Electromagnetic Induction
Electromagnetic Induction
Objectives: To study the production of induced emf's via the relative motion of closed loops of I wires
and magnetic fields.
Apparatus:
Gilley Induction Coil set
bar magnet
horseshoe magnet
1 1/2 volt battery
resistance box
rheostat
tap key
galvanometer
connecting wires
Background:
Among the most significant discoveries in the field of electricity of the nineteenth century were
those having to do with the mutual relationships between electricity and magnetism. The first of these
was the observation, by H.C. Oersted in 1820, that a wire in which there is a current has associated with
it a magnetic field.
The second significant discovery in this field was made by Michael Faraday in a series of
experiments lasting from 1824 to 1831. Faraday reasoned that if a current has an associated magnetic
field, then conversely, if a magnetic field is set up around a wire, the wire should acquire a current.
However, when Faraday attempted to find such a current he met with failure as long as the field was
static relative to the wire. He finally observed that the expected current is produced when there is motion
of the wire relative to the field or when the field is changing in strength.
Whenever a conductor moves through a magnetic field in such a manner as to cut across the
magnetic flux, an emf is induced in the conductor. The magnitude of the induced emf is directly
proportional to the rate of change of flux. Similarly, an induced emf is produced whenever there is a
change in the flux threading a coil. In symbols:
ε=−
Δφ
Δt
where: φ = the magnetic flux
Δφ = the change in magnetic flux
€time over which the flux has changed
Δt = the elapsed
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Electromagnetic Induction
Note: The magnetic flux, φ , linking an area is the product of the area and the component of the
magnetic intensity, B, perpendicular to the area.
B
A
φ = ABcosθ
θ
€
If the N pole of a magnet is moved toward a coil, Fig. 2a, an emf is induced in the coil and there
is a current in the closed circuit in the direction indicated by the arrows. The field of the current is
opposite in direction to that of the magnet and thus tends to oppose the increase of the field. When the N
pole is moved away from the coil, Fig. 2b, the current reverses in direction. In this case, the field caused
by the current is in the same direction as the field of the magnet and opposes the decrease in field.
Lenz's law states the general condition for the direction of an induced emf, "Whenever there is an
induced emf, it is in such a direction as to tend to oppose the change that caused it" This law is a form of
the law of conservation of energy.
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Electromagnetic Induction
Procedure:
1. In order to determine the directions of the induced currents, it is first necessary to determine
the direction of the galvanometer deflection corresponding to a known direction of current. Connect the
galvanometer into a potential divider circuit, Fig. 3. Use a dry cell as a source and set the slider of the
rheostat to tap off one or two wires of the slidewire. Note the direction of the current when the tap key is
closed, and note the direction of deflection of the galvanometer. Arrange polarity of cell so that
galvanometer deflects toward terminal marked positive on the meter. Use this information throughout
the experiment to determine the direction of the induced current.
2. Loop of wire and Bar Magnet: Attach a long wire to the terminals of the galvanometer and
make a single loop of wire. Thrust the N pole of the bar magnet quickly into the loop and note the
deflection of the galvanometer.
N
Figure 4
S
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Electromagnetic Induction
a) What is the direction of the current through the loop? (This question, and all like it, is best
answered with a drawing).
b) Use the right-hand rule to find the direction of the field due to the induced current. Is the
direction in accord with Lenz's Law?
Repeat questions a and b when you
i. Quickly pull the bar magnet out of the loop.
ii. Thrust the S pole of the magnet through the loop.
iii. Hold the bar magnet stationary and quickly thrust the loop over the N pole of the magnet,
3. Connect one of the coils to the galvanometer. Note the way the wire is wound on the coil.
Record the direction of the current each time as clockwise or counter-clockwise a viewed from the side
on which the binding posts are mounted.
Thrust the N pole of the magnet toward the coil and note the deflection of the galvanometer.
Explain deflection in terms of Lenz's law. Hold magnet stationary near the coil and note galvanometer
reading. Explain. Withdraw the N pole and explain the result. Repeat, using the S pole.
Figure 5: (for part 3)
Figure 6: (for part 4)
4. Connect the second coil in series with a dry cell, and a tap key. Connect the cell so that the
current in the coil is clockwise as viewed from the side opposite to the binding posts. Set this coil
immediately adjacent to the coil connected to the galvanometer and with their axes in line. Close the
switch and note the deflection. Is the induced current in the secondary coil in the same or opposite
direction to that in the primary coil? Explain in terms of Lenz's law. With the key closed note the reading
of the galvanometer. Explain. Open the switch and note the direction of the induced current. Is the
induced current in the same or the opposite direction to that in the primary coil? Explain in term of
Lenz's law. Repeat with the coils separated by a half inch. Explain the effect of the separation.
5. With the cells in the initial position of Step 4, close the switch of the primary circuit. After
the pointer has come to rest, quickly move the primary coil away from the secondary. Note and explain
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Electromagnetic Induction
the deflection. Quickly replace the primary coil. Note and explain the deflection. Repeat these readings
with the coil moved slowly. Explain the difference in the two effects.
6. Set the two coils in the position shown. Close the switch and note the deflection. Insert the
soft-iron core and again close the switch and note the deflection. What does this indicate as to the
effect of the presence of the iron core? Change the distance between the coils with the iron core still
joining them. Close the switch and compare the reading with the one previously taken by use of the
iron core. Does the distance between the coils materially affect the reading? Explain.
Figure 7
7. Set the two coils in the position shown. Close the switch and note the deflection. Insert the
U-shaped soft iron and again close the switch. Explain the difference. What does this experiment
show regarding the path of the flux?
Figure 8
8. Summarize the results of all these experiments. What factors are common to all the
experiments? State the conclusions that you can reach concerning the factors upon which the value
of the induced emf depends.
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