electromagnets & induction · 1. the primary coil is connected to outside power lines. current...

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Electromagnets & Induction

Vocabulary CP Physics

Term Definition Coil

Solenoid

Electric Motors

Rotor

Commutator

Armature

Brushes

Electromagnetic induction

Faraday’s Law of Induction

Generator

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17.1 Electric Current and Magnetism

What did Hans Christian Oersted conclude from his experiment?

How is magnetism created?

The picture below depicts a wire with no current flowing through it. Notice how the compasses

all point __________.

As shown below when current flows through the wire the compasses point in a __________.

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The strength of the magnetic field depends upon two things:

1.

2.

What is a solenoid?

When the current is in the same direction in two wires, they __________ each other.

If the current goes in the opposite direction, the wires __________ each other.

Will these wires attract or repel?

17.1 Check Questions

What did Hans Christian Oersted detect in his experiment?

What happens to the magnetic field as you move closer to a current carrying wire?

What effect does increasing the current in a wire have on the magnetic field?

Homework

Complete p. 394 Understanding vocabulary #’s 1-2 and Reviewing concepts #’s 1-9

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17.2 Electric Motors

Imagine a disk with magnets that can spin. To make the disk spin you must bring another

magnet close to it. The South pole of the magnet you are holding will attract the North pole

of a magnet on the disk and will repel the South pole of the other magnet on the disk.

To keep a rotor turning the electromagnet must switch from north to south as each rotor magnet

passes by. The device that makes this happen is called a __________. The commutator switches

the electromagnets from north to south and back again as well as switches the polarity of the

electromagnets.

What are the three main parts of a motor?

What is the rotating part of a motor called?

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What is the purpose of the commutator?

How does a motor spin?

In which direction will the rotating disk spin?

Where would I place the south pole of a magnet to get the disk to spin in a clockwise direction?

Homework


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17.3 Electric Generators and Transformers

A motor transforms __________ energy into __________ energy.

When does electromagnetic induction occur?

What is the name of the device that uses electromagnetic induction to produce electricity?

What happens when you move a magnet in and out of a coil of wire as shown below?

What happens if the magnet stops moving?

What does Faraday’s law of induction tell you about the relationship between current and how

fast you move a magnet in and out of a coil? (ie if you move a magnet faster in and out of a coil

of wire do you produce more or less current?)

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A generator converts __________ energy into __________ energy.

A transformer is a device used to change voltage and current. You may have noticed the gray

electrical boxes often located between two houses or buildings. These boxes protect the

transformers that “step down” high voltage from power lines (13,800 volts) to standard

household voltage (120 volts).

How a transformer works: 1. The primary coil is connected to outside power lines. Current in the primary coil creates a

magnetic field through the secondary coil.

2. The current in the primary coil changes frequently because it is alternating current.

3. As the current changes, so does the strength and direction of the magnetic field through

the secondary coil.

4. The changing magnetic field through the secondary coil induces current in the secondary

coil. The secondary coil is connected to the wiring in your home.

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Voltage relationships in a transformer:

Example:

A transformer steps down the power line voltage (13,800 volts) to standard household voltage

(120 volts). If the primary coil has 5,750 turns, how many turns must the secondary coil have?

Looking For Given Relationship Solution

When you plug in a cell phone, a transformer on the plug changes the outlet’s 120 volts to the 6 volts

needed by the battery. If the primary coil has 240 turns, how many turns must the secondary coil have? Looking For Given Relationship Solution

On a European trip, you discover that the electric outlets have 240 volts. You realize that you need a

transformer, so you quickly wrap 20 turns around the primary coil. If you need 120 volts to run your

hairdryer, how many turns do you need to wrap around the secondary?


A transformer has 20 turns on the secondary coil and 200 turns on the primary. What is the secondary

voltage if the primary voltage is 120 volts?


How many turns must the primary coil have if it steps down 13,800 volts to 120 volts with 112 turns?


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In England, standard household voltage is 240 volts. If you brought your own hair dryer on a trip

there, you would need a transformer to step down the voltage before you plug in the appliance. If

the transformer steps down voltage from 240 to 120 volts, and the primary coil has 50 turns, how

many turns does the secondary coil have?


You are planning a trip to Singapore. Your travel agent gives you the proper transformer to step

down the voltage so you can use your electric appliances there. Curious, you open the case and

find that the primary coil has 46 turns and the secondary has 24 turns. Assuming the output

voltage is 120 volts, what is the standard household voltage in Singapore?


