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Extra Questions for EM Ch1 to 4
3.
Figure 3.1 shows the electric plug of a washing machine. X, Y and Z are the wires connected to
the mains and P is a small device fixed at wire X (see Figure 3.2).
(a) Which wire(s) should be connected to
(i) the metal case, and
(ii) the electric motor
(2 marks)
of the washing machine?
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(b) Explain the function of P and why it is fixed at X but not at Y.
(3 marks)
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(c) The pin connected to Z is thicker and longer than the other two pins. Give TWO reasons for
such a design.
(2 marks)
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5.
An electron moving with a speed of 6 × 107 m s-1 enters a magnetic field ABCD and is deflected as
shown.
(a) What is the direction of the magnetic field?
(1 mark)
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(b) Calculate the magnitude of the magnetic field.
(2 marks)
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(c) How long does the electron stay in the magnetic field?
(2 marks)
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(d) An electric field is applied so that the electron moves without deflection. Determine the magnitude
and direction of the electric field.
(2 marks)
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6.
A simple motor consists of a flat rectangular coil with n turns in a magnetic field B as shown.
The coil has an area of 0.01 m2 and carries a current of 1 A. The motor drives a pulley of diameter
20 cm, and weights can be hung from either side of the pulley at point P or point Q.
(a) In order to prevent rotation, should a weight be hung at P or at Q?
(1 mark)
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(b) What is the magnitude of the torque provided by a mass of 0.2 kg suspended from either P or Q?
(2 marks)
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(c) If the motor is just stopped by a mass of 0.2 kg, how many turns does the coil have? (2 marks)
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7. (a) The circuits in Figure 7.1 each contains two resistors connected in series with a 6 V battery of
negligible internal resistance. The resistors in circuit I are 10 k each while those in circuit II are
100  each.
A voltmeter of internal resistance RV = 10 k is used to measure the potential difference across one of
the resistors as shown.
(i) What would be the respective voltmeter readings?
(3 marks)
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(ii) In fact, the potential difference across each resistor before connecting the voltmeter is 3 V in both
circuits. Explain why this voltmeter gives a relatively inaccurate value for circuit I. Hence state the
general principle of selecting a suitable voltmeter for such measurement.
(2 marks)
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(b) Circuit III shows a possible method for measuring resistance using a voltmeter and an ammeter. The
internal resistances of the voltmeter and the ammeter are Rv and RA respectively and their readings
V
Vm and Im give the measured resistance Rm  m . The true resistance value of the resistor is R.
Im
(i) State which reading(s), Vm, Im or both, do(es) NOT give the true voltage across the resistor and/or
the true current passing through the resistor. Hence write down an equation relating RA, Rm and R.
(2 marks)
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(ii) Find the percentage error associated with Rm when measuring the resistance of this resistor.
Given: Rv =10 k, RA = 1 and R = 10 .
(2 marks)
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Answers
3. (a) (i) Z
(1A)
(ii) X and Y
(1A)
(b) Ｐ will be blown when a larger current passes through it.
(1A)
Thus P protects the appliance from further damage. (1A)
It is connected toＸ because Ｘis at a high voltage. By cutting this connection，the damaged
appliance is not live anymore.(1A)
(c) The appliance is earthed before being connected to a live wire / to open the shutters of neutral
and live holes. (1A)
The plug cannot be inserted in wrong way.
(1A)
5. (a) magnetic field pointing into paper.(1A)
(b) qvB = mv 2 / r
B = mv / qr
(1M)
= 9.11x10-31 x 6 x 107 (1.6 x 10-19 x 0.1)
= 3.42x10-3 T
(1A)
(c) t 
period 1 2 2r
2  0.1
 


4
4 
4v 4  6  10 7
(1M)
= 2.6210-9 s
(1A)
(d) electric field downward.
(1A)
qvB = qE
E = 6x1073.42 x10-3
= 2.05 x105 N C-1
6. (i) P
(1A)
(1A)
Torque =mg r
(1M)
= 0.2x9.81x0.1
(ii)
= 0.1962
= 0.196 Nm
(1A)
Torque  = NBAI
(1M)
0.1962 = N x 0.05 x 0.01 x 1
N = 392.4 = 392 turns
(1A)
(iii)
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7.(a) (i) R = 10 k (circuit I)
(ii) Resistance of circuit / that part of circuit would be lowered / altered significantly when
introducing the voltmeter (i.e. loading effect). Or , the resistance of the voltmeter
is comparable to the resistance of resistor R being mearsured. (1A)
Resistance of voltmeter should be much higher than the resistance of the part of the circuit
under study. (1A)
(b) (i) Vm does NOT give the true voltage for the resistor. (1A)
Rm =RA+R
(1A)
(ii) Rm =RA+R
= 1+10=11
Percentage error = 100%
= 10%
(1A)
(1M)
8 / 19