Download IV Ch 5

4
Electricity and Magnetism
5
Chapter 5 Electromagnetic Induction
Electromagnetic Induction
Practice 5.1 (p. 229)
F = BIl
1
A
= 1.8  2.7  103  0.1
2
D
= 4.86  104 N
3
(a)
(iv) The brightness of the bulb can be
increased by moving the metal rod
faster and by increasing the magnetic
field.
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(b) (i)
(a) At t = 0,
 = BA = 1.2  0.052 = 3  103 Wb
The induced current will increase.
At t = 5 s,
(ii) The direction of the induced current
 = BA = 2.4  0.052 = 6  103 Wb
will reverse.
(b) Induced e.m.f. in the coil

=
t
(iii) The induced current will beome zero.
4
(a) There is e.m.f. induced across AB. This is
because the metal rod cuts through
=
magnetic field lines.
Magnitude of induced e.m.f. 
= 6  104 V
= Bvl = 1.8  0.03  0.1 = 5.4  103 V
The induced e.m.f. in the coil is 6  104 V.
ε
(c) I =
R
6 10 4
=
0.5
However, no current is induced because the
circuit is not complete.
(b) (i)
6 10 3  3 10 3
50
The current will flow through around
the loop ACDB.
= 1.2  103 A (anticlockwise)
The direction can be determined by
Fleming’s right-hand rule.
Practice 5.2 (p. 243)
(ii) Power dissipated by the bulb
1
C
2
C
5.4 10 
3
D
2
4
D
= 1.458  105 W
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(a) When a piece of metal moves in a magnetic
=
ε
2
R
3 2
=
field, or when it is placed in a changing
(iii) Yes. The current induced generates an
opposite magnetic force against the
magnetic field, an induced e.m.f. will be
motion of the metal rod.
ε 5.4 10 3
I= =
= 2.7  103 A
2
R
produced. This will give rise to induced
current circulating within the body of the
metal. This induced current is called eddy
current.
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 Oxford University Press 2010
4
Electricity and Magnetism
Chapter 5 Electromagnetic Induction
(b) An induction cooker uses coils of wire with
4
D
high frequency a.c. to set up a fast
5
D
changing magnetic field. The changing
6
A
magnetic field then induces large currents
7
(HKALE 2002 Paper II Q27)
in the metal cooking pot placed above. The
8
(HKALE 2002 Paper II Q28)
pot gets hot due to the heating effect of the
9
(HKCEE 2007 Paper II Q42)
current and cooks the food.
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Conventional (p. 248)
(a) A bicycle dynamo has a simpler structure,
as carbon brushes and slip rings are not
1
(a)
required. Besides, current does not have to
flow through moving contacts.
(b) This is because some of the kinetic energy
required to drive the bicycle is converted
into electrical energy by the dynamo.
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(a)
(Correct shape and direction of the field
lines.)
(1A)
(Correct separation between field lines.)
(1A)
(b) If an a.c. flows along the wire, the direction
of the magnetic field changes alternatively
(b)
(1A)
while the shape of the field lines remains
the same.
(1A)
(c) Since the coil intercepts a changing
(c)
magnetic field produced by the wire
connected to an a.c. power supply, (1A)
a current is induced in the coil by Lenz’s
2
D
3
B
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the light bulb gives out light.
(1A)
(1A)
To determine whether current is a.c. or d.c.
Multiple-choice (p. 247)
A
(1A)
(d) Any one of the following:
Revision exercise 5
1
law and
To compare the size of the a.c. flowing in
different wires.
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2
Electricity and Magnetism
Chapter 5 Electromagnetic Induction
(a) When the bar magnet spins, the magnetic
By the conservation of energy, the elastic
field surrounding the magnet changes
potential energy of elastic threads changes
alternatively.
into the kinetic energy of the spinning
(1A)
By Lenz’s law, current is induced in the
magnet, and in turn the kinetic energy of
coil to oppose the motion of the magnet
the magnet is converted into the light
and the light bulb gives out light.
energy of the light bulb.
(1A)
(1A)
The magnet will stop spinning when all
(b)
elastic potential energy of the elastic thread
is converted into light energy.
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(1A))
(a) The coil rotates in clockwise direction.
(1A)
(b)
(Correct labelled axes.)
(1A)
(Correct curve showing alternating positive
and negative voltage.)
(1A)
(Correct positions of the magnet, the poles
can be interchanged.)
(c) Any two of the following:
(1A)
(2  1A)
Use a solenoid with more turns.
Use thicker wire for the solenoid.
(Correct labelled axes.)
Use a stronger magnet.
(Correct curve showing alternating positive
Spin the magnet at a faster speed.
(d) I do not agree.
and negative voltage.)
(1A)
of the coil.)
induced in the solenoid to oppose the
(c) Any two of the following:
motion of the magnet, i.e. a force acts on
Increase the area of the coil in the magnetic
field.
(1A)
Wind the coil on a soft-iron core.