A businessman from Zimbabwe buys a transformer so that he can use his own electric appliances

on a trip to the United States. The input coil has 60 turns while the output coil has 110 turns.

Assuming the input voltage is 120 volts, what is the output voltage necessary for his appliances

to work properly? (This is the standard household output voltage in Zimbabwe.)


A family from Finland, where standard household voltage is 220 volts, is planning a trip to

Japan. The transformer they need to use their appliances in Japan has an input coil with 250 turns

and an output coil with 550 turns. What is the standard household voltage in Japan?


An engineer in India (standard household voltage = 220 volts) is designing a transformer for use

on her upcoming trip to Canada (standard household voltage = 120 volts). If her input coil has

240 turns, how many turns should her output coil have?


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What is a generator?

What type of energy does it produce?

What happens as you move a magnet toward a coil of wire in terms of electricity?

What is Faraday’s law of induction?

What is a transformer?

When does electromagnetic induction occur?

How do you get the greatest induced current to flow in a coil?

How do you step up voltage?

Homework


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Faraday’s Law of Induction CP Physics

Directions: Type the answers to the following questions in complete sentences.

Go to http://phet.colorado.edu/simulations/sims.php?sim=Faradays_Electromagnetic_Lab

Bar Magnet Tab – General Magnetism

1. Click on the Bar Magnet Tab and you should see a bar magnet and compass on the

screen. Please note that the color red refers to North and white refers to South. Place the

compass at the North end of the bar magnet and observe which way the “red tip” of the

compass points. Move the compass to the South end and observe where the “red tip” of

the compass points. What can you say about where the north (red) tip of a compass

points?

2. Use your response to #1 to explain why the geographic north pole is the magnetic south

pole.

Pickup Coil Tab – General Electromagnetic Induction

3. Set the number of loops to “1” and note what happens to the light bulb when

The magnet is not moving and is not in the loop –

The magnet is moving and is not in the loop –

The magnet is not moving and is in the loop –

The magnet is moving and is in the loop -

4. Does the speed of the magnet affect your results to #3? If so, describe how.

5. Increase the number of loops to “3” and see if it affects your results from #3. If so,

describe how.

6. Increase the loop area to “100” and see if it affects your results from #3. If so, describe

how.

http://phet.colorado.edu/simulations/sims.php?sim=Faradays_Electromagnetic_Lab

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Electromagnet Tab – Is Electromagnetism Reversible?

7. You should see a battery attached to a loop of coil (an electromagnet) and a compass on

the screen. Move the electromagnet around the screen and describe what the compass

does.

8. Move the compass around the electromagnet in order to determine the North and South

poles. Draw a picture and label the North and South Poles.

9. Change your current source from DC to AC and describe what the compass does.

10. Observe the electrons in the AC current source and compare their movement to those in

the DC current source. Explain the difference between DC and AC in terms of electron

movement.

Transformer Tab – Can We Use Electromagnetism?

11. The last tab showed us that current can create a magnetic field. Can this magnetic field

generate electricity? That is, can we use electricity to generate more electricity? Move

the electromagnetic back and forth and note what happens.

12. Can electricity be used to create more electricity? Explain how.

13. Change to an AC source. Note what happens while the electromagnet is not moving.

Why does the light bulb light up? Do the electrons in the light bulb move as fast as the

AC source?

Generator Tab – Putting it All Together

14. Turn on the water faucet and describe what happens.

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Investigation 17A CP Physics

Research Question

How does an electric motor work?

Electric motors are used in many devices you use every day. In this Investigation you will build a

simple electric motor and learn how it works. The concepts you learn with the simple motor also

apply to other electric motors.

Procedure

1. Bend the two paper clips so they look like the drawing below.

2. Fasten them with rubber bands so they contact the positive and negative terminals of the

battery.

3. Set the battery on a lump of clay so it stands up.

4. Stick a magnet to the top of the battery with another lump of clay.

5. Your motor base is complete!

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Making the motor work

1. Set the coil into the paper clips so it is free to spin.

2. Adjust the height of the paper clips until the coil rotates just above

the magnet.

3. The motor should spin! Adjust the balance by bending the wires

or paper clips.

