Electrical work, power and energy for Electrical engineering PDF

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Electrical work, power, and energy study materials for electrical engineering students. It helps competitive exams. This is multiple choice questions and answers...

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b) 200 W bulb will be fused c) both bulb will be fused

1. A kilowatt hour is the unit of a) energy b) power c) electric charge d) electric current 2. Heat produced in a conductor depends a) inversely on time b) directly on time c) both a and b d) none of the above 3. If potential difference across a conductor of a material of resistivity ρ remains constant, then heat produced in the conductor is proportional to a) ρ

b) ρ2

c)

1 √ρ

d)

𝟏 𝛒

4. One kilowatt hour is equal to a) 36 × 103 J b)103 J c) 𝟑𝟔 × 𝟏𝟎𝟓 𝐉 d) 105 J 5. The heater element in an electric iron is made of a) nichrome b) iron c) tungsten d) constantan 6. The heat produced (in kcals) in a resistance R ohms when a current I amperes flows through it for t seconds is given by a) 𝐼 2 𝑅 𝑡

b)

c) 𝐼 2 𝑅 𝑡 × 4.2

d)

𝑰𝟐 𝑹 𝒕 𝟒.𝟐×𝟏𝟎𝟑 𝐼𝑅𝑡 𝐽

7. Three electric bulbs 40 W, 60 W and 100 W are designed to work on 220 V mains. Which bulb will burn most brightly if they are connected in series across 220 V mains? a) 60 W b) 40 W c) 100 W d) All bulbs will burn quality brightly 8. Two electric bulbs have tungsten filament of the same length. If one of them gives 60 W and the other 100 W, then, a) 100 W bulb has thicker filament b) 60 W bulb has thicker filament c) both filaments are of the same thickness d) information insufficient 9. Two electric lamps of 40 W each are connected in parallel. The power consumed by the combination is a) 20 W b) 60 W c) 80 W d) 100 W 10. The time required for 1 kW heater to raise the temperature of 10 litres of water through 10⁰C is a) 210 sec b) 20 W c) 160 W d) 40 W 12. Two bulbs, 100 W, 250 V and 200 W, 250 V, are connected in parallel across a 500 V line. Then, a) 100 W bulb will be fused b) 200 W bulb will be fused c) both bulb will be fused d) no bulb will be fused 13. Two bulbs, 100 W, 250 V and 200 W, 250 V, are connected in series to a 500 V line. Then, a) 100 W bulb will be fused

d) no bulb will be fused 15. You are given the following electrical appliances i) 1 kW, 250 V electric heater ii) 1 kW, 250 V electric kettle iii) 1 kW, 250 V electric bulb which of these has the highest resistance? a) heater b) kettle c) bulb d) all have equal resistances 16. If current in an electric bulb drops by 2%, then power decreases by a) 2% b) 4% c) 1% d) 16% 17. Two bulbs of 500 W and 300 W are manufactured to operate on 220 V line. The ratio resistance of 500 W bulb to heat of 300 W bulb is a) 5 : 3 b) 9 : 25 c) 3 : 5 d) 25 : 3 18. The same mass of copper is drawn into two wires 1 mm thick and 3 mm thick. Two wires are connected in series and current is passed. Heat produced in the wires is in the ratio a) 81 : 1 b) 1 : 81 c) 3 : 1 d) 9 : 1 19. The power of a heater is 500 W at 800⁰C. What will be its power at 200⁰C? Temperature coefficient o resistance is 4 × 10−4 /⁰C a) 484 W b) 672 W c) 526 W d) 620 W 20. A constant voltage is applied between two ends a uniform metallic wire. Some heat is developed. If both length and radius of wire are halved, the heat developed in the same duration is a) half b) twice c) one-fourth d) same 21. The heat produced in a conductor of resistance 4.2 Ω with 10 A flowing in it is a) 250 W b) 500 W c) 200 W d) 400 W 23. Two electric bulbs marked 500 W. 220 V are put in series with 110 V line. The power dissipated in each bulb is a)

25 4

W

b)

225 4

W

c)

325 4

W

d)

