode set and the lemon electrolyte give a linear equation of:

Y = 0.628X (1)

The result from the graphing calculators show that the r2 fit is 0.96, indicating a good linear fit for the voltage versus number of cells result. X is the number of cells and Y is the voltage produced by the assembly.

Construct a data table of the total voltage versus the number of cells in the stack. Enter the data into the graphing calculator and study the plot of voltage versus number of cells. Is the plot linear? Use the statistical analysis for a linear least squares regression and determine the level of fit for the data. Use the equation of the plot to answer question 1 at the end of this activity. QUESTIONS

1, How many cells such as shown in figure 1 will be needed to light a 120 volt light bulb? Use equation 1 to predict this result.

2. The electric eel can produce several hundred volts of potential capable of causing a severe shock for anyone in the water nearby. Describe how you think the eel can produce such large potentials along its body.

3. Try the set up described in figure 1 by using a sliced Irish potato for the electrolyte. What voltage do you read for six cells constructed with the sliced potato and copper and silver for electrodes?

4. If the electrodes in figure 1 are replaced with iron and copper what potential do you read with the data collector?

5. Examine the table of electrochemical potentials provided in the list and find the two electrodes that will produce the maximum potential for a single cell.

6. Replace the electrolyte (lemon) with salt water. What potential do you find for a single cell of copper, silver and salt water?

7. Chemical batteries fail after a period of time. Describe the life cycle of a carbon, zinc and ammonia paste battery. Will it fail suddenly or will it slowly die over time?

8. A nickel and cadmium (NiCd) battery has special characteristics for its life span. This is shown in the plot of voltage versus time provided above. Describe the difference between the two types of batteries, carbon cells and NiCd composition batteries in their nature of failure.

9. Study the following table and determine the electrode combination that will produce the maximum theoretical potential difference.

10. Metal tanks, gas transmission lines and water pipes corrode in the soil in which they are buried. Discuss how this process works in the light of what you have learned in this exercise.

Figure 3. A close up view of the zinc, Copper and Lemon battery composed of six cells in it. The surface area of each plate (electrode) is 2.5 cm by 2.5 cm.

Light Output Experiment and Current Relationships in a Light Bulb Using Technology and Probes By Jim Roberts, Professor of Physics and Material Science, The University of North Texas

OBJECTIVE: This experiment is designed to establish the validity of intensity of light versus current square law using probes and data analyzers (EA-200) and graphing calculators like the fx-9750G.

INTRODUCTION

This experiment uses Ohm’s Law, which is one of the most fundamental laws of electric circuits. The light circuit is analyzed using this law. The law of electricity that showed the relationship for voltage, current and resistance was first worked out by G. S. Ohm and is now called Ohm’s Law. It shows how the three quantities relate. All circuit analysis rests upon the mutual dependency of these three quantities, voltage V, current I and resistance R. The circuit below is set up using the data collector/analyzer (EA-200) with two voltage probes attached. One probe is used to measure voltage and the second probe is used to measure current in the following way. One voltage probe is placed across a one-Ohm resistor. When the voltage is measured across this resistance, the measurement is scaled to change the unit of voltage into current by the ratio of Ohm’s law. I = V/R.

Figure 1. A set up to test the validity of the light intensity law by using the EA-200 data collector to obtain current and voltage. The voltage probe (current) is inserted in channel 1. A one-Ohm resistor R1 serves to convert the voltage reading into current. A pictorial set up is shown in the right of the figure. The light intensity is measured using the light probe provided with the EA-200. The graphing calculator is ready to receive the data

1 from the EA-200 after it has been collected. Note the opaque tube surrounding the light source to shield the probe from stray light.

The voltage from a variable power supply (shown with an arrow across it to indicate a variable source) is varied in increments that will provide current and voltage variations in the closed circuit. The intensity of the light is measured using a light probe coupled to the EA-200 Data Collector/Analyzer. (See figure 1, right.) When the circuit has been set up, each setting of the voltage is made at increments of one

volt to set the independent variable and the current (voltage across R1) is measured (dependent variable). The two values are tabulated in a data table to measure the pattern of voltage versus current. The data can be transferred from the EA-2oo to the fx-9750G (See figure 1.) Further analysis can be made by transferring the data sheet into a spread sheet such as Excel.

