Download Test - Yu Chun Keung Memorial College No. 2

P.1/4/11/T11/
Yu Chun Keung Memorial College No.2
F.7 Physics Test (2)
2005-2006
Time allowed: 1.5 hrs.
Answer ALL Questions.
1.
1980IIB8
In Figure 10 the primary circuit consists of alternating voltage in series with a long solenoid. The
secondary circuit consists of a few turns of wire wound round the solenoid as shown and
connected to the Y-pates of a cathode ray-oscilloscope. The wave-form produced on the screen is
shown in Figure 11.
Figure 11
Figure 10
(a) The voltage source is now adjusted at constant frequency until half the original current
flows in the primary. In the space below copy the original waveform shown above and then draw
the new waveform. (Assume the triggering remains the same.)
Explain your reasoning.
(b) The voltage source is now adjusted so that the frequency is one half of its original value and
the current is restored to its original value. In the space below copy the original waveform and
then draw the new waveform.
(c) Returning to the initial conditions of operation of the circuit prior to (a) and considering that
the long solenoid acts purely as an inductance (i.e., of negligible resistance), determine the effect
on the waveform of reducing the frequency by half, if the source voltage is kept constant . Draw
below the original waveform and the new waveform.
Explaining your reasoning.
1
1982IIB7
2.
S
Current / A
10
R
200 V
M
motor
armature
resistance
=5
5
A
0
2
Figure 6
4
6
Time / s
Figure 7
A d.c. source of 200 V supplies power to a circuit containing a motor with armature resistance 5
, a switch S, a rheostat R and an ammeter A, all connected in series. After the switch S is
closed, the reading on the ammeter changes with time as shown by the curve in Figure 7.
(a)
(b)
(i)
(ii)
(iii)
Calculate the resistance of the rheostat R.
After the motor has reached constant speed, calculate
the back e.m.f. of the motor,
the mechanical power output of the motor, and
the power dissipated as heat in the armature.
1987IIB5
3. A student wishes to construct a simple microammeter to measure small direct electric currents. A
horse-shoe magnet with curved pole pieces is available and a light framework, or former, upon
which to wind a coil. In addition there is a choice of two fibres, A and B, to suspend the coil, and
a choice of two insulated conducting wires, C and D, from which to wind the coil.
The fibres A and B are of equal length but A can reliably support double the load of B. The
restoring torque provided by A is double that of B for the same angle of twist.
There is an effectively unlimited supply of wires C and D, which are made from the same
material. The area of cross-section of wire C is double that of wire D.
(a) The student tries to compare the four possible combinations of wires and fibres and drawn
up the following table :
2
maximum
corresponding
Combination of fibre and wire
no. of turns
internal
torsional constant
current
Fibre
in coil
resistance
of suspension
sensitivity
Wire
of coil
A
C
N
R
c
s
A
D
_____
_____
_____
_____
B
C
_____
_____
_____
_____
B
D
_____
_____
_____
_____
(i) Copy the above table in your answer book and fill in the blanks in it.
(ii) Given that the resistance of the suspension fibres is negligible, use the completed table to
decide which of the four choices is the best one. Explain your answer briefly.
(b) Draw a sketch, in the space provided, showing the internal resistance of the microammeter.
(c) List the features that would be necessary to ensure that the deflection is proportional to the
current.
1986IIB5
4.
X
search coil
R
to CRO
a.c.
source
r
A
Y
Figure 5
3
A student fixes a straight wire XY, about 1 m long, on a table and connects it in series with an a.c.
power supply, a rheostat and an ammeter, as shown in Figure 5. He also connects a search coil to
a CRO. Starting at a position near the middle of the straight wire, he places the search coil flat on
the table at various distances r from the wire and measures the corresponding peak-to-peak
voltages V on the CRO.
__
1 /V -1
V
1 000
900
800
700
600
500
400
300
200
100
0
0.1
0.2
0.3
0.4
0.5
r /m
Figure 6
(a) The student plots 1/V against r, as shown in Figure 6.
(i) It is found that the first six data points lie on a straight line. What experimental conclusion
can you draw about the variation of the magnetic flux density due to the alternating current at
various distances from the wire ?
(ii) Explain why the data points for > 0.3 m depart from the fitted straight line.
(b)
(i) In this experiment, the variation of current with time t in the long straight wire
is Io sin(2ft ) , where Io is the peak current and f the a.c. frequency. If the search coil is of area A
and consists of N turns, find an expression for the peak-to-peak voltage V induced in the search
coil when r  0.3 m.
(ii) If Io = 14.1 A, f = 50 Hz, A = 3.14  10-4 m2 and o = 1.26  10-6 H m-1, estimate the
number of turns in the search coil.
(c) Another student argues that the results obtained in this experiment are not accurate because
the influence of the earth's magnetic field has not been considered. Is this argument correct ?
Give a brief explanation.
***** End of Test *****
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