Electromagnetic Induction©98

Experiment 7

Objective: To become familiar with the character of electromagnetic induction through the use of a galvanometer, magnets and coils.

DISCUSSION:

If there is a changing magnetic field in a certain region of space, then an electric field is induced in that region. The changing magnetic field may be produced by the motion of a permanent magnet, or by the fluctuation of a current. (Recall that all electrical currents are accompanied by a magnetic field, the strength of which depends, among other things, upon the current.)

The electric field generated by the changing magnetic field can be detected by the electric force which charged bodies in the region experience. If there is a conductor which is part of a closed circuit in the region, it is possible that the induced electric field will drive negative charges along the conductor and thus generate an electric current in the circuit. (Nearly all electrical currents generated for commercial use are produced in this fashion.)

If a solenoid is connected in series with a galvanometer so as to form a closed circuit and if a permanent magnet is inserted into or withdrawn from the solenoid, a current is induced in the solenoid and is maintained as long as the motion of the magnet continues. The direction of the current in the solenoid (CW-clockwise, CCW- counterclockwise when viewed from one end of the coil) is determined by the direction of the motion of the magnet and the position of its poles relative to the end of the coil.

The rule is as follows: If the solenoid is viewed on end, as shown in Fig. 1, and if the north pole of the magnet is moved toward the observer so that it approaches the far side of the solenoid, then a positive clockwise current is induced in the coil. A single reversal of the poles or motion of the magnet reverses the direction of the current. For example, if the north pole of the magnet is moved away from the observer on the far side of the solenoid, the current induced is counterclockwise, or if the south pole is moved toward the observer, the current is counterclockwise.

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The magnet need not be permanent. Identical results are produced by the magnetic field associated with currents. In particular, a current carrying solenoid acts like a bar magnet in that one end of the coil is a magnetic north and the other the magnetic south. The rule is that if the observer views the solenoid on end, when the positive current is clockwise, the far end of the solenoid is the magnetic north end.

A magnetic field surrounding a solenoid may be changed by moving the solenoid in the space or by altering the current in the solenoid. For example, a decrease in the current in the solenoid weakens the magnetic field and is roughly equivalent to withdrawing the solenoid from the region.

EXERCISES:

Explain and describe fully the behavior of the galvanometer in each of the following exercises. Use diagrams freely. Before proceeding, examine the galvanometer to determine how the direction in which the needle moves is influenced by the direction of the current. Also note the polarity of the magnets, and the direction of winding of the coils.

1. Connect one of the coils in series with the galvanometer and hold the coil still against the magnet. Does electricity flow in the wires? Explain.

2. With the magnet fixed, move the coil slowly so that the magnet passes through the coil. Withdraw the coil slowly. Repeat with a more rapid motion. Is there a current in the wires? Explain. Repeat this exercise with the poles of the magnet reversed.

3. With the coil fixed, move the magnet in and out of the coil. Repeat this exercise with the poles of the magnet reversed.

4. The transformer. a. Connect one of the coils, henceforth called the

primary, to the lab jacks as shown in Figure 2. This current is designed so that the current in the coil can be turned on and off and its direction reversed. Connect the other coil, called the secondary, to the galvanometer as in Exercise 1.

b. Place the coils next to one another along a common axis.i) Start the current in the primary.ii) Stop the current in the primary.iii) Start the current in the opposite direction in

the primary.iv) Stop the current in the primary.

c. Repeat the above exercises with an iron core inserted in the centers of the coils.

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5. Connect the secondary in series with an AC milliammeter.

a. Energize the primary with the output from a 6-volt transformer, without an iron core, and note the current (if any) in the secondary as the latter is moved toward and away from the primary.

b. Repeat with the iron core in the primary.

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