Exam examples for DC Circuits

1. DC Circuits
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1. A bulb rated 110 V, 60 watts is connected with another bulb rated 110 V, 100 W across a 220 V
mains. Calculate the resistance which should be joined in parallel with the first bulb so that
both the bulbs may take their rated power.
A. 302.5 Ω
B. 302 Ω
C. 303 Ω
D. 301.5 Ω
2. Two coils connected in parallel across 100 V supply mains, take 10 A from the line. The power
dissipated in one coil is 600 W. What is the resistance of the other coil?
A. 24 Ω
B. 26 Ω
C. 25 Ω
D. 27 Ω
3. Two resistors have a resistance of 25 Ω when connected in series and 6 Ω when connected in
parallel. Calculate their resistances.
A. 20 Ω, 5 Ω
B. 18 Ω, 7 Ω
C. 13 Ω, 12 Ω
D. 10 Ω, 15 Ω
4. Eight cells are connected in series to an external resistance of 4 Ω but one of the cells is
connected in opposition. If the emf and internal resistance/cell are 2 volts and 3 Ω, respectively,
find the current in the 4 Ω resistor.
A. 3/7 A
B. 6/7 A
C. 7/3 A
D. 7/6 A
5. A shunt is to be used with a galvanometer of resistance 20 Ω so that only 1% of the line current
passes through the galvanometer. Find the value of shunt resistance.
A. 20/101 Ω
B. 10/99 Ω
C. 20/99 Ω
D. 10/101 Ω

2. DC Circuits
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6. Twenty tones of water are heated in 12 hours. If water Is to be heated from 25 deg C to 95 deg
C, find the KW rating of the heater if its efficiency is 92%.
A. 50 KW
B. 175 KW
C. 100 KW
D. 150 KW
7. An isolated capacitor of unknown capacitance has been charged to a potential difference of 100
v. When charged capacitor is then connected in parallel to an uncharged 10-microfarad
capacitor, the potential difference is 30 v. Find the unknown capacitance in microfarad.
A. 6.67
B. 3.33
C. 4.29
D. 5.38
8. A 10-microfarad capacitor is charged by a constant current source and its voltage is increased to
2 volts in 5 seconds. Calculate the value of the constant current source in microamperes.
A. 5
B. 6
C. 4
D. 3
9. What extends the range of a dc ammeter?
A. Inductance
B. Capacitor
C. Current transformer
D. Shunt resistance
10. To increase the range of a voltmeter, a ___________.
A. Low resistance is connected in series
B. Low resistance is connected in parallel
C. High R is connected in series
D. High R is connected in parallel
11. Two meters A and B require 50 mA and 60 mA, respectively, to give full scale deflection. Which
is more sensitive?
A. A
B. B
C. A and B
D. Cannot be determined

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12. A 0-10 ampere-meter has an internal resistance of 0.01 Ω. What shunt resistance to the
ammeter must be connected to have a reading of 0-100?
A. 1.11 x 10-3
Ω
B. 1.0 x 10-3
Ω
C. 0.111 Ω
D. 0.1 Ω
13. A DC 0-50 millivoltmeter is used to measure 5 amperes current by means of a shunt. The meter
was tested and found to have a terminal resistance of 4.17 Ω. Determine the resistance to be
connected to indicate a full-scale deflection of 5 A.
A. 0.01 Ω
B. 0.042 Ω
C. 1735 Ω
D. 413 Ω
14. A 75-volt voltmeter has a resistance of 7,500 Ω. What will be the multiplying factor of the
instrument if a resistance of 7,500 Ω is connected in series?
A. 10
B. 6
C. 4
D. 2
15. An ampere-hour meter is calibrated to give a reading of energy in kilowatt-hour on a 220 v
circuit. When used on a 250 v circuit, it records 845 KWHR in a certain time. Calculate the
energy actually supplied.
A. 920
B. 960
C. 950
D. 955
16. To test the accuracy of a kilowatt-hour meter, an electrician used a wattmeter. He found out
that the meter makes 5 revolutions in 18 seconds. The meter constant is 3.2. What is the
percentage error if the wattmeter reading is 3368?
A. 8% slower
B. 8% faster
C. 5% slower
D. 5% faster

