Experiment 5b
Class:
Name:
(
) Date:
5b Moving a conductor across a steady
magnetic field
Objective
To study the induced e.m.f. in a conductor when the conductor is
moved across a steady magnetic field.
Background information
An e.m.f. is induced in a conductor when there is a relative motion of the
conductor and a magnet which results in a change in the magnetic field
experienced by the conductor.
Apparatus
❏ 1 light-beam galvanometer
❏ several PVC-coated copper wires
❏ 3 pairs of slab-shaped magnets on
❏ several connecting leads
a steel yoke
Procedure
✐ To produce more
1
(a) Connect a U-shaped wire to a light-beam galvanometer.
dramatic effect, the
Eclipse Major magnet
may be used instead of
the slab-shaped magnets
mounted on a steel yoke.
This produces a much
larger induced current
even with a very small
movement of the wire.
(b) Move the wire to and fro between two pairs of slab-shaped
magnets with unlike poles facing each other (Fig 5b-1). Note the
deflection of the light-spot on the galvanometer.
U-shaped wire
light-beam
galvanometer
two pairs of slab-shaped
magnets on a steel yoke
Fig 5b-1
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Class:
Name:
(
✎
Experiment 5b
) Date:
When the wire is moved to and fro between the magnets,
the light-spot moves to and fro about the centre zero.
Note
✎
induced current
The deflection of the galvanometer shows that an __________________
is produced in the wire.
2
Repeat by fixing one more pair of slab-shaped magnets on the
steel yoke to increase the strength of the magnetic field (Fig 5b-2).
Compare the induced e.m.f. produced.
Fixing more slab-shaped
magnets on the steel
yoke will
1increase the number
of magnets,
U-shaped wire
2reduce the separation
between the magnets.
Both factors increase the
strength of the magnetic
field between the magnets,
but the former is the
dominant factor in this
case.
to light-beam
galvanometer
three pairs of slab-shaped
magnets on a steel yoke
Fig 5b-2
larger
✎
The induced e.m.f. is ______________________ (smaller/larger/the same)
when the strength of the magnetic field is increased.
3
Move the wire to and fro slowly and then quickly between the two
pairs of slab-shaped magnets. Compare the induced e.m.f. produced.
✎
larger
The induced e.m.f. is ______________________
(smaller/larger/the same)
when the wire is moved to and fro faster between the magnets.
4
(a) Connect a coil of several turns to the galvanometer.
(b) Move the coil to and fro between two pairs of slab-shaped
magnets (Fig 5b-3). Compare the induced e.m.f. produced.
coil of several turns
to light-beam
galvanometer
two pairs of slab-shaped magnets
on a steel yoke
Fig 5b-3
✎
larger
The induced e.m.f. is ______________________ (smaller/larger/the
same) when the length of the wire moving between the magnets
is increased.
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Experiment 5b
Class:
Name:
(
) Date:
5
(a) Connect a wire of 0.5 m to the galvanometer.
(b) Move the wire sideways between two pairs of slab-shaped
magnets (Fig 5b-4). Note the deflection of the light-spot on the
galvanometer.
to light-beam
galvanometer
two pairs of slab-shaped
magnets on a steel yoke
Fig 5b-4
✎
When the wire is moved sideways between the magnets,
the light-spot does not move much.
Discussion
✎
✐ Indeed a closed circuit
is needed for generating
an induced current. If the
circuit is open, only an
e.m.f. is induced and no
current flows in it.
How should the wire be moved in the magnetic field for producing
an induced current?
A current is induced when the wire is cutting through magnetic field lines.
✎
State three ways to increase the deflection of the galvanometer in this
experiment.
(i) Increase the strength of the magnetic field which the wire cuts through.
(ii) Increase the length of the wire cutting through the magnetic field lines.
(iii) Increase the speed of the wire cutting through the magnetic field lines.
1 An e.m.f. is induced in a conductor when the conductor is
magnetic field lines
cutting through ____________________________________________ .
2 The induced e.m.f. in a conductor can be increased by
increasing
_______________________________
the speed of the conductor
increasing
cutting through the magnetic field lines, ____________________
the strength of the magnetic field which the conductor cuts
increasing
through and ________________________________
the length of the
conductor cutting through the magnetic field lines.
94
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Class:
Name:
(
) Date:
Experiment 5b
Further thinking
✎
What will be the result if a student moves a wire of shape as shown
in Figure 5b-5 between two slab-shaped magnets with unlike poles
facing each other? Explain your answer.
light-beam
galvanometer
Fig 5b-5
The light-spot does not move much. This is because the induced e.m.f. from the
first vertical segment of the wire cancels out with the induced e.m.f. from the
second vertical segment of the wire. Similar situation happens in the third and fourth
segments of the wire.
New Physics at Work (Second Edition)
© Oxford University Press 2007
95