| a. |
Faraday’s
Law of electromagnetic induction may be summarized by the equation
Explain the physical
meaning of this equation, using a single turn coil of wire as an example. |
2
marks |
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For single turn of wire, the magnetic flux
is |
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where 
is the magnetic flux density at right angle to the plane of coil and A
is the area. |
0.5 |
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The induced e.m.f. across the wire is equal
to the rate of change of magnetic flux. |
1 |
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The negative sign in the expression shows that
the e.m.f. is induced to oppose the change. |
0.5 |
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| b. |
Explain,
using the case of a N-pole of a magnet approaching a closed loop of wire,
why Lenz’s Law is a consequence of the law of conservation of energy. |
2
marks |
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When the magnet is moved towards the wire,
a current would be induced in the wire. By Lenz's Law, the motion of the
magnet is opposed. |
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Work is done on the magnet to keep it moving at constant
speed against the retarding force. This is also the internal energy produced
as the current flows in the wire. Energy is conserved. |
1 |
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| c. |
Describe
the phenomenon of electromagnetic induction in a coil which involves
i)
physical movement of the coil |
4
marks |
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1 |
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When the coil is moved towards a N-pole, the magnetic flux
in A increases suddenly. According to Faraday's Law, a current is induced
in a clockwise direction, generating a N-pole to oppose the change. |
1 |
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ii)
no physical movement of the coil. |
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1 |
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When coil B is closed suddenly, a current flows
anti-clockwise in B. The magnetic flux in A increases suddenly. According
to Faraday's Law, a current is induced in A to oppose the change. The induced
current is clockwise. |
1 |
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| d. |
Explain
the effect of electromagnetic induction on the switching-off of a current
supply to an electromagnet. Give a diagram showing the instantaneous direction
of the induced e.m.f. |
4
marks |
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When the current of an electromagnet is switched off suddenly,
its magnetic flux falls to zero in a very short time. According to Faraday's
Law, a very large e.m.f. would be induced in a way to oppose the drop in
flux linkage. |
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2 |
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1 |
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The induced e.m.f. is directed so that it "tends to" restore
the current. |
1 |
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| e. |
Explain
how the law of conservation of energy is applied to the effect described
in (d). Briefly explain suitable precautions which can be taken to minimize
the detrimental effects produced by switching off a circuit with large
inductors. |
4
marks |
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When a coil (or an inductor) carries a current, it also
stores up magnetic energy. |
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where L is the inductance of the coil and I
is the current. |
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When the current stops suddenly, this energy would be released
and converted into other form, e.g. sparks occurs across the switch, giving
sound and light. |
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Switch design |
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Use spring-loaded switch arm so that the separation of contact is done
rapidly.
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Connect a capacitor across the switch of a d.c. circuit with large inductance.
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Sparks can be blown off by jets of compressed air.
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Sparks can be blown off by immersing the contacts in non-inflammable oil.
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In electronic circuit, diodes are connected across the magnetic relay coil.
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