Table I. Data table to record data taken for a resistor using the EA-200 modified to make current and voltage measurements and light intensity for a light bulb.

VOTAGE CURRENT INTENSITY

Table I shows the (x,y) pairs of voltage (V1) and current (V2/1Ω) and to records the intensity of the light. The one Ohm resistor converts the units from voltage to current as per the scheme shown in figure 2 (left).

When the data have been collected by the EA-200 it is input into the CFX 9850GB plus graphing calculator for observation and analysis. The students should be required to plot the data in a standard way using graph paper and suitable axes. This procedure will demonstrate the power of technology in the classroom. One of the requirements for students is to gain experience in graphing as part of the TEKS.

Figure 2. (Left) A sketch of the symbol of the IEEE used to show how voltage can be converted to current by using a scaling resistor. (Right) A picture of the screen of the fx- 9750G graphing calculator showing the intensity versus current for a light bulb. The voltage was changed and the current read for fixed increments of the voltage. Note the “glitch” in the curve. This is due to the interference of the room light as the brightness of the light changes relative to the room.

7 6 5 4 3 2 1 LIGHT ARB UNITS ARB LIGHT 0 0 0.95 1.95 2.95 3.95 4.95 5.95 6.95 7.95 8.95 9.95 11 12 CURRENT (ma)

Figure 3. An Excel plot of the data taking every fifth point from the graphing calculator to show the trend in the data for the light versus current pattern. The light intensity varies as the square of the input current to the filament. The equation for the data fit is y = aX2 + b. This trend can be checked with the graphing calculator.

3 QUESTIONS

1. Discuss what happens in a circuit when current flows through a resistance to produce light.

2. How does the light intensity vary with current?

3. How does the light intensity vary with voltage?

4. Why does the light bulb show a change in voltage and current when it begins to glow as the voltage is raised to its rated value?

5. Would the same method work for a. c. current to produce light output from the bulb?

6. If two bulbs are placed in parallel for the same voltage, what will be the intensity of the light output?

7. If two identical bulbs are placed in series for the same voltage as for one bulb, what will be the intensity of the lights?

8. Using what you have learned about the way current heats the filament in a light bulb, show the expected intensity can be calculated using the equation Watts = I2 R.

4 STUDYING POLARIZED LIGHT

By Jim Roberts, Professor of Physics and Material Science The University of North Texas

OBJECTIVE: This experiment is designed to show how light polarized by one lens of Polaroid® sun glasses can be studied by using the second lens to analyze the light as the lenses are rotated relative to each other with the light intensity measured using a light probe. The ensuing light change with relative angle of rotation is plotted on one of the family of fX-9750G graphing calculators.

INTRODUCTION Ordinary light coming from most sources is “vibrating” with equal probability through an angle of 360°relative to the direction of propagation. When this non-polarized light is subjected to special media, reflected or selectively refracted, it will become polarized. Francois Arago observed that as light passes through certain liquids with suspended materials in them the light vector leaving the solution is polarized in accordance with the nature and amount of the suspended materials in solution. This discovery has provided a powerful tool of analysis for studying solutions. It has been observed that some solutions rotate the light vectors clockwise and some rotate the light vector counter clockwise. The angle of rotation is dependent upon the concentration of the solution for a given solute. It has also been noted that certain materials such as dichroic crystals will produce the same effect. Polaroid® sunglasses with these selectively absorbing materials will produce polarized light. In this activity we show how the polarized glasses can be used to polarize light with one lens acting as the polarizer and the second acting as the analyzer. The light intensity is measured with a light probe as one lens is held stationary and the second rotated about its axis in fixed increments of angle.

