Electromagnetism
Lenz’s Law
Effects of Changing Magnetic Fields
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-when a wire moves through a changing magnetic
field, a current is induced in the wire
-the induced current in the wire generates a
magnetic field about itself which is called an induced
field
-the induced magnetic field will in turn interact with
the external magnetic field of bar magnets through
which the wire is moved
- the magnetic fields oppose each other which acts
to slow down or reduce the velocity of the wire
Interaction of the two Magnetic Fields
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-use the 3rd LHR to indicate the direction of Fm on
wire with the induced current
- Fm is directed downwards which is opposite to the
velocity of the wire (electric motor effect- Fm acting
onto a current carrying wire)
Lenz’s Law
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the direction of the induced current is
such that the magnetic field resulting
from the induced current opposes the
action of the inducing field
KEY: The velocity vector, v, will always be
in the opposite direction of the magnetic
force vector, Fm.
http://www.launc.tased.edu.au/ONLINE/SC
IENCES/physics/Lenz's.html
Eg) dropping a magnet through an
aluminum tube (conductor)
http://micro.magnet.fsu.edu/electromag/java/lenzla
w/index.html
http://www.launc.tased.edu.au/online/sciences/physics/Lenz's.html
Lab Demo:
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Determine the magnetic force exerted onto
the falling magnet
mass of magnet 0.0549 kg
Weight of magnet Fg =mg Fg = 0.5386 N
Length of tube 1.52m
Time to Fall t = 8.00s
Acceleration of the magnet down the
tube
1 2
d  vi t  at
2
1
2
1.52m  0  a(8.0 s )
2
2
a  0.0475m / s
Fnet  ma
 0.0549kg (o.o475m / s 2 )
 0.0026078 N
Fnet  Fm  Fg
0.0026078 N  Fm  0.5386 N
Fm  0.5359923 N
Fm  0.536 N
Examples With Lenz’s Law
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Eg) a North pole of a magnet is moved into the right side of a coil as
illustrated below
As the magnet is moved past the coils, an induced current in the
coils (generator effect) will generate a magnetic field to oppose the
motion of the magnet
The right end of the coil will become a north pole to exert Fm to the
right onto the magnet moved left.
When the magnet is pulled out to the right, the induced current in the
coils is reversed and the N pole becomes a S pole
Lenz’s Law Applied To an Electric Motor
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-when a current carrying wire moves through an
electric field, a back voltage is generated which
opposes the direction of current flow from the
voltage source
Electric Motor Effect
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as the wire is forced
upwards, the velocity of the
wire is upwards and the
induced current in the wire
is generated in the opposite
direction of the electron flow
from the voltage supply.
Summary
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the voltage induced in an electric circuit
always acts in such a direction that the
current it drives around a closed circuit
produces a magnetic field which opposes the
change in magnetic flux."