4. DC Circuits
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17. An 800 W electric iron operates on 110 V. How many electrons pass the iron per second?
A. 4.54 x 1019
B. 6.4 x 109
C. 70.4 x 106
D. 3.25 x 1019
18. Two equally charged spheres repel each other with a force of 0.1 kg. If their centers are 20 cm
apart. Find the charge on each.
A. 2.09 x 10-6
coul
B. 2.09 x 106
coul
C. 2.00 x 10-6
coul
D. 2.00 x 106
coul
19. How many electrons must be placed on each of two spheres separated by 4 mm in order to
produce a repulsive force of 500 N?
A. 5.89 x 1012
B. 5.27 x 1012
C. 6.42 x 1012
D. 6.97 x 1012
20. A charge of 20 units is placed 5 cm from a like charge of 3 units in mica. The force of repulsion is
0.52 dynes. Find the electric constant of mica.
A. 2.4
B. 4.61
C. 6.6
D. 1.25
21. An electric dipole consists of two pole charges, 12 nC, placed 10 cm apart. Determine the
potential at a point in the middle of the electric dipole.
A. 0 V
B. 2160 V
C. 4320 V
D. 1080 V
22. Two metallic plates separated by 1 cm are connected across a 12-V battery. A certain plastic
material is inserted completely filling the space between the plates and the charge on each plate
is observed to double. What is the dielectric constant of the plastic material?
A. 0.5
B. 4
C. 2
D. 6

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23. A two parallel-plate capacitor charged to 15 KV has a capacitance of 4.5 x 10-4
µF. If the plates
of a capacitor 200 square centimeter each are separated by a distance of 1 mm, what is the
permittivity of the dielectric medium?
A. 2.8
B. 2.65
C. 2.55
D. 2.54
24. A multi-plate capacitor is made up of 501 sheets of aluminum 25 cm x 30 cm with an insulation
of paraffin paper 0.0015 cm thick. What is the capacitance in microfarads of the capacitor? (k
for paraffin paper is 3).
A. 50
B. 55
C. 35
D. 67
25. Two capacitors C1 = 50 µF and C2 = 30 µF are connected in series. Find the equivalent
capacitance in microfarad.
A. 187.5
B. 18.75
C. 1.875
D. 1875
26. If the charge accumulated on two parallel conductors charged to 4 V is 500 pC, what is the
capacitance of parallel conductors?
A. 1,000 pF
B. 2,000 pF
C. 250 pF
D. 125 pF
27. A certain electric generating station has an efficiency of 25%. If coal consumption is 4500 kg/hr
and the calorific value of coal is 6,000 kcal/kg, find the total electric energy produced per day.
A. 188,200 kWh
B. 188,400 kWh
C. 188,300 kWh
D. 188,500 kWh

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28. A 3-tonne electric motor operated vehicle is being is being driven at a speed of 6 m/s upon an
incline of 1 in 20. The tractive resistance may be taken as 25 kg/tone. Assuming a motor
efficiency of 85% and the mechanical efficiency between the motor and the road wheels of 80%,
calculate the output of the motor and the current taken by the motor if it gets power from a 220
V source.
A. 16,515 W, 88.36 A
B. 16,535 W, 89.36 A
C. 16,525 W, 88.36 A
D. 16,500 W, 87.36 A
29. An electric kettle was marked 500 W, 230 V and was found to take 15 minute to bring one kg of
water at 15 deg C to the boiling point. Determine the heat efficiency of the kettle.
A. 79%
B. 80%
C. 78%
D. 81%
30. If a 220 V heater is used on a 110 V supply, Heat produced by it will be ___________ as much.
A. One-half
B. Twice
C. One-fourth
D. Four times
31. The electric energy required to raise the temperature of a given amount of water is 1000 kWh.
If heat losses are 25%, the total heat energy required is _____________ kWh.
A. 1500
B. 1250
C. 1333
D. 1000
32. A 110 V electric motor drives a rope brake which is cooled by a steam of water. Eighty-five
percent of generated heat raises 2767 kg of water/hour through 10 deg C. Find the motor
output in kW and its efficiency if it is taking 400 A.
A. 38 kW, 86.4%
B. 38 kW, 84.3%
C. 37 kW, 84.5%
D. 39 kW, 84%