1 PROCEDURE

Obtain some sunglasses that have the ability to polarize light and not just “dark

glasses”. Usually, the identifying label of Polaroid® will indicate this property. The lenses can be separated so that one can be rotated with respect to the other one and the light output measured as the angle or rotation is changed. One lens is fixed in rotation while the other is rotated in increments of about 10º. The light should vary in intensity as the rotation takes place if the glasses chosen have polarizing capability.

1. Obtain two lenses from discarded Polaroid® sun glasses.

2. Fashion an arrow along the edge of each lens so the relative orientation can be

measured. Just cut an arrow shape from sticky labels and paste the labels onto the

edge of each lens. One lens becomes the analyzer and the second the polarizer.

Either lens will serve to act as a polarizer or an analyzer since the light vector position

is relative. This procedure can be improved by gluing the lenses to two cylinders that

fit inside each other. The tubes can then be rotated easily, after the glasses have been

trimmed to fit the ends of the tubes.

3. Turn on the light source and rotate the lens about its axis in increments of 10º as the

light output is measured with the light probe. Use a 180º protractor to measure the

angle of rotation of one lens as it is rotated about its axis.

4. Record the angle of rotation and the light intensity that goes with each angle in a data

table for future input to the fX-9850G graphing calculator for analysis.

5. Use the EA-100 Data Collector/Analyzer to read the light intensity for each angle.

6. Plot the data for X and Y with X the angle of rotation and Y the light intensity.

Figure 1. (Left) A schematic setup of the experiment with the two lenses in place and the incident light polarized by one lens and analyzed by the other lens. The light probe is placed in the exit beam of the light. (Right) A picture of the setup with the light source, light tube with polarizer and analyzer attached so one tube can be rotated as the light is measured for each angle.

The relative rotation of the lenses can be achieved by use of two tubes that fit inside each other. This assembly is shown in Figure 1 on the right side of the figure. The plastic lenses are cut to fit the ends of each tube and then glued in place so they can be rotated relative to each other. The angle of rotation is determined by use of a pointer attached to the rotating tube. The data can now be obtained by fixing the tube assembly so that the bright light shines into the tube at the front end of the assembly.

When the light intensity versus angle of rotation has been measured and tabulated, the data are input into the graphing calculator for analysis to determine the nature of the light behavior as the analyzer is rotated relative to the polarizer. These data are tabulated below for one experiment. Figure 2 shows the plot of a sine wave, an exponential plot and the experimental data for comparison.

3 Table I. Experimental data and trial fitting results for polarized light using Polaroid® sunglasses’ lens to polarize and analyze the light.

ANGLE INTENSITY 950Sin[2π(Θ+10)/360] 950Exp[2π(Θ-84)/360] 0 104 168 111 10 179 212 179 20 248 306 273 30 372 437 391 40 492 584 527 50 656 725 668 60 791 840 797 70 859 911 895 80 917 927 945 90 938 886 940 100 824 794 879 110 776 665 773 120 565 518 640 130 440 376 499 140 299 259 366 150 228 186 252 160 202 167 164 170 211 205 135

1000 800 600 400

INTENSITY 200 0 0 20 40 60 80 100 120 140 160 RELATIVE ANGLE (DEG)

4 Figure 2. An Excel plot of the raw data ♦, a Gaussian fit of the data X and a Sine wave plot of the data ■. The “best fit” for the data is given by the Sine wave.

______

Figure 3. A plot of the experimental data with a Sine curve passing through the points. The FX-9750G Plus graphing calculator determines the “best fit” sine curve for the data. This is achieved easily by choosing the sin plot in the menu shown on the picture above. The cursor needs to be moved one setting to the right to find the sine function to fit the data, The data fit the curve y = 381 Sin(0.039X-1.684)+547.

Once all of the data have been collected, the data analysis can be made using the appropriate functions in the graphing calculator. Many applications can be made using the properties of polarization of light.

5 QUESTIONS

1. Find a digital watch and analyze the light coming from the dial by rotating one of the

Polaroid ® sun glass lenses around its axis as you view the numbers on the dial.

