20'2 Lenzs Law

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20.2 Lenzs Law

• Lenzs law
• Motional emf
• Some applications of Faradays Law

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Faradays Law gives a means to generate a current
using a changing magnetic flux. What is the
direction of the current? How much power can be
generated? Where does the power come from? How
can we use the changing flux to generate voltage?
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Lenzs Law The polarity of the induced emf is
such that it produces a current whose magnetic
field opposes the change in magnetic flux through
the loop. i.e. the current flows to maintain the
original flux through the loop.
B increasing in loop
Bin
B
Bin acts to decrease flux
-
?
Current direction that produces Bin is as shown
(right hand rule)

I
V
Emf has the polarity shown. e drives current
in external circuit.
4
Now reverse the motion of the magnet
The current reverses direction
B decreasing in loop
Bin
Bin acts to increase flux

?
B
Current direction that produces Bin is as shown
(right hand rule)
-
I
V
Emf has the polarity shown.
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Lenzs Law and Reaction Forces
Fmc
Fcm
A force is exerted by the magnet on loop to
produce the current
Bin
B
A force must be exerted by the current on the
magnet to oppose the change
I
V
The current flowing in the direction shown
induces a magnetic dipole in the current
loop that creates a force in the opposite
direction
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Example. Eddy currents
A magnet is dropped down a cylindrical
conductor. Currents are induced in the conductor
to oppose the movement of the magnet
S
N
B increasing
Bin
Induced magnetic field
I
creates magnetic dipole that opposes the movement
of magnet
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Example. Eddy currents
What about the region behind the magnet?
creates magnetic dipole that opposes the movement
of magnet
Bin
I
B decreasing
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Work
Fcm
Work done against the reaction force by the force
moving the magnet drives the current flow.
Bin
B
induced magnetic dipole
I
V
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Motional emf
A voltage is produced by a conductor moving in a
magnetic field
Charges in the conductor experience a force
upward
x x x x x x
x x x x x x
x x x x x
x x x
The work done in moving a charge from bottom to
top
F
v
L
The potential difference is
B into the page
10
The potential difference can drive a current
through a circuit
wire
I
x x x x x x
x x x x x x
x x x x x
x x x
F
v
R
L
B into the page
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18. R 6.0 O and L1.2 m and B2.5 T. a) What
speed should the bar be moving to generate a
current of 0.50A in the resistor? b) How much
power is dissipated in R? c) Where does the power
come from?
wire
a)
I
x x x x x x
x x x x x x
x x x x x
x x x
F
v
R
b)
L
c) Work is done by the force moving the bar
B into the page
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Two coils are connected by an iron core. Is the
emf from a to b positive or negative when the
switch is closed?
Bin
Iin
I
B
increasing
a
b
-

Iin
I
A
current flows from b to a Iab is negative
therefore eab is negative
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59. Magnetic field due to a lightning bolt
A bolt of lightning passes 200 m from a 100 turn
coil oriented as shown. If the current in the
lightning bolt falls from 6x106 A to zero in 10
µs what is the average voltage induced in the
coil.
r0.8m
R200m
Bout?0
I?0
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Application of Faradays Law Generation of
voltage by a tape recorder
As magnetic tape moves under the gap it induces a
changing flux in the coil. The changing flux
produces a voltage that is amplified to give the
signal.
Iron core electromagnet
Gap
Magnetic tape
V