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1
CHAPTER 30
INDUCTANCE
BASIC CONCEPTS
Mutual Inductance
Self Inductance
Magnetic Field Energy
Circuits with Inductors
2
Mutual Inductance
I1 #1
If I1 changes with time t
3
Put second coil at P
I1 #1 #2
4
If I1 changes with time
For example
Then
And
B
5
Or
And
I1 #1 #2
I2
6
be the flux through
coil #2 due to B of coil #1.
Let be the number of turns
of coil #2.
Then depends on the
magnitude of B.
Magnitude of B depends on
7
Therefore
depends on
In fact
Call constant of proportionality
Then
is the Mutual Inductance
8
Solve for
Faraday’s Law
9
is a constant depending only on
geometry‐ position, size and shape.
By Symmetry
And
is Mutual Inductance.
Units are Henry (H).
10
Example
11
From Chapter 28
12
Only depends on geometry, number of
turns, area and length.
13
Self Inductance
If the current is changing then there will be
a changing magnetic flux through the coil.
14
A changing magnetic flux will induce an
emf.
Therefore we define in analogy to the
mutual inductance
This is Self‐Inductance
Solve for
15
Faraday’s Law
Self‐Induced emf
16
What is the inductance, , of the black coil?
B
Area
A
17
Magnetic Field Energy
Consider the coil
Current starts at zero and increases to .
Or
18
And
Power is work per unit time.
19
How much work to bring current through
inductor from zero to ?
Integrate.
20
Apply to solenoid
21
So
Put into equation for
22
And
So
Energy stored in magnetic field .
23
R‐L Circuit
Consider this circuit
24
First consider that switch is closed and
calculate how the current will increase with
time.
Second, then while there is a current in the
circuit consider opening and closing
and calculate how the current will decay.
Close
25
At time close switch.
Kirchkoff’s loop starting at the switch and
going ccw.
26
Integrate
27
Raise to the power of each side
Solve for i
28
Let to get
29
30
The L‐C Circuit
The capacitor has been charged to an initial
charge and then placed across the
inductor.
31
Kirchhoff’s Law
A solution of this differential equation is
Where is the initial charge on C.
32
Differentiate to get current
Remember energy in
Capacitor
And
Inductor
Then study
33