Describe what you see.

2. Repeat the activity in problem #1 by looking through one of the Polaroid ® sun glasses

and rotating it as you view the digital display on the gas station pumps. Describe

what you see as the lens is rotated about its axis.

3. Try the same activity in question #2 as you view an ordinary light bulb. Describe what

you see.

4. What property do you think makes the difference in what you view in questions 1 and

2 that are different in problem 3?

5. Go to the web site and learn what you can about a Polarimeter.

6. Allow a beam of light from a flashlight to be reflected from a plane mirror with the exit

ray leaving the surface at about 45º. Take one of the lenses and rotate it about its axis

while viewing the reflected beam of light. Describe what you see.

7. Late in the evening when the sun is red looking and setting or in the morning when a

similar effect is observed, observe the sun through the lenses of the sunglasses.

Describe what the light does as the lens is rotated as in the experiment above.

8. What is happening to the light from the sun?

6 Using a Data Collector-Analyzer and Graphing Calculator to Show How the Sun’s Energy can be Concentrated

By Jim Roberts Professor of Physics and Material Science University of North Texas

OBJECTIVE: This activity is designed to show how solar energy can be focused to produce highly concentrated energy flux using a data collector/analyzer and a graphing calculator.

INTRODUCTION

It is well known that light waves travel in straight lines. Any wave can be changed in direction by reflection and by using the lens effect to make the rays that represent the wave converge or diverge from the source. In this activity it is demonstrated that energy from the sun can be brought to focus and high temperatures can be produced. Test tubes of water are used to determine the amount of heat energy gain produced by focusing the rays of heat coming from the sun. Both test tubes are the same volume and composition to simplify the comparison for the temperature changes.

EXPERIMENTAL ARRANGEMENT

The EA-200 is programmed for the time and number of points to be gathered in the experiment. Two temperature probes are used so the experiment can be conducted more rapidly. Also, by taking data for both the focused and non-focused energy at the same time all other variables are fixed and a simple comparison of the data can be made.

Figure 1. (Left) A drawing of the lay out of the experiment. Two types of rays are shown, one focused (top) and the other not focused (bottom). (Right) A picture of the experiment in operation. Aluminum foil is used to make the flat reflector (left side of the box) and curved surface (right side of the box). The bottom of the curved surface should be at the same distance from the test tubes of water ad the flat surface. The picture was taken using the QV7000 Casio digital camera. Figure 1 is shows the experimental set up with both a drawing of the focusing apparatus and a picture of the actual experimental set up. The experiment was run for 15 minutes.

In figure 1, the rays coming from the sun, left represented by the arrows, will land on the curved surface and will be focused at A to increase the concentration of energy. The case for a non-curved surface will not concentrate the energy but will reflect it off the surface. Thus, the area at A in the curved surface case will have a greater temperature for the same time exposure. This experiment is to show that the concentration of energy can be used to significantly raise the temperature at A. The test tubes filled with water are placed at A, the focal distance of the curved surface. The flat reflector is placed the same distance from the second test tube.

) 23.98

23.48

22.98

TEMPERATURE (C 22.48 1 13 25 37 49 61 73 85 97 109 121 TIME

Figure 2. (Bottom) An Excel plot of the temperature rise with time for the flat reflector (lower curve) and the curved, focused, reflector (top curve). Note that the two curves appear to level off after a time t. This appears to be due to the fact that the wind was blowing across the apparatus and the heat was being carried away by the airflow. The Excel plot shows only125 of the 255 points taken with the fx-9750G graphing calculator. The top view of the figure shows a picture of the display for all of the data points for the two temperature channels. (Left top) Unfocused light. Note that the data are not continuing to rise as rapidly as the focused energy shown at the top right. The picture was taken using the QV7000 Casio digital camera.

When the data are collected by the EA-200 they are transferred into the CFX-9850GC Plus calculator for display and analysis. These data curves for the flat reflector and the curved reflector are given in figure 2. Data were also input into an excel spreadsheet for further analysis.

