Basic Physics II

Lab 6: Electromagnetic Induction (October 28 & 29)

In this lab you will study electromagnetic induction due to changing electric currents and/or magnetic fields.

APPARATUS

Coils, switch, galvanometer, bar magnets, and compass

THEORY

Electric currents produce magnetic fields, which can be observed by their effect on a compass needle. The needle tends to align itself in the direction of the magnetic field. It turns out that a changing magnetic field can also cause a current. This is called electromagnetic induction. In this experiment, we will study both of these effects.

Lenz's law allows us to analyze many situations of electromagnetic induction quantitatively. It states that the direction of an induced emf is always such that it opposes the change producing it. We can use Lenz’s law to analyze induction through the following steps:

1. Determine whether the magnetic flux is increasing or decreasing.

2. Find the direction an induced magnetic field must have in order to oppose the change in flux by adding to or subtracting from the original field.

3. Having found the direction of the induced magnetic field, use the right hand rule to determine the direction of induced current.

PROCEDURE

Part I. A moving magnet causes electric current in a coil.

We will call the back face of the coil the one with two connectors at the top. When you watch the coil from the front face as fig.1, the connector on the right goes to the inner lead of the coil while the left one goes to the outer lead. A current entering the outer lead and leaving the inner lead will be in a counterclockwise direction when looking at the front face. (Note that the front face is the one that does NOT have the wire terminals connected to it.)

Connect the outer lead from the coil to the (-) terminal of the galvanometer and the inner lead to the (+) terminal. Note that the current flowing into the galvanometer on the (+) terminal causes a deflection to the right (see Fig.2)

Perform the following experiments and record the direction and amount of the deflection observed in the galvanometer. Explain the direction of the deflection in terms of Lenz's law. Discuss the factors influencing the amount of deflection of the galvanometer.

1. Insert the N pole of your magnet from the front face of the coil to the back.

2. Retract the magnet from the position in step 1.

3. Insert the S pole from the front of the coil to the back.

4. Retract the magnet from the position in step 3.

5. Repeat 1-4 but insert the magnet from the back to the front of the coil instead of from the front to the back.

6.. Pass the magnet all the way through from front to back with the N pole leading. (It does not have to be moved rapidly.) Describe the galvanometer motion. Discuss the effect of the rate of movement on the galvanometer deflection.

7. Pass two bar magnets with poles aligned and compare it to the deflection produced by one magnet.

8. Repeat step 6 with the two magnets oppositely aligned. Is there still some deflection? If so, why?

Part II. A current in a wire produces a magnetic field.

1. Connect a coil (with the compass in the middle) to the dc power supply as follows: the inner lead of the coil to the + (red) terminal and the outer lead to the - (black) terminal. Sketch the magnetic field lines for the coil by placing the compass at various points inside and outside the coil. Indicate the current direction in the coil, and which face is the front face.

2. Reverse the connections to the dc power supply and describe what happens to the magnetic field.

3. Note that the coil acts as a magnet. From the sketch of the magnetic field obtained in step 1, indicate which end behaves as a north pole.

Part III. Induction between coils

In this section of the experiment, there will be no wires connecting the two coils directly. The coupling between them will be magnetic. Place the two coils parallel, as close together as possible and facing the same direction. Connect the front coil to a 5-volt dc power supply through a switch: inner lead to the - (black) and outer lead to the switch and the + (red) source. Connect the rear coil to the galvanometer, the outer lead to the + terminal, and the inner lead to the - terminal. Fig.3 shows the connections for the two coils.

Perform the following experiments and record both the direction and amount of deflection observed in galvanometer connected to the rear coil. Explain the direction of the deflection in terms of Lenz's law.

1. Quickly close the switch to the front coil.

2. Leave the switch closed for a few seconds.

3. Now open the switch quickly.

4. Insert a soft iron rod in two coils to enhance the coupling between two coils. Repeat steps 1 to 3.

5. Turn the rear coil at right angle to the front coil, repeat steps 1 to 3. Try to explain the magnitude of the deflection in this case.

For Part I, you need to tell me whether a + deflection on the galvanometer represents a clockwise or a counterclockwise current in the coil (with the front facing you) and whether this represents an induced magnetic field coming out of the front or going into the front of the coil.

Make a table for part I that is like the one below.

Direction of field from magnet	Increasing or decreasing in strength within the coil	Deflection of galvinometer	Direction of Induced Current	Direction of Induced Field
(In or Out of front face)		(+ or − )	(CW or CCW)	(Into or out of front face)

Then you need to say whether the direction of the induced field is in agreement with Lenz’s law, given the direction of the magnet’s field and whether it is increasing or decreasing within the coil.

A similar table would also be useful in part III, except the first two columns will refer to the field from the first coil that is connected to the power supply, and you will need to tell me whether a + current in the galvanometer represents a CW or CCW induced current in the second coil and whether this represents an induced magnetic field coming out of the front or going into the front of that coil.