Questions

a. Explain the force acting on the coil when there is current flowing. What creates the force?

b. Why was it important to sand only one side of the wire on one end of the coil.

c. Perform an experiment to measure the speed of your motor. Remember we were able to

calculate the speed of an object traveling in a circle: 2∏r/t.

d. Try adding a second magnet. Does this make the motor go faster, slower, or about the

same? What observations did you make that support your answer?

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Ripcord Generators Purpose After reading the lab, create your own purpose below:

______________________________________________________________________________

______________________________________________________________________________

Pre-Lab Discussion Changing electric currents can cause magnets to move, as in the electric motor. The reverse is

also true. Changing magnetism can cause electric currents to flow. This is the principle on which

the electric generator works. The spinning disk of magnets, the rotor, can induce a current

through the electromagnet. The device you will build is similar to the ones used to power cities

all over the world.

Materials Magnets

Rotor Assembly

Multimeter

Wire Leads

Stopwatch

Part 1 – Making the Generator

1. Place the coil around the

large nail and attach it to

the generator assembly.

2. Connect the meter to the

coil using the two wire

leads. Make sure the leads

contact the metal on both

ends.

3. Unscrew the rotor

assembly. Put four magnets

in the rotor, evenly spaced and alternating north/south. Screw the top back on the rotor.

4. Wrap a string around the spindle and pull it to set the rotor turning.

5. Use a stopwatch to keep time from when the rotor starts to spin. Every second, observe

the voltage with the meter set to “Volts DC.” Record the values in Table 1.

6. Repeat steps 4 and 5, but switch the meter to “Volts AC.” Record voltage values every

second in Table 1.

Figure 1

Figure 1

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Part 2 – New Configurations

7. Follow Fig. 2 to configure the rotor with a different magnet setup.

8. Repeat steps 4 through 6 for each configuration. Record your data in Table 2.

Fig. 2

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Part 3 – Voltage and Speed

9. Open the rotor and include 12 magnets, alternating north and south. Screw the

cover back on top.

10. Use the cutout in the board to place a photogate under the rotor. Make sure the

light beam passes through the

slot in the cover of the rotor.

11. Set the multimeter to measure

“Volts AC.” Set the Timer to

measure “Frequency.” This tells

you how many times per second

the light beam is broken. (Note:

The beam is broken once per

turn. See picture at right.)

12. Start the rotor spinning as in

step 4. Have one member watch the frequency on the timer, one watch the voltage

on the multimeter, and another record the data.

13. When the frequency reaches 35 Hz, record the voltage from the multimeter in

Table 3.

14. Continue to monitor the meters and find the voltages for the other frequencies

listed in Table 3.

Data Part 1 – Building the Generator

Table 1 – Generator Voltage in Time Intervals

Time

(seconds)

AC

(volts)

DC

(volts)

0

1

2

3

4

5

6

Fig. 3

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Table 2 – Generator Voltage in Time Intervals

6 Magnets –

Alternating

12 Magnets –

Alternating

12 Magnets –

Same Side Out

Time

(seconds)

AC

(volts)

AC

(volts)

AC

(volts)

0

1

2

3

4

5

6


Table 3–Voltage vs. Speed

Voltage produced

(volts)

Frequency from Timer

(Hertz)

Rotational Speed

(revolutions per second)

35 35

30 30

25 25

20 20

15 15

5 5

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Questions Part 1 – Building the Generator

a. Explain the process by which your generator produced electricity.

b. What type of current was produced by your generator? How did you know?

c. Make a graph of “Voltage vs. Time” for your AC current values. On the same

graph, plot “Voltage vs. Time” for your DC current values in another color or

design. Include all necessary labels and a legend indicating which line is which

type of current.

d. According to your graph, what happens to AC voltage as time goes on? Why is

this?


e. Compare the 3 configurations. What is different about them? Explain the

differences using what you know about induction.


f. Does the voltage produced depend on the speed? Support your answer with your

observations.

g. Make a graph of voltage versus speed. Voltage should be plotted on the y-axis

and speed on the x-axis. Provide a title for your graph, label the axes with units,

add a trendline, and display the equation and R2 value. Is the graph a straight line

or a curve?

h. What can you conclude about the relationship between voltage and speed? Does

this relationship support Faraday’s law of induction?

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Conclusion Questions

1. Which direction will the motor spin below? Explain.

2. To cause the rotor below to spin in a counter-clockwise direction, the north pole

of a magnet should be placed at which position? Explain.

N S

A B

C

D

electromagnets & induction · 1. the primary coil is connected to outside power lines. current...

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