The experiment can be varied by changing the curvature of the reflecting surface. Changing the composition and reflective properties of the surface will also change the outcome of the experiment. Try different types of set ups to find the best conditions for reflecting the hear energy (infrared energy) from the sun onto the test tube. William Herschel, an astronomer discovered this range of energy from the sun by accident in the 19th century.

QUESTIONS

1. Can you estimate how hot the area in A will get if you know the time of the experiment? Discuss how you could devise a hot dog cooker that would cook wieners rapidly and yet not burn them.

2. How much difference in temperature was found for the two containers of water located at A for the curved surface and for the flat surface?

3. If you reverse the curvature of the surface and compare the temperature produced at A for the flat surface and the curved surface, how much do you expect the temperature to change?

4. If we change the color of transparent paper and place it over the apparatus to shield different color of light, how do you think the data will change?

5. Since heat energy is responsible for the rise in temperature, can you devise an experiment that will stop the light and let the heat through? Discuss this set-up.

6. Can you describe how what you have learned from this experiment can enable you to construct a solar wiener cooker?

7. If the airflow across the apparatus is stopped, what will change in the curves?

8. Try dull metal that will not reflect the light and do the experiment. What do you predict will happen?

9. Try using screen mesh under the heat reservoirs to deflect the radiant energy. Does the mesh size change the results? Think about radar dishes and what wave lengths they reflect and focus.

10. Would similar results be found if the energy from the sun is focused by using glass lenses? Discuss this. Using a Data Collector-Analyzer and Graphing Calculator to Find the Speed of Sound in Air

By Jim Roberts Professor of Physics and Material Science University of North Texas

OBJECTIVE: This activity is designed to show how the speed of sound propagated in a garden hose can be determined by using an EA-200 data collector/analyzer to collect data and a graphing calculator to display the signals.

INTRODUCTION

Have you wondered how fast sound travels in air? When one watches a baseball game there is always a delay between the time that the batter strikes the ball and the sound arrived at the ear of the people in the stands. Light travels so rapidly that it appears that the bat strikes the ball at the same time that we see the batter swing. How can we find out how fast the sound travels to us from the bat? This experiment shows how that can be done using an ordinary garden hose to guide the sound and two microphones to determine the delay as the sound travels through the garden hose.

In order to understand more about the experiment, we need to understand the meaning of time. Time is just an interval between two events. It may be the time from sunrise to sunset. How long it takes for the moon to return to its initial position in the sky…etc. Time can be measured by using the internal sampling rate of the EA-200 Data Collector/Analyzer. The “ticks” can be scaled to a speed that allows us to capture signals from two positions, one at the input end of a garden hose and another at the output end of the garden hose.

Garden hoses come in different lengths, 25 ft., 50 ft., etc. The longer the hose the more time it takes for the sound impulse to travel through the hose.

Procedure

A garden hose is set up according o the picture shown in figure 1. Two microphones are needed, one to record the signal at the input end of the hose and a second to record the signal at the output end of the hose. Since the length of the hose is known and the time of travel can be measured, the only remaining unknown is the rate of travel or speed of sound inside the hose.

In figure 2 is shown the format for setting up the timer base in the EA-200 to determine the interval of time between the two events, one when the sound enters the hose and the second when the sound impulse arrives at the other end of the hose.

Figure 1. A picture of the apparatus for measuring the speed of sound in air. The sensors for the input and exit signals are condenser microphones #270-092. (obtained from Radio Shack) The microphones are powered by a 9 volt battery. Picture taken with the QV-5500SX digital camera.

Figure 2. The “spike” at position 1 starts the time interval and the spike at position 2 ends the time interval during which the sound travels through the garden hose.

0.4

0.2

0 1 4 7 1013161922252831 -0.2 PULSE AMPLITUDE PULSE

-0.4 TIME (10XmSec)

Figure 3. Experimental data for the time of flight of a sound pulse through a 50 ft. garden hose. The horizontal axis, X, is the interval of time for the scan. In this case only 300 mSecs of time are displayed. Note that the echo pulse shows some dispersion (broadening of the signal) as it travels through the hose.

