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ELECTROMAGNETIC INDUCTION
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All conductors that carry a current produce a magnetic
field
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As the magnet is moved in and out of a coil of wire in a
closed circuit an induced current will be produced
All dynamos and generators produce electricity using
this effect
Electromagnetic induction takes place when the
magnetic field around a conductor changes
If the magnetic field is made to change quickly, the size
of the current induced is larger
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INDUCTANCE
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Inductance is a phenomenon in which a changing
current in a circuit builds up a magnetic field which
induces an electromotive force either in the same
circuit and opposing the current (self-inductance) or in
another circuit (mutual inductance)
A component designed to introduce inductance into a
circuit is called an inductor
It is usually in the form of a coil of wire
The energy stored in the magnetic field of the coil is
proportional to its inductance and the current flowing
through it
The magnitude of the voltage induced in a coil depends
directly on the rate of change of the current through it,
where L is the inductance
Symbol
Unit of inductance is the henry (henries) (H)
I
V L
t
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FUNCTION OF AN INDUCTOR (1)
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Consider the following circuit
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Without inductor, the bulb lights up when the switch is
closed
Light bulb is a resistor – resistance creates heat to
make bulb filament glow
The coil of wire around a piece of iron in the inductor
has a much lower resistance
When the is switch closed, the bulb burns brightly, and
gets dimmer
When the switch is opened, the bulb burns very brightly
then quickly goes out
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FUNCTION OF AN INDUCTOR (2)
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When current starts flowing in the coil (inductor), the
coil wants to build up a magnetic field
As the field builds up, the coil inhibits current flow
Once the field is built, current flows normally through
the wound wire
When the switch is opened, the magnetic field around
the coil maintains current flow in the coil until the field
collapses
The current keeps the bulb lit for a period of time, even
though the switch is open
In other words, an inductor can store energy in its
magnetic field
An inductor also tends to resist any change in the
amount of current flowing through it
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VOLTAGE ACROSS AND CURRENT
THROUGH INDUCTOR EXAMPLE
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Sketch the voltage across a 9.87mH inductor, when the
current through it changes with time as shown in the
graph below
I(A)
0.5
8
0
4
6
9
t(ms)
-0.3
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PRACTICAL EXAMPLE OF AN
INDUCTOR
Switch
contacts
Battery
12V
iL
Air gap in
spark plug
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VL
High
voltage
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An inductor is used in a car ignition system
Objective is to produce a high voltage that’s sent to the
air gap in the spark plugs
Its operation depends on the fact that the induced
voltage is directly proportional to the rate of change of
current (Δi / Δt) through it
For large Δi / Δt, DC voltage from the battery is
switched into and out of the circuit at a high rate
Switching is accomplished by the contacts in the
distributor
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INDUCTORS IN CIRCUITS
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The total equivalent inductance for series connected
inductors is
LT = L1 + L2 +…+ Ln
For parallel connected inductors
1/LT = 1/L1 + 1/L2 +…+ 1/ Ln
When an inductor having no current flowing through it
is first switched into a circuit, it behaves like and open
circuit because the current cannot change
instantaneously from its zero initial value
After the circuit has been switched on for a long time,
the current has reached a state where its value is not
changing (steady state value) anymore, hence the
inductor acts as a short circuit
The energy stored (W) in an inductor with inductance
(L) is given by
W = (1/2)LI2 Joules
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INDUCTORS IN CIRCUITS
EXAMPLES
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Find the initial and steady-state (final) voltage across
and current through every component after the switch is
closed at t = 0.
t=0
6kΩ
3kΩ
1.5kΩ
28V
L2
5kΩ
L1
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The inductor in the circuit below has an inductance of
0.2H, and a winding resistance of 400Ω. Find the
energy stored in the inductor and the rate at which
energy is dissipated by the winding under steady-state
conditions.
t=0
600Ω
100V
L
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