Understand Electricity

part 1: Static electricity

part 2: Current electricity

Don Mathewson Kwantlen University College [email protected] www.donmathewson.com www3.telus.net/mathewsonphysics lessons 1. Charge 2. Plus and Minus Drawings 3. How TV works 4. How lightning happens 5. Voltage and Current 6. Resistance: Insulators and Conductors 7. Short Circuits 8. Ohm's Law 9. Circuits in the Home (series) 10. Circuits in the Home (parallel)

©Don Mathewson 2006 1 equipment list lesson 1: several rolls of scotch tape Van de Graaf (VdG) generator () lesson 2: electroscope (); electrostatics kit () balloon, puffed rice, plank, watchglass VdG, pie plates, Volta's hail storm, wand lesson 3: demonstration CRT+ wires (Boreal ) old TV cut-away lesson 4: Tesla coil (Boreal ) Plasma ball (Radio Shack) VdG and fluorescent tube lesson 5: 1.5 Volt bulb + holder; 1.5 Vcell and holder;tap switch; wires old tennis balls (free from local tennis club); juice; device potato clock or lemon clock (s17science.com) lesson 6: 1.5 Volt bulb + holder; 1.5 Vcell and holder;tap switch; wires common materials for conductivity testing digital multimeter (candian tire) lesson 7: 1.5 Volt bulb + holder; 1.5 Vcell and holder;tap switch; wires common materials for ohm's law testing digital multimeter (candian tire) analog ammeter lesson 8: 1.5 Volt bulb + holder; 1.5 Vcell and holder;tap switch; wires analog ammeter steel wool; lantern battery; fireproof tray lesson 9:

©Don Mathewson 2006 2 Electricity lesson plan 1: charge idea: charge is a property of matter there are 2 types of charge that we call + and - like charges repel, unlike charges attract if an object has equal + and 0 charges, it is neutral. activity: two tape lab (when two tape strips are placed atop each other and ripped apart, charge is separated since the glue and tape surface have different 'electron affinity') demonstration: VanDe Graaf generator- like charges repel.

©Don Mathewson 2006 3 Charge Lab name: ______date: ______Equipment: scotch tape strips, a pencil

Preparation 1. Place a 20 cm long strip of clear transparent tape on a desk. Bend over a bit (0.5 cm) at the end to make a 'handle' 2. Place a slightly shorter tape strip directly on top of the first-make a small handle for it. 3. Using a pen label B for bottom tape, T for top tape - write on the handle. 4. Pull sharply to remove top tape. Stick it -perhaps to the side of your desk-so that most of the tape hangs freely in the air 5. Remove the bottom tape and attach it in the same way close by. 6. Prepare another T and B tape in the same manner.

Questions 1. Use one of the T tapes. (the other 3 tapes dangling from your desk) a) Bring it close to the other T tape. What happens?

b) Bring it close to a B tape. What happens?

c) Now try the other B tape. What happens?

©Don Mathewson 2006 4 2. Now use a B tape (the other 3 tapes dangling from your desk) a) Bring it close to a T tape. What happens?

b) Bring it close to the other T tape. What happens?

c) Bring it close to a B tape. What happens?

3. Fill in the blanks: a) a T tape ______another T tape a T tape ______a B tape b) a B tape ______another B tape a B tape ______a T tape c) like tapes (T,T or B,B) ______unlike tapes (T,B or B,T) )______

4. What conclusions can we draw from this lab?

©Don Mathewson 2006 5 lesson plan 2: Plus and Minus Drawings idea: there are tiny particles called atoms that make up matter. Atoms themselves are made up of electrons and protons. electrons and protons have opposite charge. We can draw these charges using small + and -.

Charge drawing activity: 1. remind students of the key principles of charge - there are 2 types of charge -like charge repels/ unlike attracts 2. draw picture of a neutral object 3. draw picture of T and B tapes 4. draw the polarization of a neutral object to explain attraction (demo: balloon-rub on hair then -pick up puffed rice -attract student hair -stick to wall demo: plank on watchglass) lab: rods and fur and electroscopes 1. show students how to charge a black rod - by rubbing with fur (emphasize that in most cases, we use fake fur these days) 2. have students bring the rod close to (but don't touch) an electroscope and observe what happens 3. ask them to explain their observation using small drawings of + and - 4. explain 5. next have them touch the rod to the electroscope, observe and explain demonstration: VandeGraaf generator -pie plates -volta's hail storm -discharge wand

The charge of your VandeGraaf

©Don Mathewson 2006 6 Charge Drawing Notes name: ______date: ______

There are two types of charge: + and -

In matter, the + charges are basically fixed. Only the - can move.