When the balloon at the left end of the hose is stretched and released it will make a sharp “snap”. This impulse will travel through the hose to the far end in a time t. On the left side of figure 3 is a picture of the screen display of a graphing calculator showing the spikes for the sound pulses. The number of data points needs to be reduced and the scan range expanded around the two lower spikes to determine the time interval between the two spikes. This reduction in number of points is shown in the excel plot in the right side of the figure 1.

When we know the time interval for the sound traveling between position 1 and position 2, we can use the standard equation for rate, time and distance to find the speed from equation (1).

Rate X Time = Distance, vt = d Eq (1)

When the value is obtained, the accuracy of the measurement can be checked by compared the experimental value with the standard value. This is done using equation (2).

Percent Error =[|true value – experimental value|]/{true value} X 100 Eq. (2)

Remember that the speed depends on the temperature, so you need to measure the temperature using the EA-200 and the temperature probe. Compare the value of the speed of sound with the correct temperature. Use your graphing calculator to calculate all values in the experiment.

From figure 3 we can find the time between the two pulses, the one at the entrance of the garden hose and the one at the exit end of the garden hose. This interval is about 45 mSecs. This gives a speed for sound about 330 Meters/sec. All of the distance needs to be converted to the SI system. The hose length is 15.24 meters. Calculating the speed using meters and seconds gives 339 meters/sec. The percent error, assuming 330 meters/sec is:

{[|330-339|]/330}X100% ≈ 3% Eq. (3)

Part of the error can be attributed to the temperature effect on the speed of sound. In that the value of 330 meters/sec is for 20 degrees C, the value expected should be higher in that the temperature on the day of the measurement was higher than the standard reference temperature.

QUESTIONS

1. Since time is an interval between two events, can you list a number of examples? Galileo used his heart beat rate to measure the period of the chandelier in the Duomo in Pisa. Is this a good metric for time?

2. What is the system that determines the standard interval of time today?

3. Can you devise a scheme for determining the speed of a baseball?

4. Light travels many times faster than sound. Can you devise a way to use the EA-200 to measure the speed of light? If not, what are the limitations?

5. If the hose is filled with another gas than air, will the speed of sound change?

6. If the pressure of air in the hose is changed, describe how the speed may vary?

7. Sound travels at about 1100 feet/second. If you see a flash of lightening and hear the thunder one second later, how far away is the lightening?

8. The hose can be filled with different gasses and the speed of sound measured for each gas. Would you think low mass gas molecules will have a different speed for sound, at the same pressure and temperature, than high mass gasses? Explain your answer.

9. The “half-second” rule is used when observing lightening displays. How far away is a bolt of lightening if the half-second rule holds?

10. Olaus Roemer, a Danish scientist, measured the speed of light by observing the change in occultation time of Io as the earth and Jupiter moved around the sun. Use the internet to find information about this experiment and explain how this experiment is like the one described in this activity.

Studying the Rays of Energy from the Sun By Jim Roberts, Professor of Physics and Material Science University of North Texas

OBJECTIVE: This activity is designed to show how the rays from the Sun vary as the angle of incidence changes with daylight and darkness and over the seasons. Data collectors and graphing calculators are used to show how the data can be collected and then displayed for analysis.

Introduction

It is well known that the rays of the Sun change angles during the passing of day as well as for long periods during the various seasons. The change over the seasons is due to the fact that the spin axis of the earth is tilted 23.5° relative to the orbital plane of the Earth. This tilt is responsible for the differing temperatures around the globe as we go from one season to another. In this experiment we will take data on the heat exchange between the Sun and the Earth as the solar energy falls on the surface at different angles. This effect can be studied by taking temperature data on surfaces of the same type materials with areas that make different slopes with respect to the Sun’s incoming rays of energy. Examples of such surfaces can be found in the design of some buildings.