7. DC Circuits
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33. Two equally charged spheres repel each other with a force of 0.1 kg. If their centers are 20 cm
apart, find the charge on each.
A. 2.9 x 10-6
C
B. 2.09 x 10-5
C
C. 2 x 10-6
C
D. 2.09 x 10-6
C
34. Find the radius of an isolated sphere capable to being charged to 1 million volt potential before
sparking into the air. Given the breakdown voltage of air is 30 kV/cm.
A. 0.33 m
B. 0.44 m
C. 0.55 m
D. 0.22 m
35. The total capacitance of two capacitors is 0.03 µF when joined in series and 0.16 µF when
connected in parallel. Find the capacitance of each capacitor.
A. 0.10 µF, 0.04 µF
B. 0.12 µF, 0.04 µF
C. 15 µF, 0.03 µF
D. 0.12 µF, 0.05 µF
36. A plate 2 µF capacitor charged to 100 V is connected by a fine wire to a plate of a 0.01 µF
capacitor charged to 200 V, the other plates of the two capacitors being joined together and
connected to earth. How much charge passes along the wire and how much energy disappears?
A. 1.96 x 10-3
J
B. 1.76 x 10-5
J
C. 1.786 x 10-4
J
D. 1.796 x 10-4
J
37. Calculate the energy stored in a parallel-plate capacitor which consists of two metal plates, each
60 cm2
separated by a dielectric 1.5 mm thick and of relative permittivity 3.5 if a p.d. of 1000 V is
applied across it.
A. 62 µJ
B. 61 µJ
C. 63 µJ
D. 60 µJ

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38. A capacitor of 8 µF capacitance is connected to a direct current source through a resistance of 1
MΩ. Calculate the time taken by the capacitor to receive 75% of its final charge.
A. 11.80 sec
B. 11.18 sec
C. 11.08 sec
D. 11.25 sec
39. An iron ring, 100 cm mean circumference is made from an iron rod of round cross-section 10
cm2
. It is wound with 200 turns. What current would be necessary to produce a flux of 1 mWb.
Relative permeability of iron = 500.
A. 8.96 A
B. 7.96 A
C. 6.96 A
D. 9.96 A
40. A coil of resistance 100 Ω is placed in a magnetic field of 1 mWb. The coil has 100 turns and a
galvanometer of 400 Ω resistance is connected in series with it. Find the average emf and the
current if the coil is moved in 0.1 second from the given field to one of strength 0.2 mWb.
A. 1 V, 1.5 mA
B. 1.8 V, 2.6 mA
C. 0.8 V, 1.6 mA
D. 1.08 V, 1.06 mA
41. A conductor 10 cm long and carrying a current of 50 A lies perpendicular to a field strength 1000
AT/m. Calculate the force acting on the conductor.
A. 4π x 10-3
N
B. 5π x 10-3
N
C. 6π x 10-3
N
D. 2π x 10-3
N
42. A soft iron ring is wound with a coil of 600 turns. When the current through the coil is
decreased from 12 to 7 A, the flux changes from 965 to 960 µWb. Calculate the self inductance
of the coil.
A. 0.5 mH
B. 0.6 mH
C. 0.7 mH
D. 0.8 mH

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43. An air-cored toroidal coil has 450 turns, a mean diameter of 30 cm and a cross-sectional area of
3 cm2
. Calculate the average emf induced if a current of 2 A is reversed in 0.04 second.
A. 8.0 mV
B. 8.4 mV
C. 9.1 mV
D. 8.1 mV
44. Two identical coils X and Y of 1000 turns each lie in parallel planes such that 80% of flux
produced by one coil links with the other. If a current of 5 A flowing in X produces a flux of 0.05
Wb in it, find the mutual inductance between X and Y.
A. 9 x 10-2
H
B. 10 x 10-3
H
C. 8 x 10-3
H
D. 9 x 10-4
H
45. A wooden ring has mean length of magnetic path of 100 cm and a cross-section of 10 cm2
. A coil
of 2000 turns of wire is wound over it uniformly. Closely wound over this is another coil of 4000
turns. The terminals of each coil are brought out separately. What is the inductance of the first
coil alone?
A. 5.15 H
B. 5.50 H
C. 5.03 H
D. 5.08 H
46. In CGS system, what is the unit of emf when I is in abampere nad P is in erg per second?
A. Millivolt
B. Kilovolt
C. Abvolt
D. Volt
47. Two 10 Ω parallel resistors are connected in series with a 5 Ω resistor. The combination is then
connected across a 24 volts battery. Find the voltage drop across the 5 Ω resistor.
A. 6 V
B. 18 V
C. 12 V
D. 20 V

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48. One KWHR is equal to __________ BTU.
A. 3412
B. 4186
C. 2520
D. 3930
49. A certain generator generates 1,500,000 joules per minute. What is the output in kW?
A. 50
B. 500
C. 25
D. 125
50. Which of the following is a bilateral circuit?
A. Vacuum tube rectifier
B. Diode
C. Transmission line
D. Transistor