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CH 23-2
Review
Changing the magnetic flux in a coil induces an emf around the coil. (As
long as the coil is connected in a complete circuit, a current is
induced in the coil.) This effect is called electromagnetic
induction.
Two typical ways to induce a current in a pickup coil is:
(1) change the magnetic field in the plane of the pickup coil
(a) by moving a magnet back and forth
(b) by changing the current in a primary coil
(2) change the orientation of the pickup coil in a uniform magnetic field
(or alternatively, rotate a magnet and keep the coil fixed)
Example
A primary coil has a current of 0.5 A, radius 0.02 m, and
20 loops of wire. It creates a magnetic field in the plane
of a pickup coil of 0.001 T. If the current in the primary
coil decreases to zero in 0.1 s, what is the magnitude of
the emf induced in the pickup coil if its radius is 0.01 m
and it is made of 100 loops of wire? (Assume that the
two coils are aligned along the same axis.)
Direction of the induced current -- energy
must be conserved
Lenz’s Law
The induced current will flow so as to produce a
magnetic field opposing the change in flux.
If the flux in the pickup coil is increasing, then the
induced current produces a magnetic field that is in the
opposite direction as the applied magnetic field.
If the flux in the pickup coil is decreasing, then the
induced current produces a magnetic field that is in the
same direction as the applied magnetic field.
Example
move the magnet toward the coil
N
S
Example
move the magnet away from the coil
N
S
Example
move the magnet toward the coil
S
N
Example
move the magnet away from the coil
S
N
Example
increase the current in the primary coil
I (+z)
I (z)
Example
decrease the current in the primary coil
I (+z)
I (z)
CIRCUIT 1
a
b
CIRCUIT 2
c
d
The coils in these two circuits are wound around a common
core, but the circuits are electrically insulated
from each other. According to Lenz’s law, the current
induced in Circuit 1 flows ________ when the switch is
closed, and then, with the switch kept closed, it flows
________ as the sliding contact of the variable resistor in
Circuit 2 is moved from point c to point d.
A. from a to b … from b to a
B. from b to a … from a to b
C. from a to b … from a to b
D. from b to a … from b to a
A battery is connected to
a wire loop, with a switch
in the circuit. When the
switch is closing, what
direction does current
flow in the pickup coil?
A battery is connected to
a wire loop, with a switch
in the circuit. When the
switch is opening, what
direction does current
flow in the pickup coil?