2 Electric Current and Electrical Energy

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CHAPTER 17 Introduction to Electricity SECTION 2 Electric Current and Electrical Energy

National Science BEFORE YOU READ Education Standards After you read this section, you should be able to answer PS 3a these questions: • What is electric current? • What is voltage? • What is resistance? • How does a cell produce electrical energy? • What are thermocouples and photocells?

What Is Electric Current? We use electricity every day to watch TV, use a STUDY TIP computer, or turn on a light. Electricity makes all of Reading Organizer As you these things work. read this section, make a table. The table should de- Electrical energy is the energy of electric charges. In scribe what electric current, most of the things that use electrical energy, the charges voltage, and resistance are (electrons) flow through wires. and how one property affects The drawing below shows electrons moving through a another property. wire when it is connected to a battery. This movement is called an electric current. Electric currents provide the energy to things that use electrical energy. READING CHECK 1. Describe What movement causes an electric current in a wire? + When a wire is connected to a battery, electrons will move through the wire. Electrons moving through a wire make up current. This provides energy to the things that you use every day.

An electric current is the rate at which charges pass a given point. Electric current is like the current of a river or a stream. When the current is high, more charges pass READING CHECK the point each second, like a fast-flowing river. We talk 2. Name What unit is used about electric current in units called amperes, or amps. to measure electric current? The symbol for ampere is A. In equations, the symbol for What is its symbol? current is the letter I.

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STANDARDS CHECK MAKING CHARGES MOVE PS 3a Energy is a property of When you flip the switch on a flashlight, the light comes many substances and is associ- on quickly. But do charges in the battery instantly reach the ated with heat, light, electricity, mechanical motion, sound, bulb? No, they don’t. When you flip the switch, it sets up an nuclei, and the nature of a electric field in the wire at the speed of light. The electric chemical. Energy is transferred in many ways. field makes the free electrons in the wire move. The energy of each electron is transferred instantly to the next electron. 3. Explain What makes electrons move when you turn on a lamp? COMMANDING ELECTRONS TO MOVE When you flip a switch, the electric field is created very quickly. All of the electrons start moving through the wire at the same time. Think of the electric field as a command to the electrons to move ahead. The light comes on right away because all of the electrons obey the command at the same time. So, the current that lights the bulb starts very quickly even though each electron moves slowly. A single electron may take more than an hour to travel through one meter of wire.

AC AND DC There are two kinds of electric current—direct current (DC) and alternating current (AC). In the drawing below you can see that in direct current, the charges always flow in the same direction. In alternating current, the direction of the charges continually changes. It moves in one direction, then in the opposite direction.

ONE WAY

Direct Current

TWO WAY

Alternating Current

Charges move in one direction in DC, but charges continually change direction in AC. Critical Thinking The electric current from the batteries in a camera or 4. Describe What kind of a flashlight is DC. The current from outlets in your current makes a refrigerator home is AC. In the United States, the alternating current run and in what direction do the charges move? changes directions 120 times each second. That means it has 60 cycles each second. Both kinds of current give you electrical energy. For example, if you connect a flashlight bulb to a battery, the bulb will light. And, you can light a household bulb by putting it in a lamp and turning it on.

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What Is Voltage? If you are on a bike at the top of a hill, you can roll down to the bottom. This happens because of the difference in height between the two points. The “hill” that causes charges in a circuit to move is voltage. This pro- cess can occur in an electric circuit. Voltage is the potential difference between two points in a circuit. We use volts, V, as to measure voltage. We write voltage in equations with the letter V.

VOLTAGE AND ENERGY Voltage is a measure of how much work is needed to move a charge between two points. Voltage is like the water pressure in your plumbing system. More volts means more energy pushing electrons through the wire.

VOLTAGE AND ELECTRIC CURRENT The voltage between two points on a wire causes charges to flow through the wire. The size of the current depends on the voltage. If the voltage between two points on a wire is increased, more current will flow in the wire. READING CHECK 5. Describe What happens to DIFFERENT KINDS OF VOLTAGE the electric current in a wire Different devices need different amounts of current to when voltage is increased? run. So, batteries are made with many different voltages. For example, it takes a large current to start a car. So, the battery in a car has a high voltage, 12 V. Things that run on batteries often need a low voltage. For example, a portable radio might need only 3 V. If you have a device that uses direct current, you would probably use one of the batteries in the photo below.

12 V

6 V Batteries are made with various voltages for use in many different devices. Critical Thinking 1.5 V 9 V 6. Analyze Why do batteries come in different sizes and voltages?

Most devices in your home use alternating current from an outlet. In the United States, outlets usually have AC at 120 V. So, things like TVs, toasters, and clocks run on 120 V.

