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Lecture 181
1
Self Induction
•
The changing current in a coil also induces an emf in itself
This phenomenon is called
selfinduction
The resulting emf is termed the
selfinduced emf
•
Faraday’s Law of Induction:
the selfinduced emf for an inductor is
given by
•
Lenz’s Law
provides the
direction
of the selfinduced emf
•
The minus sign
expresses that the induced emf always
opposes any
change in current
V
emf
,
L
d N
B
dt
d Li
dt
L
di
dt
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View Full Document Lecture 182
SelfInductance: increasing or decreasing current i
•
As current i through coil
increases,
magnetic flux through itself
increases.
This in turn induces
counter EMF
in
the coil itself
•
When current i is
decreasing,
EMF is induced again in the coil itself
in such a way as
to slow the decrease.
Selfinduction
B
N
L
i
(if flux linked)
Lecture 183
Examples of Induction
Switch has been
open for some time:
Nothing happening
Switch is just closed:
EMF induced in Coil 2

+

Switch is just opened:
EMF is induced again

+
Switch is just closed:
EMF is induced in coil
+

Back emf
(counter emf)
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View Full Document Lecture 184
Mutual Inductance (1)
2
1
2
1
1
,2
2
()
ind
dN
di
dt
dt
VM
For two coils, total magnetic flux
through coil 2 due to the field created
by coil 1:
Then,
12
1
i
2
1
N
M
i
Define mutual inductance of
coil 2 with respect to coil 1
unit: henry H
2
T m
V s
H
AA
,
B
ind L
d Li
di
VL
dt
dt
dt
From Faraday’s law of induction, the seldinduced potential difference
for a inductor (or coil) is given by
1 2 1
2
1 2
i
M
N
Lecture 185
Mutual Inductance (2)
1
2 1
,1
2 1
2
()
ind
dN
di
VM
dt
dt
Reciprocity
– a fundamental symmetry
2
1
1
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This note was uploaded on 12/07/2011 for the course PHYS 241 taught by Professor Wei during the Fall '08 term at Purdue UniversityWest Lafayette.
 Fall '08
 Wei
 Current

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