# Lenzs Law and Motional EMF

```4/29/2014
Electromagnetism
Lenz’s Law
Motional EMF
Lenz’s Law
Lenz’s Law: An
induced current
always flows in a
direction that opposes
the change that
caused it.
– Therefore, if the
magnetic field is
increasing, the
magnetic field created
by the induced current
will be in the opposite
direction; if decreasing,
it will be in the same
direction.
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Example – Lenz’s Law
A coil of wire is placed near an
electromagnet, as shown.
–
Find the direction of the induced current in the
coil
a)
b)
c)
at the instant the switch is closed
after the switch has been closed for several
seconds
when the switch is open
N
S
Example – Lenz’s Law
A coil of wire is placed near an
electromagnet, as shown.
–
Find the direction of the induced current in the
coil
a)
b)
c)
at the instant the switch is closed
after the switch has been closed for several
seconds
when the switch is open
Solution:
N
S
a) at the instant the
switch is closed
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Example – Lenz’s Law
A coil of wire is placed near an
electromagnet, as shown.
–
Find the direction of the induced current in the
coil
a)
b)
c)
at the instant the switch is closed
after the switch has been closed for several
seconds
when the switch is open
Solution:
b) after the switch has been closed for several seconds
NO CURRENT IS INDUCED!
Example – Lenz’s Law
A coil of wire is placed near an
electromagnet, as shown.
–
Find the direction of the induced current in the
coil
a)
b)
c)
at the instant the switch is closed
after the switch has been closed for several
seconds
when the switch is open
Solution:
c) When the switch is open
N
S
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Conceptual Checkpoint
The Direction of Induced Current
Consider a system in
which a metal ring is
falling out of a region
with a magnetic field
and into a field-free
region, as shown in
our sketch to the
right. According to
Lenz’s law, is the
induced current in the
ring (a) clockwise or
(b) counterclockwise?
Conceptual Checkpoint
The Direction of Induced Current
The induced current is counterclockwise.
Note, the induced current generates an upward
magnetic force on the ring and there is no
magnetic force on the bottom, causing the
motion of the ring to be retarded (slowed down).
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Motional EMF
Motional emf
– The emf created when a conductor is
moving through a magnetic field.
– The conductor must move perpendicular
to the magnetic field.
Motional emf
ε = Blv
Example - Motional EMF
The Electrified Airplane Wing
An airplane with a wing span of 30.0 m
flies parallel to the Earth’s surface at a
location at which the downward
component of the Earth’s magnetic field is
0.60 x 10-4 T.
– Find the difference in potential between the wing
tips when the speed of the plane is 250 m/s.
Solution:
ε = Blv = (0.60 x10-4 T)(30.0 m)(250 m/s) = 0.45 V
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