Download Electromagnetic Induction

Chap 28
ELECTROMAGNETIC INDUCTION

When current passes through a wire it
generates a magnetic field.

Similarly, if a changing magnetic field
passes through a coil, an emf appears in
the coil. This phenomenon is called
electromagnetic induction
EXP.

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To demonstrate electromagnetic induction
(a) coil and magnet
(b) coil and coil-one of coils connected to ac
supply – changing magnetic field is generated in this
coil which then induces a current in the second coil
NOTE: The current depends on the fact that the
magnetic field is changing .
MAGNETIC FLUX Φ

Magnetic flux can be
thought of as the total no.
of magnetic field lines
passing through a
particular area.
Φ = BA
 B = magnetic flux density,
A = area

Magnetic flux is a scalar quantity
 Unit = weber (Wb)


The magnetic flux is 1 Weber if the
magnetic flux density over an area of 1m2
is 1T
FARADAY’S LAWS OF ELECTROMAGNETIC
INDUCTION

When a changing magnetic field is applied to a
coil an emf is induced in the coil

The size of the induced emf is directly
proportional to the rate of change of flux

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Induced emf =
change in flux / time
E = (final flux – initial flux) / time
E = - dΦ / dt
The minus sign indicates the direction of the
induced emf, and can be ignored in calculations
of E
Do questions p315
LENZ’S LAW

States that the direction of the induced emf is
such as to oppose the motion causing it.

E.g. if the current is caused by a north pole
approaching the coil, this is the motion the
current seeks to oppose→ the current flows in
such a direction as to make that end of the coil
into a north pole (i.e. opposes the north pole
approaching). This means work must be done to
push the magnet towards the coil. This loss of
energy = gain of electrical energy of the coil

If however, the north pole is moving away
from the coil, the current flows such that a
south pole is created at that end of the
coil ( this will attract the north pole back,
i.e. oppose the north pole moving away)

This is why as the magnet moves in and
out the induced current changes direction

If Lenz’s law were not true, then as the
north pole approaches, a south pole could
be induced. This would attract the magnet
even faster, causing a bigger current,
causing it to go faster etc.

Thus both magnet and coil would gain
energy. This energy would be coming from
nowhere and so violate the principle of
conservation of energy
ELECTRIC GENERATOR
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-converts mechanical energy
to electrical energy
Opposite system to dc motor
In this system the coil is
turned mechanically in the
magnetic field
As the coil turns an emf is produced
across the coil, and a current flows.
 Each ½ cycle the direction of the induced
current changes direction, thus
generating A.C.

If d.c. is required a split ring commutator
is used to change the direction every ½
cycle
 Generators are found in power stations,
alternators in cars, dynamo of a bike

ALTERNATING CURRENT
Current of 50 Hz means 50 complete cycles per
sec.
 This means the current changes direction 100
times per sec.

AC VALUES

AC is continuously changing. When values are
quoted for a.c. (e.g. 3A a.c.) what it actually
means is that this quantity of a.c. generates
heat at the same rate as a direct current of 3A
would generate
MEASURING AC
Since a.c. is always changing, when a
value of a.c. current is quoted, it must be
an average of sorts. The average used is
the root mean square value

Irms = Io / 2

Vrms = V0 / 2
 I0 or V0 = maximum values
 Do questions p321
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MUTUAL INDUCTION

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If you place 2 coils near each other, a changing
magnetic field in one will induce an emf, and
hence current in the other.
This induced current is a.c. and so induces a
changing magnetic field in the second coil.
Thus the first coil is now in the changing
magnetic field due to the second coil, so an emf
is induced in the first coil
SELF INDUCTION
When ac flows through a coil, a magnetic
field is created around the coil. Thus the
coil itself is now sitting in a changing
magnetic field, so an emf is induced in
the coil.
 This induced emf opposes the driving
emf, according to Lenz’s law.

BACK EMF
The effect of this induced current is to
reduce the effective current flowing in the
coil.
 When motors are being made, they take
into account that by virtue of the fact that
a motor spins, the coil will be in a
changing magnetic field, and so there will
be an induced back emf.

Thus the current which will actually flow
through the wire is less than the initial
forward current.
 The coil is manufactured to handle this
reduced current.
 If a motor is not allowed to spin freely,
there is no back emf, and so the current
then flowing will be bigger than what the
coil was manufactured to carry → the coil
will burn out
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A.C. AND INDUCTORS.
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If an inductor (coil) is used in a circuit with an
a.c. source, since the current through the coil is
always changing, there will always be a back
emf and current which will oppose the driving
current.
If a soft iron core is inserted in the coil, the
induced magnetic field, and hence back emf is
greater, so the overall current is even less.

If an induction coil is used with a d.c.
source it has no effect (since there is no
induced changing magnetic field except
for the initial instant while the current is
building up to its maximum value)
A.C. AND CAPACITORS

If a capacitor is used with an a.c. source,
the capacitor starts to charge from one
side, then when the current changes
direction the capacitor empties from that
side and starts to charge from the other
side. This happens repeatedly so the
capacitor never gets fully charged

Since a capacitor only blocks current once
it is fully charged, capacitors don’t block
a.c

In the case of d.c., other than the initial
period when the capacitor is charging,
capacitors block d.c.
TRANSFORMERS
- a device to change the value of an alternating
voltage.
 Consists of 2 coils of wire wound around a soft
iron core (to increase the magnetic effect)

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An alternating voltage (Vin) and current is
applied to the primary coil. This generates
a changing magnetic field around this coil.

The nearby secondary coil is now in a
changing magnetic field → an induced
emf (Vout) and current is generated across
the secondary coil.
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The relative sizes of the input and output
voltages depend on the no. of turns of
wire in primary and secondary coils.
Vin / Vout = Np / Ns
If there are no energy losses in the
transformer then power stays the same

Pin = Pout
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But P = V I
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Vin Iin = Vout Iout

Vin / Vout = Iout / Iin

Do questions p327
USES OF TRANSFORMERS
Used by power stations to minimise heat losses
in cables by transforming voltages to very high
values.
 Used in many everyday household electrical
items to supply the necessary voltages to
various parts
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