In the T/B tape lab, before we pull quickly, both tapes are neutral (they have equal + and -, so no net charge)

When we pull off a top tape, it leaves some of its electrons on the B tape. So the bottom tape has extra - and the top tape has a lack of - (net +). We say the objects are charged (ie they have net charge).

Many objects we use every day have neutral charge

ex: pen balloon

But it is possible to charge them by rubbing.

©Don Mathewson 2006 7 If we bring a neutral object close to a charged B (or T tape), we see there is attraction. Why does that happen? Is 'neutral' another type of charge? NO. We can explain this attraction using +/- drawings.

since the unlike charges are closer, the attraction is stronger than the repulsion.

This process explains why neutral dust particles are attracted to your computer screen and why a charged balloon will stick to a neutral blackboard.

+ + + - + - wall charged balloon

©Don Mathewson 2006 8 Lab: Electroscopes name: ______date: ______

Equipment list: electroscope, rod, fur Note: Treat the electroscopes gently. Above all do not pull them open since the metal 'leaves' are extremely fragile and expensive. They will rip easily with the force of a gentle breeze.

Procedure: 1. Charge a rod using the fake fur like your teacher shows you. 2. Do not touch the rod to the electroscope. Bring it in close to the electroscope and observe/draw what happens using the diagram below

met al ball and rod

met al leaves

3. Try to explain your observation. Write a sentence or two. Use the theory of charge (+ and -)

4. Class discussion

met al ball and rod

met al leaves

©Don Mathewson 2006 9 5. Now very gently touch the rod to the electroscope. Observe/draw what happens using the diagram below

met al ball and rod

met al leaves

6. Try to explain your observation. Write a sentence or two. Use the theory of charge (+ and -)

7. Class discussion

met al ball and rod

met al leaves

©Don Mathewson 2006 10 The Charge of the Van deGraaf Generator

The Van deGraaf generator works by transferring - charge either: a) from the dome via the belt to metal case and on to the ground this leaves the dome with excess + charge b) from the ground through the metal case via the belt to the dome this leaves the dome with excess + charge

Your Van deGraaf will be either + or - and you can do the simple experiment below to find out which.

1) charge a ballon by rubbing it with fur. In this process, - charges are transferred FROM THE FUR TO THE BALLOON. You can look up "the Tribo-electric Series" to confirm this.

so the fur is charged ______and the balloon is charged ______

2) Now prepare T and B tapes as before. Attach these to a pencil. Then bring the pencil close to the - charged balloon (but try to keep the tapes from touching). This confirms that the

T tape is charged ______

B tape is charged ______

3) Now start the Van deGraaf and bring the pencil close to the charged dome. Look closely since the Van deGraaf will tend to quickly discharge to the tapes, giving them the same charge as the dome. One tape should be attracted to the dome, and one should be repelled.

the ______tape is attracted so the Van deGraaf dome is charged ______

©Don Mathewson 2006 11 lesson plan 3: how does TV work?? notes: CRT demonstration: CRT, TV cut-away writing activity: Produce a super-hero comic strip where you describe the journey of "Electron Man" as he journeys through the CRT (from start to screen). Your comic should have between 5 and 10 frames and be drawn carefully. It should present a creative, but conceptually correct description of how a CRT works. It should be "suitable for all audiences" (ie no blood).

©Don Mathewson 2006 12 Television- Cathode Ray Tube (CRT) Notes name: ______date: ______

•inside your TV (assuming you don't have LCD or plasma), you have a cathode ray tube which looks like this

++++++++++++

vi =0 -

------

fluorescent screen

•recall that matter is made up of small particles called atoms. These atoms in turn are made up of protons (+) and electrons(-).

•historically electrons were called cathode rays, since they came from the cathode (-) terminal of a vacuum tube.

TV step 1: "electron gun": use a big positive voltage to get the electrons going fast

- + - + elect ron int ially at rest - - - - + - + - + large + volt age

©Don Mathewson 2006 13 TV step 2: the screen is fluorescent (when struck by electrons, it glows)

vi =0 -

fluorescent screen

TV step 3: how do we make the picture more interesting? we need to move the beam around on the screen

++++++++++++

vi =0 -

------

fluorescent screen

©Don Mathewson 2006 14 TV step 4: make a horizontal sweep (a line on the screen) upper

left right

upper upper

+ left + left + +

lower lower

upper upper

+ + + +

lower lower

©Don Mathewson 2006 15 TV step 5: now hit the entire screen. "bump up and sweep horizontal"

left right

©Don Mathewson 2006 16 TV step 6: how do we get an image now? Using pixels.

left right

lower the TV screen is made up of pixels - the TV signal turns the beam on and off to make the pixels white (on) or black (off) or grey (partly on). This produces an image!!