Figure 1 (Left) A sketch of the angles for three surfaces used to test the angle of incidence for incoming rays from the Sun. (Right) A picture of the apparatus used to make the required measurements to test for the effect of the angle of incidence for the incoming energy. Picture taken with a QV 7000 digital camera.

Figure 1 shows an example of how the rays of the Sun can be monitored by taking the temperature change over time for two different angle surfaces. The material tested must be of the same composition to make a proper test. Both rates of cooling and rates of heating of the surfaces should be studied. This can be set up by finding a place where the Sun is casting its rays onto the surface for a time and then has passed overhead so that the rays no longer fall on the two surfaces. They should begin to cool. At this time the data collector is set to take data for a new time interval to test the rate of change of temperature for about 30 minutes. This can be especially pronounced if the experiment is conducted for cooling rates just after the sun has set. The picture is a bit more complex then in that the earth will cool by radiation as the heat energy escapes from the surfaces and cools them. Interesting effects occur on the Earth through the variable exchange of energy between land mass and water masses. This simple experiment can be used to show how the thermal energy of the Sun can warm land and water bodies at different rates and then when the Sun goes down the bodies cool at different rates causing the winds to blow in patterns that vary from the evening to morning times. Moreover, the nature of cooling of the earth can be better understood when we speak of cooling of the earth from night to day. The flow of heat energy out of the system leads to cooling at night and then warming during the day as new solar energy flows into the earth. The rule is very clear: Heat lost is equal to heat gained as the heat energy is exchanged in the area.

Figure 2. A schematic diagram to show the basic law of heat exchange. Energy will flow from the warmer to the cooler body. The rates of cooling or rates of heating will determine the direction of the heat energy flow as the energizing source is turned off. This heat exchange may be considered as a system for the purpose of analysis.

40 ) 38

30 TEMPERATURE (DEG C 1 3 5 7 9 11 13 15 17 19 21 TIME (X5)

Figure 3. (Left) An Excel plot of data taken over a period of 100 seconds to collect energy from the Sun. The upper curve is for the slanted part of the wall and the lower curve is temperature change for the vertical wall. In the right side of the figure is shown the display of the fx-9750G for a longer temperature cycle. Note the effects of wind currents cooling the system.

Procedure

Find a location where two walls make different angles with respect to the incoming rays of the Sun. Make sure the walls are made of the same material (i.e., bricks, stone, etc.) This reduces the number of variables in the experiment. Set up the two temperature probes such that they are parallel to the surfaces and lie along the direction of the rays of the Sun. This can be done by following the shadow of a pencil placed normal to the horizontal surface. The two temperature probes are then aligned with the shadow and vertical to the shadow to establish the angle to collect data.

The data presented in figure 3 were taken with the Sun making an angle 27° relative to the North –South Meridian and 35° South of overhead. Although the experiment is designed to show relative changes in the ability of the surfaces to absorb heat energy from the sun, it is part of completeness in the experiment to consider all variables in the study. It is also important to determine if the wind is blowing as it will carry away some of the heat energy. This effect can be seen in the extended data plot shown in the right of figure 3. When you have completed all of the activities above, please answer the questions provided below.

QUESTIONS

1. As the Earth rotates the angle of the Sun’s rays will change a small amount. Can you guess what mathematical expression can show how this change in angle will change the average temperature over time?

2. Are the results in this experiment expected to be the same for a cloudy day as for a clear day? Explain.

3. Lakes and streams lie horizontal to the surface of the Earth. This allows the rays of the Sun to fall across the surfaces at the same angle. As the Sun appears to move across the sky, how do you expect the temperature to change for lakes as the Sun rises and sets? Can you devise an experiment that will allow you to use the data collector to prove your assumption?

4. In question 3, how does the temperature behave after the Sun sets and the energy of the Sun is no longer providing energy to the surfaces?

5. What is the plot of the data as the temperature is rising in the experiment? i. e., Is it linear, quadratic or some other form? Use your graphing calculator to show the nature of the curve.

