Download Experiment 7 - WFU Physics

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circuit and if a permanent magnet is inserted into or withdrawn from the solenoid, a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, CCWcounterclockwise 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
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of the solenoid, the current induced is counterclockwise, or if the south pole is moved
toward the observer, the current is counterclockwise.
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.
Lab Jack
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Figure 2. Connection of primary
coil to the transformer
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c. Repeat the above exercises with an iron core inserted in the centers of the coils.
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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