TV step 7: colour TV has 3 electron beams- one that hits blue dots one that hits green one that hits red dots

the various colours are made up of RGB mixtures (ex: 10% blue, 5% red, 60% green)

©Don Mathewson 2006 17 Lesson plan 4: Lightning notes: where does lightning come from? demonstration: tesla coil, plasma ball, VdG and fluorescent tube research project: Find five interesting facts about lightning beyond what we have learned in class. Two of the five facts should be quantitative (involve a number). Present each fact in a short paragraph in your own words. Use correct vocabulary and grammar. Include with your report two pictures that relate to lightning. optional demo: LCD

©Don Mathewson 2006 18 Lightning Notes name: ______date: ______

•atoms are made up of protons (+) and electrons (-) that orbit the protons like planets around the sun.

What happens if we take an atom and put it next to a really big + charge?

++++ ++++ +++ big + charge posit ive ++++ negat ive prot ons elect ron ++++

•When an atom is ripped apart like that, we say it has been ionized.

•It takes millions of volts to ionize the air around us. This may seem like a lot but in actual fact ionization of the air occurs frequently: i) the crackle we hear when we pull apart clothes that come out of the clothes dryer ii) the 'shock' we get when we walk across a carpet on a cold winter day and then reach for a doorknob iii) lightning

©Don Mathewson 2006 19 •each of these effects is accompanied by light. The light occurs because the ionized electron does not remain free. It will quickly fall into an orbit around a neighbouring atom. When the electron does this, some of it's energy is converted to light energy.

ionized elect ron posit ive rejoins an at om and prot ons emit s light t o conserve energy

•Lightning happens because clouds are charged.

The charged cloud polarizes the ground, pulshing the electrons further from the surface.

©Don Mathewson 2006 20 The air is normally an "insulator" and does not allow the charge to flow to the ground. However, if enough charge builds up on the clouds, ionization will occur

Once ionization occurs, the circuit is "completed" and a large current flows

•where do you not want to be in a lightning storm?

charged clouds

boat ground

©Don Mathewson 2006 21 LCD Notes name: ______date: ______

This is a (very) simplified model of how a Liquid Crystal Display (LCD) works.

Liquid crystal molecules are long and thin and they have a charge at the end. A large number of them are found inside every 'pixel' of the display

With the molecules randomly aligned, they allow the light to pass through the display pixel.

When we apply a voltage to the pixel, the molecules of liquid crystal will line up and block the passage of light. This makes the pixel dark.

©Don Mathewson 2006 22 The correct combination of pixels produces the number or letter required

©Don Mathewson 2006 23 lesson plan 5: Voltage and Current drawing activity: voltage, current demonstration-simple cell (ex: potato clock) lab activity-light the light bulb activity-tennis ball circuit writing activity- describe the journey of the charge around the circuit.

©Don Mathewson 2006 24 Voltage and Current Notes name: ______date: ______Voltage

a) definition:

b) units:

c) a 'dry cell' (commonly/incorrectly called 'battery') produces voltage (we say it is a 'source of voltage')

d) the circuit symbol for a voltage source:

•the voltage at a point in the circuit is like the height at a certain point on the mountain. The voltage gain in the cell is like the height a skier gains on the chairlift. The voltage drop in a bulb is like the height lost as a skier moves down a ski run

©Don Mathewson 2006 25 Current a) what happens if we connect a wire to one side of a voltage source(cell) ?

b) what if the wire forms a continuous loop?

definition:

units:

•current is like the 'flow' of skiers on a ski hill

©Don Mathewson 2006 26 Lightbulb Lab name: ______date: ______equipment: 1.5 Volt bulb + holder; 1.5 V cell and holder;switch; wires

1. Follow the directions carefully 2. connect a wire to each side of your cell holder. 3. connect one of the wires to the switch and the other wire to the lightbulb. 4. Use a third wire to connect "other side" of lightbulb to "other side" of switch 5. put a cell in the cell holder 6. push the switch and see the lightbulb glow

wire 1 bulb

wire 3

wire 2

swit ch

Questions 1. What provides energy to this circuit?