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What Is Resistance? Have you ever tried to run in a swimming pool? The water makes it hard for you to move ahead. That’s a type of resistance. With electricity, resistance is a factor that determines the amount of current in a wire. Resistance is the opposition to the flow of electric charge. Opposition means “to work against.” So, a material that is resistant lowers the flow of current. We use ohms () as a measure of resistance. In READING CHECK equations, the symbol for resistance is the letter R. You can think of resistance as “electrical friction.” The 7. Describe What is resistance and how does it affect higher the resistance of a material, the lower the current the ﬂ ow of current? flowing through it. So, if the voltage does not change, as resistance goes up, the current goes down. Resistance depends on four things: an object’s thickness, length, the material it is made of, and temperature.

RESISTANCE, THICKNESS, AND LENGTH The figure below shows how the thickness and length READING CHECK of a wire affect the wire’s resistance. This figure com- 8. List What are the four fac- pares the flow of electricity through a wire to water that tors that affect resistance? flows through a pipe. The pipe filled with gravel is like a wire. The water is like the flow of electric charges.

A thick pipe has far less resistance than a thin pipe does. This is because there are more spaces between pieces of gravel in a thick pipe for water to ﬂ ow around.

A short pipe has less resistance than a long pipe does. This is because the water in a short pipe does not have to move around as many pieces of gravel.

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RESISTANCE AND MATERIAL Materials with different amounts of resistance can do different jobs. Good conductors, such as copper, have low resistance. Electricity flows easily through them. That’s why we use good conductors to make electrical wire. Poor conductors, such as iron, have higher resistance. Materials with high resistance are useful. For example, Critical Thinking the tiny wire in a light bulb is made of resistant material. 9. Infer Toasters and space It heats up and gives off light heaters heat up when current ﬂ ows through them. Do they use material with low RESISTANCE AND TEMPERATURE resistance or high resistance? Resistance also depends on temperature. The resistance of metals usually increases as temperature rises. The atoms vibrate faster at higher temperatures. They get in the way of the flowing electric charges. If you cool certain materials to a very low temperature, resistance will drop to 0 . Materials in this state are superconductors. Very little energy is wasted when electric charges move in a superconductor. However, it takes a large amount of energy to cool them. Scientists are studying how to use superconductors to store and transmit energy. One possible application of superconductors is maglev trains. These are trains that use magnets made by an electric current in coils of wire. The magnetic field produced raises the train above the track and pulls it along. If the coils could be made from a superconducting material, operating the train would cost Say It much less than it does today. Investigate Research how superconductors work and their potential uses and report to the class.

One interesting property of superconductors is that they repel magnets. The superconductor in this photograph is repelling the magnet so strongly that the magnet is ﬂ oating.

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How Do We Generate Electrical Energy? Recall that energy cannot be created or destroyed. It can only be changed into other kinds of energy. Many things change energy into electrical energy. For example, generators convert mechanical energy (the energy of motion) into electrical energy. Cells convert chemical energy or radiant energy from the sun into electrical energy. Batteries are made of one or more cells.

How a Cell Works

a A chemical reaction Flow with the electrolyte leaves extra electrons c on one electrode. This If the electrodes electrode is made of are connected by a zinc. wire, electrons ﬂ ow TAKE A LOOK through the wire and ions move in the 10. Identify What are the b electrolyte. The mov- parts of a cell? A different chemical ing charges make an reaction causes elec- electric current. trons to be pulled off the other electrode. In this cell, this electrode is made of copper.

PARTS OF A CELL A cell, such as the one in the picture above, contains a mixture of chemicals called an electrolyte. Electrolytes allow charges to flow. Every cell also has a pair of electrodes made from conducting materials. Charges enter or exit electrodes through wires. Chemical changes between the electrolyte and the electrodes convert chemical energy into electrical energy.

KINDS OF CELLS Two kinds of cells are wet cells and dry cells. Wet cells, such as the one above, have liquid electrolytes. A car battery is made of several wet cells that use sulfuric acid as the electrolyte. You can make your own wet cell by pok- ing strips of zinc and copper into a lemon. When the metal strips are connected, the cell generates enough electrical READING CHECK energy to run a small clock or light a small light. 11. Name What are two Dry cells work in a similar way. But the electrolytes in kinds of cells? dry cells are solid or like paste. Radios and flashlights use batteries with dry cells.

