Phy132/222 Experiment
Electromagnetic Induction and Transformers
(revised 6/24/08T)
Name: _______________________________
Date: _____________
Lab Partners: ________________________________________
Introduction: After studying the properties of magnetic and electric fields you have discovered some
similarities between the two fields and may have wondered if these two phenomenons were somehow
related. As we have seen, both the electrostatic force and the magnetic force act at a distance, the properties
of magnetic and electric field lines are virtually identical, and a current in a wire produces a magnetic field.
This last observation makes a direct connection between electric charges and magnetic fields. The apparently
close connection between electric and magnetic fields and the fact that a magnetic field could be produced
from a current of charged particles led scientists to ask if the opposite could be true. Is it possible to create a
current with a magnetic field? Keep in mind that you need an electric field to create a current. So if a
magnetic field causes a current, then it is also creating an electric field. In this lab we will be exploring this
connection between electric and magnetic fields. Is it possible to create a current using a magnetic field? If
so, what factors are important? And what practical application can this phenomenon have?
Part I: Electromagnetic Induction
You will need the following: a coil box set, a bar magnetic, a couple wires, and a galvanometer. A
galvanometer is the basic component for an analog ammeter. If a current is flowing in a circuit the needle
will deflect to the right or to the left, depending on the direction of the current. The magnitude of the current
is determined by how far the needle deflects. The coil box set contains several solenoid coils. The number of
times the copper wire has been wrapped around the coil is indicated by a number on the side of the coil. For
example, a label of 200 means that the copper wire was wrapped around the coil 200 times. There is also an
iron loop in the box which can be used to give the coil an iron core. Leave this in the box for now; you won’t
be using it in Part I.
Connect the 400 turn coil (labeled 400 on the side) to the galvanometer with your wires. Then proceed to the
questions section.
Figure 1
1
Part II: Electric Generators
You will need the following: an electric motor, a couple of wires, and a galvanometer. Connect the
galvanometer to the electric motor, and then proceed to the questions section.
Part III: Transformers
You will need the following: a coil box set, wires, two DMM’s, and an AC Power Supply. A transformer
consists of a primary coil and a secondary coil. In this experiment the primary coil is the one attached to the
power supply. The secondary coil will be set next to the primary or “connected” to it via a straight iron bar, a
U-shaped iron bar, or an iron loop. When using the iron loop, make sure the connecting screw is secured.
~
Set the dial of a DMM to V and connect a 400 turn coil to it. This will be your secondary coil for now.
Connect the other 400 turn coil to the power supply connections marked AC. Then connect the other DMM
~
in parallel to the power supply and set the dial to V . This is your primary coil.
Place the two coils next to each other as shown in the figure below. Then turn the power on just long enough
to measure the voltage across the primary and the secondary. DO NOT leave the power on longer than a few
seconds. If you do leave the power on too long your primary coil will become very hot. If this happens you
will have to wait five minutes for the coil to cool down before you can continue the lab.
Figure 2
Repeat the above procedure with a straight iron bar connecting the two coils, then the U-shaped bar, and
finally the full loop. See figures below.
2
Figure 3
Now let’s see what effect the number of turns has on the voltages across the primary and secondary coils.
For this part we will use the full loop for all measurements. Connect a 200 turn coil primary to a 400 turn
secondary using the full loop, see figure below. Turn on the power just long enough to measure the voltage
across the primary and the secondary. Repeat for the other primary and secondary coil combinations listed in
the data table.
Figure 4
3
Data Part III: Transformer
Primary Voltage
Secondary Voltage
Primary Voltage
Secondary Voltage
Air
Bar alone
U-Shape
Full Loop
200 Primary 400 Secondary
400 Primary 200 Secondary
200 Primary 800 Secondary
800 Primary 200 Secondary
400 Primary 800 Secondary
800 Primary 400 Secondary
4
Questions
Part I
1.
Move the magnetic in and out of the coil. Then move the coil back and forth with the magnet inside (do
not let the magnet move, only the coil).
Does it matter whether the magnet moves or the coil moves?
2.
Observe the direction that the needle deflects when you move the magnet in and out of the coil.
Does the needle deflect in the same direction as the magnet moves in and out?
3.
Move the magnet rapidly through the coil. Then move the magnet slowly through the coil.
How does the speed of the magnet affect the deflection of the galvanometer needle?
4.
Connect the coils with more and less than 400 turns to the galvanometer and move the magnet in and
out of the coils.
How does the number of turns of the coil loop affect the deflection of the needle?
5
5.
Let the bar magnet lie flat on the table. Then rotate the coil about its center at the end of the magnet.
Try to keep the distance between the coil and the magnet constant.
Describe what happens.
6.
If there is a current in the wire, then there must be a potential difference present to cause the current.
The larger the potential difference, the larger the resulting current which will be indicated by the
magnitude of the deflection of the galvanometer needle. From the observations you made above can
you determine what measurable quantities affect the potential difference in the circuit.
7.
Derive a relationship (an equation) between the potential difference (induced Emf) and the physical
quantities that affect it.
Part II
8.
Rotate the coil in the electric motor. Does the needle of the galvanometer move? Which direction?
9.
Rotate the coil in the electric motor in the opposite direction. How does this affect the needle of the
galvanometer?
10.
Explain why the needle deflects when the coil in the electric motor rotates.
6
Part III
11.
In the first part of the transformer experiment, why does the straight iron bar move out of the coils
when the power is turned?
12.
Calculate the ratio of Ns/Np for each combination of coils. Then calculate Vs/Vp for each combination
of coils. Compare the ratio to each other.
13.
Using the ratios above, determine a simple relationship between the number of turns of the coils on the
primary and secondary coils and the voltage across the primary and secondary coils.
14.
What are some practical applications of electromagnetic induction and transformers?
7