2. Why do we use the switch?

3. If we hold down the switch for a long time, what will happen?

©Don Mathewson 2006 27 4. Draw the path that current follows in this circuit. Be sure to show the flow through the bulb

+ - - + - bulb + + -

cell -

- + + - - + swit ch closed

5. When a bulb burns out what happens in a) the bulb itself

b) the above circuit

6. When the cell is 'drained' what happens in a) the cell itself b) the circuit

©Don Mathewson 2006 28 Tennis Ball Circuit Activity name: ______date: ______materials: a box of tennis balls something mechanical for the balls to do several co-operative students a juice box procedure 1. assign roles and responsibilities. 1 student will be the cell 5-10 students will be the wire 2 students will be the 'bulb' (or motor, or "electrical load") 1 student is the switch 2. when the switch closes, run the circuit for some time. Then discuss.

©Don Mathewson 2006 29 Questions 1. Why are hands kept high for the wire leading to the load?

2. Why are hands kept low for the wire leading to the load?

3. a) Where did the balls change from high to low? b) Why did this happen? c) How is this similar to the light bulb circuit?

©Don Mathewson 2006 30 4. a) Where did the balls change from low to high? b) Why did this happen? c) How is this similar to the light bulb circuit?

5. a) Why did we give the cell some juice to drink? b) How is this similar to the lightbulb circuit

6. Fill in the blanks using the words "voltage" or "current" a) the height of the tennis balls is a model for ______in a circuit b) the flow of the tennis balls is a model for ______in a circuit.

7. As the tennis balls move around the circuit, the tennis balls do not disappear, they are merely passed on. What does this tell us about real circuits?

©Don Mathewson 2006 31 lesson plan 6: Resistance; Conductors and Insulators notes: resistance lab activity-conductors and insulators lab activity: measuring resistance using a digital meter

©Don Mathewson 2006 32 Resistance Notes name: ______date: ______

As charges flow through the wires, they collide with other atoms. In these collisions, the charges lose energy/voltage. The resistance of a material tells us how many collisions the charges have as the flow

high resistance means:

low resistance means:

The symbol for a resistor looks as follows

the jagged lines suggest that the charges have to 'bump around' to get through the resistor.

In the ski hill analogy, resistance is like the depth of the snow. Deeper snow is like high resistance since skiers flow slowly. Hard packed snow is like low resistance since skiers flow fast

©Don Mathewson 2006 33 The units of resistance are Ohms. One Ohm of resistance means that a voltage drop of 1 volt will give a current of 1 Ampere. 1  = 1 Good conductors have low resistance, and poor conductors (insulators) have large resistance.

Resistance can be measured using a digital meter. Take the readings below

©Don Mathewson 2006 34 Conductor or Insulator Lab name: ______date: ______

equipment: 1.5 Volt bulb & holder; 1.5 V cell & holder; switch; wires common materials for conductivity testing (penny, quarter, button, paper clip, soda pop, lemon juice, string, wooden dowel, pop can,....)

procedure 1. build the conductivity tester below 2. obtain a sample from the teacher for testing 3. connect the wires to the sample (one to each end) 4. observe whether the sample is a conductor (allows current flow) or insulator (blocks current flow) 5. record your observations and conclusion (conductor/insulator) in the chart on the next page 6. repeat for another sample

wire 1 bulb

wire 3

wire 2

sample

©Don Mathewson 2006 35 data sample observations conclusion

©Don Mathewson 2006 36 Resistance Measurements name: ______date: ______

Equipment: common materials, digital meter

Procedure: 1. Use the multimeter to measure the resistance of each of the common materials (touch a lead to each end of the material; it doesn't matter which end) 2. Record the resistance and the units (note k means 1000 ) If the meter shows _____ that means the resistance is infinite (a very, very large number) 3. Use the results from our previous lab to fill in whether the material is a conductor or an insulator

Data: sample resistance conductor/insulator

©Don Mathewson 2006 37 Lesson plan 7: Ohm's Law notes: ohm's Law worksheet: problems lab activity: Ohm's Law

©Don Mathewson 2006 38 Ohm's Law Notes name: ______date: ______question 1: How does resistance affect current flow? a) if we have large resistance the flow will be ______b) if we have small resistance the flow will be ______

This suggests that resistance is in the "bottom" of the current fraction (since a bigger "bottom" makes a smaller number)

question 2: How does voltage affect current flow?

a) if we have large voltage the flow will be ______b) if we have small voltage the flow will be ______

This suggests that voltage is in the "top" of the current fraction (since a bigger "top" makes a bigger number)

©Don Mathewson 2006 39 These two results together are known as Ohm's Law.

