J2 H2 Physics 2008 prelim questions from top junior colleges(JCs). Topic is Alternating current.

1. 1. In an a.c. generator, a coil turns at a steady frequency of 20 Hz in a uniform magnetic field of 6.0 x 10
-2
T. This produces a
sinusoidal p.d. with a peak output of 2.0 V across its output terminals. If the frequency of rotation of the coil is increased to
30 Hz and the magnetic flux density is reduced to 5.0 x 10
-2
T, the peak output p.d. will be
A 0.8 V B 1.6 V C 2.5 V D 3.0 V
RJC 08P1Q32
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2. A 2.0 Ω resistor is connected to an a.c. supply. An oscilloscope records a peak voltage of 10 V when connected across
the resistor. What is the r.m.s. current flowing through the resistor?
A 3.5 A B 5.0 A C 7.1 A D 14 A
RJC 08P1Q33
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3. The diagram shows an a.c. supply connected to four diodes 1, 2, 3 and 4 and a resistor R.
If diode 1 is removed, leaving a break in the circuit, which of the following traces would be seen on an oscilloscope
connected across R?
RJC 08P1Q34
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4. The function of a transformer is to convert
A an alternating voltage to a direct voltage.
B a direct voltage to an alternating voltage.
C one direct voltage to another direct voltage of a different magnitude.
D one alternating voltage to another alternating voltage of a different magnitude.
SAJC 08P1Q31
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5. Which one of the following statement about an alternating current or voltage is true?
A A source labeled “6 V AC” supply means that it has an alternating current with a peak voltage of 6V.
B An ammeter registers the mean current value of an alternating current.
C A “6 V AC, 220 Hz”, a “6 V AC, 240 Hz” and a “6 V DC” provides the same power supply to the same resistor.
D None of the above
SRJC 08P1Q34
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2. 6. An alternating current I/A varies with time t/s according to the equation I = 5 sin (100πt). What is the mean power
developed by the current in a resistive load of 10 Ω?
A 125 W B 160 W C 250 W D 500 W
PJC 08P1Q30
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7. The voltage V of an a.c. source varying with time t as shown in the figure is applied across a resistor of resistance 2.0 Ω.
The r.m.s. current, Irms through the resistor is
A 1.1 A B 1.4 A C 2.0 A D 2.2 A
CJC 08P1Q29
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8. In the circuit below, the potential at A is positive with respect to B in the first half of the cycle. The waveform across the
diode shown on the screen of a cathode ray oscilloscope (c.r.o.) should be
AJC 08P1Q33
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9. The diagram below shows a varying voltage signal.
What is the root mean square value of this signal?
A 2.00 mV B 2.24 mV C 3.16 mV D 4.00 mV
ACJC 08P1Q33
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3. 10. Electric Power P is transmitted from the power station to a substation via a pair of transmission cables of total resistance R
after being stepped up to a potential difference of V. The percentage power loss due to the cables is given by
ACJC 08P1Q34
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11. (a) A hair blower used in Singapore is rated at 240 V r.m.s., 1000 W. A student plans to bring the blower to the United
States of America (USA), where the voltage is . 120 V r.m.s.
(i) Explain why the current from the mains is alternating current rather than direct current. [2]
(ii) It was suggested that the student needs to bring a transformer along to USA in order to operate the blower.
Determine the transformer’s turns ratio S
P
N
N
. [1]
(iii) Determine the r.m.s. current which the transformer will draw from the USA outlet when the blower is operating at
1000 W. State any assumption you have made in your working. [3]
(b) Electricity is transmitted along the national grid using a combination of overhead and underground cables.
Suggest practical reasons why
(i) overhead cables are used outside cities. [1]
(ii) underground cables are used in cities. [1]
RJC 08P3Q4
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