6. If the walls are replaced with highly reflecting surfaces, what will change in the experiment? Test your assumption by using the data collector and placing aluminum foil on each surface.

7. If the wall material is a conducting material, will there be a difference in the rate of heating of the vertical and angled surfaces?

8. Try several experiments with different kinds of material shielding the rays of the Sun. This can be done by using plastic materials that have different colors. When you have taken data for several kinds of materials, determine what “color” light seems most responsible for raising the temperature of the surfaces.

Using a Data Collector-Analyzer and Graphing Calculator to Show How a Television control unit sends signals to the TV or VCR Using Infrared Signals

By Jim Roberts Professor of Physics and Material Science University of North Texas

OBJECTIVE: This activity is designed to show how invisible infrared signals can be detected and analyzed by using an EA-200 data collector/analyzer to collect data and a graphing calculator to display the signals.

INTRODUCTION

Have you ever wondered how the remote control communicates messages to the TV or VCR? The technique is really very simple. It works using a standard procedure that has a messenger (carrier), a message (signal) and a receiver (listener) who understands the messages. All communication takes place this way; the only differences are what each role is played for the three, carrier, message and receiver. This experiments shows how this can be done by using a data collector, display graphing calculator, TV control unit and an infrared receiver diode.

PROCEDURE

A photo diode that receives infrared IR can be purchased from such stores as Radio Shack. The unit used for this experiment is a matched pair of transmitter and receiver diodes. The stock number is 276-142 and the item is Infrared emitter and detector. The emitter (transmitter) is not used in this activity. Only the detector is needed. This device is a diode that produces a voltage when the IR signal falls on it.

Connect the receiving diode to the voltage probe of the EA-200 and insert the plug in channel 1. The instrument is now ready to receive any IR signals in the near distance. It will be necessary to shield the detector by using a tube wrapped around the diode so that only the signal from the TV control and energize the diode. The setup and the schematic are shown in figure 1.

Once the apparatus is set up and the diode shielded so that it receives only IR from the TV sending unit, the data are taken according to the program in the EA-200. Experiment with different number and function inputs to see if you can discover the code used by the manufacturer. These codes are similar to the Morse Code that relies on dots and dashes or pulses of long and short time duration.

Figure 1. (Left) A diagram of the apparatus for collecting the data and displaying it visually. (Right) A picture of the apparatus in place ready to conduct the experiment. The pictures were taken with the QV 7000 Casio digital camera.

2 1.5 1 0.5 0 VOLTAGE OUTPUT 1 15 29 43 57 71 85 99

-0.5 113 127 141 155 169 183 TIME (0.1 SEC)

Figure 2. An Excel plot of the signals generated by the TV control box. Note the on-off appearance as well as amplitude changes. The signals are encoded to represent on-off, channel number, etc., according to the manufacturer. The signals displayed out to channel 157 are for on-off and the latter signals are various channel numbers. It is possible to identify each channel signal by producing such bar graphs as shown above. At the right is shown the display screen for the graphing calculator.

Each manufacturer builds in a code for their TV programmers and the TV has to be programmed to respond to each control. Can you explain how what you have learned can be used to determine the code for a particular TV so that you can control it. Redo this experiment using a garage controller and see if you can compare the TV control activities to how garage door controls work.

QUESTIONS

1. Why do you think IR signals are used rather than light pulses in the visible range of our eyes?

2. Are all of the manufacturers using the same set of coding inputs? Explain why.

3. Experiment with different time sequences and study the shape of each pulse for duration and amplitude.

4. What do you expect to change if you use different color plastic sheets between the output of the TV controller and the infrared detector?

5. Will paper sheets stop the transmission?

6. Will cloth stop the transmission?

7. What do the signals look like for each of the numbers on the TV control?

8. The message to the TV is “carried” by the IR signal and the number code is the message. What kind of carrier wave is used to transmit the TV pictures to the TV?

9. Is the signal frequency for garage doors in the same range as a TV control?

10. Do you think you could control a garage door with a TV control? How?