(Or
New Senior Secondary Physics at Work
(2  1A)
Increase the number of turns in the coil.
in this case and the magnet will slow down
I do not agree.
(2A)
Use a stronger magnet.
(1A)
Therefore, Newton’s first law is not valid
and stop.
(1A)
(Correct horizontal and vertical positions
When the magnet spins, a current is
the magnet by the solenoid.
(1A)
Rotate the coil at a higher speed.
(1A)
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4
4
Electricity and Magnetism
Chapter 5 Electromagnetic Induction
(a) The magnet is momentarily at rest at 0,
0.02 s, 0.04 s, 0.06 s, etc.
(Note that the p.d. due to the accumulation
(1A)
of electrons is opposite to the induced
(2  1A)
(b) Any two of the following:
e.m.f., see Example 5 on p.225.)
Move the magnet with a larger amplitude.
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(a) When the coil rotates, it cuts the magnetic
Use a stronger magnet.
field lines at different angles.
Use a solenoid with more turns.
Therefore, different sizes of current are
Use a thicker wire for the solenoid.
induced in the coil.
(c) As the magnet oscillates, an e.m.f. is
induced across the solenoid.
(1A)
(b) Every time the coil passes through the
(1A)
vertical, the commutator reverses the
When a wire is connected across the
connections of the coil with the outside
solenoid, the induced e.m.f. drives a
circuit.
current in the circuit.
(1A)
Therefore, the current in the outside circuit
The induced current produces an effect that
opposes the motion of the magnet.
always flows in the same direction. (1A)
(1A)
(c) (i)
The magnet experiences a force and stops
quickly.
(1A)
current / A
(1A)
(d) I do not agree.
(1A)
time / s
The solenoid still experiences a varying
magnetic field.
(1A)
Therefore, voltage is still induced across
the solenoid.
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(Correct shape and current in one
(a) The force on the electron is Bev.
(1A)
(b) As the rod moves, the electrons in the rod
are carried along and experience a
direction.)
(1A)
(Period equal to 0.5 s.)
(1A)
(ii) The waveform repeats twice every
magnetic force perpendicular to the
second.
magnetic field and to the direction of
Therefore the frequency of the current
motion of the rod.
in the coil is 2 Hz.
(1A)
The period is unchanged.
(1A)
The magnitude increases.
(1A)
(1A)
By Fleming’s left-hand rule, the magnetic
(d) (i)
force pushes the electrons towards the end
X and the electrons accumulate there.(1A)
(ii) The period decreases.
(c) Since the electric force balances the
The magnitude increases.
magnetic force, we have:
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Ee = Bev
V
, so:
l
V
= Bv  V = Bvl
l
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=
(1A)
0.6
2π  50  5000 10 4
= 3.82  103 T
4
(1A)
(1A)
(a) 0 = 2f  magnetic flux linkage
= 2f  NBA
ε0
B=
2 πfNA
(1M)
E = Bv
The electric field is given by
(1M)
(1M)
(1M)
(1A)
 Oxford University Press 2010
4
Electricity and Magnetism
Chapter 5 Electromagnetic Induction
(b) By 0 = 2f  NBA,
ε
sensitivity = 0
B
9
= 2fNA
(1M)
= 2  50  5000  10
4
= 157 V T1
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(a)
(1A)
(a) When he speaks in front of the microphone,
the sound waves set the diaphragm into
vibration.
(1A)
The coil will then be set into vibration by
the diaphragm
(1A)
inside a magnetic field.
(1A)
(Correct labelled axes.)
(1A)
(Correct maximum/minimum points.)(1A)
The vibration will induce a current in the
(Correct zero points.)
(1A)
(Correct shape.)
(1A)
sound signal in the coil inside the magnetic
(Correct labelled axes.)
(1A)
field,
(Correct shape.)
(1A)
coil
(1A)
which carries the sound signal. The signal
(b)
is then amplified and played through a
loudspeaker.
(b) Any two of the following:
(2  1A)
Use a stronger magnet.
Increase the number of turns in the coil.
Increase the area of the coil in the magnetic
field.
Use a diaphragm of a smaller mass.
(c) When there is a current which carries
(1A)
the coil vibrates as it experiences magnetic
10
(HKCEE 2002 Paper I Q6)
force.
(1A)
11
(HKALE 2002 Paper II Q4)
This sets the (paper cone) diaphragm into
12
(HKALE 2003 Paper I Q4)
vibration. This compresses and expands the
Physics in articles (p. 252)
surrounding air, and produces a
longitudinal sound wave.
(a) There is a current induced in the coil.
(1A)
(1A)
This phenomenon is related to Lenz’s law /
Faraday’s law of electromagnetic induction.
(1A)
New Senior Secondary Physics at Work
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4
Electricity and Magnetism
(b) The student is wrong.
Chapter 5 Electromagnetic Induction
(1A)
Increasing the number of turns in the coil
increases only the magnitude of the induced
e.m.f.
(1A)
but not the frequency.
(1A)
(c) There will be no signal produced.
New Senior Secondary Physics at Work
(1A)
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