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PHOTOCELLS If you look at a solar-powered calculator, you will see a dark strip called a solar panel. This panel is made of several photocells. A photocell converts light energy into electrical energy. How do photocells work? Most photocells contain silicon atoms. As long as light shines on the photocell, electrons gain enough energy to move between atoms. The electrons can move through a wire to provide electrical energy to power a device, such as a calculator. READING CHECK With larger panels, photocells can provide energy to 12. Name What device buildings and cars. Large panels of photocells are even changes light energy into used on satellites. Photocells change light energy from electrical energy? the sun into electrical energy to power the many devices on the satellite.

THERMOCOUPLES We can change thermal energy (heat energy) into electrical energy with a thermocouple. The picture below shows a simple thermocouple. You can make a thermocouple by joining wires made of two different metals into a loop. The temperature difference in the loop causes charges to flow through the loop. You must heat one part of the loop and cool the other. The greater the temperature difference, the greater the current. READING CHECK Thermocouples do not usually generate much energy. 13. Name What device But, they are useful for monitoring the temperatures of changes heat energy into car engines, furnaces, and ovens. electrical energy?

Burner

Iron wire

Copper wire In a simple thermocouple, one section of the loop is heated and one section is cooled. The current Ice water meter shows that an electric current is produced. Current meter

Section 2 Review NSES PS 3a SECTION VOCABULARY cell in electricity, a device that produces an elec- resistance in physical science, the opposition tric current by converting chemical or radiant presented to the current by a material or energy into electrical energy device electric current the rate at which charges pass thermocouple a device that converts thermal through a given point; measured in amperes energy into electrical energy photocell a device that converts light energy voltage the potential difference between two into electrical energy points; measured in volts

1. Explain Cells can convert what types of energy to make an electrical current?

2. Apply Concepts The two wires below are made of copper. They also have the same temperature. Which wire has the lower resistance? How can you tell?

A B

3. Describe What is the difference between alternating current and direct current? What type of current runs in the wires of your house?

4. Describe How does increasing resistance affect the current?

5. Make Inferences How does the current from a 1.5 V flashlight battery compare to the current from a 12 V car battery? Explain.

6. Describe What are the three factors that would give a piece of copper wire high resistance?

13. It keeps electrical charges away from your 12. a photocell hands. 13. a thermocouple 14. Charges move easily through conductors. They cannot move easily through materials Review that are insulators. 1. chemical or radiant energy 15. rubber 2. Wire A has the lowest resistance because it 16. friction is shorter. 17. Electric discharge is the loss of static elec- 3. In direct current, the charges always flow tricity as charges move off an object. in the same direction. In alternating cur- 18. Water droplets, ice, and air move inside a rent, the charges flow in one direction, then cloud and create friction. That makes posi- reverse direction. Houses use AC. tive and negative charges build up inside the 4. It slows the current. cloud. 5. The current would be much greater with the 19. Anything that sticks up or out in an open car battery because it pushes electrons with area can provide a path for lightning. more force. 20. It hits the rod and travels to the ground 6. long length, thick diameter, high temperature through a wire. SECTION 3 ELECTRICAL CALCULATIONS Review 1. It doubles. 1. friction and induction 2. resistance They would stay the same because like 2. P V I 12 V 700 A 8,400 W charges repel each other. This is explained 3. by the law of electric charges. 4. V I R 2 A 12 24 V 3. the shock you receive when you touch a 5. The 60 W bulb gives off energy more quickly. doorknob and lightning 6. watts and kilowatts 4. No, you can’t tell if the charge is positive 7. E P t 30 W 3,600 s 108,000 J or negative, because the electroscope only 8. E P t 5 kW 8 h 40 kWh shows that a charge is present. 9. Turn off the lights; turn down heat or 5. Because rubber is an insulator, and charges air conditioning; use a fan instead of air will not pass through it. conditioning.

SECTION 2 ELECTRIC CURRENT AND Review ELECTRICAL ENERGY 1. Circuit B has the higher current. 1. movement of electrons 2. Current: increases 2. amperes or amps, A Voltage: stays the same 3. an electric field in the wire Resistance: decreases 4. Alternating current makes a refrigerator run. Energy use: increases AC charges flow first in one direction, then 3. V I R 3 A 9 27 V in the opposite direction. 4. E P t 200 W 12 h 5. Electric current is increased. E 2,400 Wh 2.4 kWh 6. They must fit into the device that uses the 5. More voltage pushes more current through battery. Each device needs a different the wire. Lower resistance lets more current voltage. flow through the wire. 7. Resistance is opposition to the flow of electric charge. It decreases electric current. SECTION 4 ELECTRIC CIRCUITS 8. material, thickness, length, and temperature 1. an energy source, wires, and a load 9. high resistance 2. First circuit should be labeled “closed 10. electrolytes and two electrodes connected circuit,” and second circuit should be by a wire labeled “open circuit.” 11. wet cells and dry cells