Ohm's Law is usually written in the following (cross-muliplied) form.

V=voltage (volts, V)

I=current (amp, A)

R=resistance (ohm, )

©Don Mathewson 2006 40 example: a) Calculate the current if we use a 6 V lantern battery to light up a 10  bulb. b) show using lines how you would connect this circuit c) indicate on the picture the direction of the current flow

©Don Mathewson 2006 41 We can use Ohm's Law to find a resistance. The way that a digital meter actually measures resistance is by applying a small voltage (9V) and by measuring the current flow.

example: a digital meter applies 9V to a small cylinder of graphite and measures a current of 0.25 A. Find the resistance of the graphite cylinder

©Don Mathewson 2006 42 In some cases, we want a certain current to flow through a device and we need to calculate the required voltage. example: A CD player has a motor inside to spin the CD. If the motor has resistance 20  and needs 0.15 A to work properly, a) how many volts are required? b) how many AA 'batteries' are required? c) draw a picture showing how the AA batteries should be placed in the device.

©Don Mathewson 2006 43 Lab: Ohm's Law

equipment: 1.5 Volt bulb & holder; 1.5 V cell & holder; switch; wires digital meter (for resistance); analog ammeter (for current); other materials for testing (small lab resistor, small electric motor, short piece of wood, penny, .....)

procedure part 1 1) measure the resistance of your bulb using the digital meter. record in the data section (remember to include units) 2) repeat the resistance measurement for several other objects. If the meter shows _____ that means the resistance is infinite (a very, very large number) item measured resistance

©Don Mathewson 2006 44 procedure part 2 1) Using the 1.5V of the cell and the measured resistance, calculate the current through the bulb based on Ohm's Law. Show all work. Enter your answer (the predicted current) in the chart below 2) repeat for the other objects you measured

work space:

item measured resistance predicted current

©Don Mathewson 2006 45 Procedure part 3

wire 1 bulb

wire 3

red pin black wire 4 wire 2 pin

swit ch ammet er 1) connect the circuit as shown 2) depress the switch to measure the current 3) if the ammeter needle moves left (the wrong way), then you have not properly connected wire 3 to the RED pin. fix this. 4) if the ammeter needle deflection is too small, 'switch to a smaller scale' 5) if the ammeter needle deflection is too large, 'switch to a larger scale' 6) when you have a 'good' reading, record it below as measured current item resistance predicted current measured current

©Don Mathewson 2006 46 Questions 1. Compare the measured current and the predicted current for the bulb. Were they equal, or almost equal (within 50 mA) ?

2. Compare the measured current and the predicted current for the other devices. Were they equal, or almost equal (within 50 mA)

3. If we switch from the bulb to one with a larger resistance , how would that change the current?

4. If the resistance is very very large, what can we say about the current?

5. If the resistance is very, very small, what can we say about the current?

©Don Mathewson 2006 47 6. If we used our original bulb, but connected it to a larger voltage, how would that affect the current?

©Don Mathewson 2006 48 Lesson plan 8: Short circuits notes: short circuits demo: short circuit a dry cell through an ammeter short circuit a lantern battery through steel wool short circuit a paper clip, a pickle !! lab: short circuit

When we discussed current, we presented the following circuit.

There is actually a problem with this circuit: there is very little resistance (no "load")

This is a better circuit, since the lightbulb filament has some resistance.

+ _ + -+ - + + - - + + - - + + + - - -

What happens when there is a short circuit? Let's apply Ohm's Law: voltage current = resistance

With very little resistance in the circuit the denominator is very small and so the current flow is very large.

A large current creates heat which can melt the wire and start a fire.

The short circuit also drains all of the power away from other parts of the circuit.

©Don Mathewson 2006 50 Lab: Short Circuit equipment: 1.5 Volt bulb & holder; 1.5 V cell & holder; switch; wires; piece of bare copper wire; analog ammeter (for current)

wire 1 bulb

wire 3

wire 2

swit ch procedure: 1) connect the circuit above 2) push the switch and verify that the bulb works 3) connect a bare copper wire from one side of the bulb to the other

cell bare wire

swit ch 4) push the switch and observe the bulb.

