22.5 Lenz’s Law
Remember Faraday’s Law:
t
N
M
Δ
ΔΦ

=
ε
, where
.
cos
φ
BA
M
=
Φ
So a changing magnetic field can produce a current.
What is the direction of
this current?
To get the direction, we must consider two magnetic fields:
The first is the original one that produced the induced current.
The second is the field created by the induced current itself.
To determine the direction of the induced current flow, we use
Lenz’s Law:
The induced emf resulting from a change in magnetic flux leads to an induced
current which produces a magnetic field to oppose the change
in flux.
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Example:
Consider a single loop of wire sitting in the plane of the paper:
Now turn on a magnetic field everywhere into the page:
Then, in all of these Lenz’s Law problems, you have to
ask yourself two questions:
First, what is the direction of the magnetic flux
?
In other words, what direction does the original magnetic field point.
Here, it points into the page
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 Spring '08
 BLACKMON
 Physics, Current, Magnetic Field, Lenz, xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx

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