Questions from Past Papers

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Questions from Past Papers
3. A coil is connected to a centre zero ammeter, as shown. A student drops a magnet so
that it falls vertically and completely through the coil.
(a) Describe what the student would observe on the ammeter as the magnet falls through
the coil.
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(2 marks
(b) If the coil were not present the magnet would accelerate downwards at the
acceleration due to gravity. State and explain how its acceleration in the studentís
experiment would be affected, if at all,
(i)
as it entered the coil,
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(ii) as it left the coil.
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(4 marks)
(c) Suppose the student forgot to connect the ammeter to the coil, therefore leaving the
circuit
incomplete, before carrying out the experiment. Describe and explain what difference
this would
make to your conclusions in part (b).
You may be awarded marks for the quality of written communication provided in your
answer.
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(3 marks)
4 A capacitor of capacitance 330 µF is charged to a potential difference of 9.0V. It is
then discharged through a resistor of resistance 470 kΩ.
Calculate
(a) the energy stored by the capacitor when it is fully charged,
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(2 marks)
(b) the time constant of the discharging circuit,
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(1 mark)
(c) the p.d. across the capacitor 60 s after the discharge has begun.
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(3 marks)
Monday 26 January 2004
A circular coil of diameter 140mm has 850 turns. It is placed so that its plane is
perpendicular to a horizontal magnetic field of uniform flux density 45mT, as shown
in Figure 5.
Calculate the magnetic flux passing through the coil when in this position.
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(2 marks)
(b) The coil is rotated through 90 ° about a vertical axis in a time of 120ms.
Calculate
(i)
the change of magnetic flux linkage produced by this rotation,
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(ii)
the average emf induced in the coil when it is rotated.
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(4 marks)
Wednesday 26 January 2005
A 680 µF capacitor is charged fully from a 12V battery. At time t = 0 the capacitor
begins to discharge through a resistor. When t = 25 s the energy remaining in the
capacitor is one quarter of the energy it stored at 12V.
(a) Determine the pd across the capacitor when t =25s.
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(2 marks)
(b) (i) Show that the time constant of the discharge circuit is 36 s.
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(iii)
Calculate the resistance of the resistor.
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(4 marks)
Friday 20 January 2006
3 (a) As a capacitor was charged from a 12V supply, a student used a coulomb meter and
a voltmeter to record the charge stored by the capacitor at a series of values of potential
difference across the capacitor. The student then plotted a graph of pd (on the y-axis)
against charge (on the x-axis).
(ii)
State what is represented by the gradient of the line.
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(iii)
State what is represented by the area enclosed by the line and the x-axis of the
graph.
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(3 marks)
(b) The student then connected the capacitor as shown in Figure 4 to carry out an
investigation into the discharge of the capacitor.
The student used a voltage sensor, datalogger and computer to obtain values for the pd
across the capacitor at various times during the discharge.
(i) At time t = 0, with switch S2 open, switch S1was moved from position A to position
B. Calculate the pd across the capacitor when t =26s.
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(ii) At time t = 26 s, as the discharge continued, the student closed switch S2. Calculate
the pd across the capacitor 40 s after switch S1 was moved from position A to position B.
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(iv)
Sketch a graph of pd against time for the studentís experiment described in
parts (b)(i) and (b)(ii).
Thursday 16 June 2005
4 (a) In an experiment to illustrate electromagnetic induction, a permanent magnet is
moved towards a coil, as shown in Figure 3, causing an emf to be induced across the
coil.
Using Faradayís law, explain why a larger emf would be induced in this experiment if
a stronger magnet were moved at the same speed.
You may be awarded additional marks to those shown in brackets for the quality of
written communication in your answer.
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(c) A conductor of length l is moved at a constant speed v so that it passes
perpendicularly through a uniform magnetic field of flux density B, as shown in
Figure 4.
(i) Give an expression for the area of the magnetic field swept out by the conductor
in time ∆t.
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(ii) Show that the induced emf, ∈, across the ends of the conductor is given by
∈ = Blv.
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(3 marks)
(c) A simple electrical generator can be made from a copper disc, which is rotated at right
angles to a uniform magnetic field, directed into the plane of the diagram (Figure 5). An
emf is developed across terminals P (connected to the axle) and Q
(connected to a contact on the edge of the disc).
The radius of the disc is 64mm and it is rotated at 16 revolutions per second in a
uniform magnetic field of flux density 28mT.
(i) Calculate the angular speed of the disc.
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(ii) Calculate the linear speed of the mid-point M of a radius of the disc.
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(iii) Hence, or otherwise, calculate the emf induced across terminals P and Q.
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(5 marks)
Thursday 15 June 2006
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