1. What happened when the bare copper wire was put in place?

2. Why do you think this was happening?

3. How could we measure what was happening in this circuit?

©Don Mathewson 2006 52 Lesson Plan 9- Circuits in the home (series) notes: series activity: tennis ball circuit -series demo: switches are in series

•In a series circuit, there is only ONE path for the current to follow. There are no junctions in the circuit. example:

bulb bulb

swit ch non-example:

bulb cell

swit ch

©Don Mathewson 2006 54 Tennis Ball Series Circuit Activity name: ______date: ______materials: a box of tennis balls something mechanical for the balls to do (2 times) several co-operative students a juice box procedure 1. assign roles and responsibilities. 1 student will be the cell 5-10 students will be the wire 2 students will be the 'bulb' (or motor, or "electrical load") 1 student is the switch 2. when the switch closes, run the circuit for some time. Then discuss.

2. What can we say about voltage in a series circuit?

equipment: two 1.5 Volt bulbs & holders; 1.5 V cell & holder; switch; wires; piece of bare copper wire; analog voltmeter (for voltage)

bulb bulb wire 1

cell red pin black pin volt met er

swit ch

procedure: 1) connect the above circuit 2) depress the switch to measure the voltage drop in the first bulb 3) if the voltmeter needle moves left (the wrong way), then you have not properly connected wire 1 to the RED pin. fix this. 4) if the voltmeter needle deflection is too small, 'switch to a smaller scale' 5) if the voltmeter needle deflection is too large, 'switch to a larger scale' 6) when you have a 'good' reading, record it below as bulb 1 voltage

bulb bulb

cell red pin

volt met er

swit ch

data: bulb 1 voltage =

bulb 2 voltage =

Questions 1. How did the bulb1 voltage and bulb 2 voltage compare?

2) Find the sum of the bulb voltages

3) What do you notice about the sum of the two voltages?

©Don Mathewson 2006 58 Lesson Plan 10- Circuits in the home (parallel) notes: parallel activity: tennis ball circuit -parallel demo: plugs are in parallel

•In a parallel circuit, there is more than one path for current to follow. The circuit has a junction and the current flow splits at this junction. It recombines at a later junction. example:

bulb junct ion

junct ion

bulb cell

swit ch non-example:

bulb bulb

swit ch

©Don Mathewson 2006 60 Tennis Ball Parallel Circuit Activity name: ______date: ______materials: a box of tennis balls something mechanical for the balls to do (2 times) several co-operative students a juice box procedure 1. assign roles and responsibilities. 1 student will be the cell 5-10 students will be the wire 2 students will be the 'bulb' (or motor, or "electrical load") 1 student is the switch 2. when the switch closes, run the circuit for some time. Then discuss.

2. What can we say about voltage in a parallel circuit?

equipment: two 1.5 Volt bulbs & holders; 1.5 V cell & holder; switch; wires; piece of bare copper wire; analog voltmeter (for voltage)

bulb junct ion black pin red pin ammet er

bulb cell

swit ch

procedure: 1) connect the above circuit 2) depress the switch to measure the current through the first bulb 3) if the ammeter needle moves left (the wrong way), then you have not properly connected wire 1 to the RED pin. fix this. 4) if the ammeter needle deflection is too small, 'switch to a smaller scale' 5) if the ammeter needle deflection is too large, 'switch to a larger scale' 6) when you have a 'good' reading, record it below as bulb 1 current

bulb junct ion

bulb cell black pin red pin ammet er

swit ch

data: bulb 1 current =

bulb 2 current =

Questions 1. How did the bulb1 current and bulb 2 current compare?

2) Find the sum of the bulb currents

3) What do you notice about the sum of the two currents?

©Don Mathewson 2006 64 Lab: The Resistance of Graphite-not working yet procedure: 1. obtain a graphite pencil with 'exposed graphite' 2. Use our conductivity tester and touch the leads to the exposed graphite (start the leads at the very ends of the graphite). Observe and record.

leads at very end

3. Move one of the leads and repeat the observation

move t his lead closer 4. Repeat until the leads are very close together.

Questions 1. Does a long piece of graphite conduct?

2. Does a short piece of graphite conduct?

4. Are there other materials that have this same property "a longer piece has more resistance" ?

5. Metals generally are good conductors and have low resistance. So why does a light bulb filament, which is metallic have a significant resistance. (hint: it's not long, but it is...... )

6. Fill in the blanks We will find a larger resistance if the piece of material is ______or if it is ______