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Orange Coast College Physics 280

Experiment #7

Electromagnetic Induction The purpose of this experiment is to help you become familiar with some of the principles of Faraday’s law of electromagnetic induction. Apparatus: Two flat wound coils Two voltmeters One ammeter One DC Power supply One AC power supply One Galvanometer One soft-iron U-shaped bar that will hold the two coils Theory Electromagnetic induction theory predicts that a voltage will be induced (also called an “emf”) every time the magnetic flux !B through a conducting loop changes, as given by the relation:

emf = ! d"Bdt

The magnetic flux !B through a coil can be expressed as:

!B =!B "d!A# = BdAcos !( )#

A change in the magnetic flux !B through a coil can be generated by: (i) changing the magnetic field through the coil, or (ii) changing the area of the coil (either by stretching or compressing the coil), or (iii) by rotating the coil relative to the direction of the magnetic field. The negative sign in the equation above means that the direction of the induced current in the conducting loop is such as to oppose whatever caused it. This is known as Lenz’s law.

In this activity you will become familiar with the qualitative response of coils when subjected to a changing magnetic flux through them. You will also build a simple transformer.

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Procedure Part I.

Connect the galvanometer, as shown in figure 1. Watch the galvanometer needle while you

thrust the north pole of a bar magnet through the stationary coil. Describe what you observe?

Use the following symbols in this report to answer the pertinent questions:

L = Left, R = Right, N = North, S = South.

Questions: (Circle the best choice, if given)

(1a) What is the direction of the galvanometer needle when you thrust the north pole of the bar

magnet (at constant speed) toward the stationary coil? _______L, R_______

when you move the coil toward the stationary bar magnet? __________L, R_______

move both the coil and bar magnet toward each other? _____________L, R______________

(1b) What general statements can you make concerning the preceding observations?

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Part II.

Next, with the same circuit of figure 1 above, place the bar magnet in the center of the coil, as

shown in figure 2 below. The coil now has a maximum amount of magnetic flux through it.

Questions:

(2a) What is the galvanometer reading? _______L, R, none_____________________

(2b) How do you account for the observed reading?

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(2c) Observe the galvanometer needle as you withdraw the bar magnet to the left at constant

speed. What is the direction of the needle deflection as the bar magnet is withdrawn? _L, R_

(2d) Vary the speed with which you withdraw the magnet from the coil. What conclusions can

you draw from this?

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Part III.

From the preceding discussion, you should be convinced by now that it is not the magnetic flux,

but the change in the magnetic flux that induces a voltage (or emf) across the coil. If we had a

method to change the magnetic flux through the coil rapidly, we would be able to observe the

generation of a rather large induced current. We can achieve this by substituting the permanent

bar magnet with an electromagnet. Arrange the set-up as shown in figure 3 below.

Use the DC power supply and set its voltage output at about 10.0 Volts.

(3a) Observe the galvanometer needle deflection as you close the circuit. What happens?

(3b) Keep a steady current through the electromagnet. What is the galvanometer needle

deflection or reading when there is a steady current in the electromagnet?

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(3c) As you approach the coil with the electromagnet, observe the galvanometer needle

deflection and describe what happens.

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(3d) As you cut the current off (by opening the circuit), what happens to the galvanometer

reading?

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(3e) Now insert the soft-iron through the center of the two coils and describe what happens when

the circuit is closed,

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and when the circuit is opened,

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(3f) Replace the soft-iron core with various other materials if they are available; a wooden

pencil, an aluminum rod, a carbon rod, etc. Describe the results using each different material.

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Part IV. The Transformer

When an alternating current passes through a coil of wire, it produces a time-changing magnetic

field that alternates in direction and varies in magnitude. This is precisely one of the conditions

needed for the electromagnetic induction to take place in a secondary coil of wire. Here you will

investigate how the number of turns and the input voltage in a primary coil (input coil) affects

the induced voltage (output voltage) in a secondary coil (output coil) as a function of the number

of turns in the secondary coil.

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Set up the coils as shown in figure 4, noting that the DC power supply has been replaced by an

AC power supply. In the diagram, the coil to the left will be referred to as the primary coil, and

the coil to the right will be referred to as the secondary coil. Note that you will put in an

alternating current to the primary coil at one voltage level, and reading the output voltage at the

secondary coil.

(4a) With the 400-turn coil as the primary coil and the 400-turn coil as the secondary coil, adjust

the input voltage to about 10 Volts. Measure the output voltage and record the result in the table

below. Repeat this for input voltages of about 13 volts and for 15 volts.

(4b) Repeat (4a) with the 400-turn coil as the primary coil and the 800-turn as the secondary coil.

(4c) Repeat (4a) with the 400-turn coil as the primary coil and the 1600-turn as the secondary

coil.

(4d) Repeat (4a) with the 400-turn coil as the primary coil and the 3200-turn as the secondary

coil.

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Number of

turns in

primary

coil

N1

Number of

turns in

secondary

coil

N2

Input

Voltage

ΔV1 (Volts)

Output

Voltage

ΔV2 (Volts)

!V2!V1

(experimental)

!V2!V1

=N2N1

(Theory)

400 400

1

1

1

400 800

2

2

2

400 1600

4

4

4

400 3200

8

8

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For a transformer, the ratio of the voltages in the coils is equal to the ratio of the number of turns

in the coils. The working transformer equation is !V2!V1

=N2N1

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Essential Question: Discuss in your report how the experimental values of the ratio ΔV2/ΔV1

compare with the theoretical values (already entered in the table above under N2/N1).