𝟏𝟐𝟓 𝟒

W

24. An electric fan and a heater are marked as 100 W, 220 V and 1000 W, 220 V respectively. The resistance of heater is a) zero b) greater than that of fan c) less than that of fan d) equal to that of fan 25. Two resistances connected in a parallel across a cell of negligible internal resistance, use 4 times the power that they would use, when in series across the same cell. If one of the resistances is 10 Ω, the resistance of the other is a) 20 Ω b) 10 Ω c) 40 Ω d) 5 Ω 27. How much energy is kilowatt-hour is consumed in operating ten 50 W bulbs for 10 hours per day in the month of June?

a) 1500 b) 15000 c) 15 d) 150 28. The water in an electric kettle begins to boil in 15 minutes after being switched on. Using the same main supply, should the length of wire used as heating element be increased or decreased if water is to be boiled in 10 minutes? a) increased b) decreased c) unchanged d) none of the above 29. A constant voltage is applied between the two ends of a metallic wire of uniform area of cross-section. Some heat is developed. The heat is doubled if a) both length and radius are halved b) both length and radius are doubled c) the radius of wire is doubled d) the length of wire id doubled 30. A uniform wire when connected directly across a 200 V line produces heat H per second. If the wire is divided into n equal parts and all the parts are connected in parallel across 200 V line, the heat produced per second is a) 𝐻 b) 𝑛𝐻 c) 𝒏𝟐 𝑯 d) 𝐻/𝑛 2 31. Three equal resistors in series across a source of emf together dissipate 10 W of power. If the same resistors are connected in parallel across the same emf source, the power dissipated will be a) 10 W b) 30 W c) 90 W d) 10/3 W 32. Two flash light electric incandescent lamps require 30 A each at 1.5 V. If they are placed in series and connected to a 6 V source, there must be connected in series with then a resistor of a) 0.1 Ω b) 12 Ω c) 6 Ω d) 3 Ω 33. A 100 W bulb will give heat and light energy of a) 100 J/s b) 200 J/s c) 50 J/s d) 150 J/s 34. Electric supply is rated at 220 V. In a house 11 bulbs of power rating 100 W each are used. The rating of the fuse should be a) 0.5 A b) 0.1 A c) 1 A d) 5 A 35. What is immaterial for a fuse? a) is resistivity b) is radius c) its length d) none of the above 36. By what percentage will the incandescence of a lamp decrease if the current drops by 2%? a) 1% b) 4% c) 2% d) 8% 37. Appliances based on heating effect of current work on a) dc only b) ac only c) both dc and ac d) none of the above 38. Two coils connected in parallel across 100 V supply take 10 A from the lien. The power dissipated in one coil is 600 W. The resistance of the other is a) 25 Ω b) 250 Ω c) 125 Ω d) 12.5 Ω

39. A 3⁰C rise of temperature is observed in a conductor by passing a certain amount of current. If current through the conductor is doubled, the rise of temperature will be a) 15⁰C b) 12⁰C c) 9⁰C d) 3⁰C 40. A house is served by a 220 V line. In a circuit protected by a fuse marked 9 A, the maximum number of 60 W lamps in parallel that can be turned on is a) 33 b) 44 c) 22 d) 20 41. A fuse wire is made of a) copper b) tungsten c) lead-tin alloy d) nichrome 42. A resistor 𝑅1 dissipated the power P when connected to a certain generator. If a resistor 𝑅2 is put in series with 𝑅1 , then power dissipated by 𝑅1 a) increases b) decreases c) remains the same d) any of above depending upon the relative values of 𝑅1 and 𝑅2 43. How many calories of heat will approximately be developed in 210 W electric bulb in 5 minutes? a) 15000 b) 1050 c) 63000 d) 8000 44. A tap supplies water at 22⁰C. A man takes 1 litre of water per minute at 37⁰C from the geyser. The power of the geyser is a) 2100 W b) 1575 W c) 525 W d) 1050 W 45. Fuse wire is a wire of a) high resistance and low melting point b) high resistance and high melting point c) low resistance and low melting point d) low resistance and high melting point 46. Lamps used for house lighting are connected in a) series b) parallel c) mixed grouping d) arbitrary manner 47. Two head lamps of a car are in parallel. They together consume 48 W with the help of a 6 V battery. The resistance of each bulb is a) 2/3 Ω b) 3 Ω c) 4 Ω d) 1.5 Ω 49. Two electric bulbs whose resistances are in the ratio 1 : 2 are connected in parallel to a constant voltage source. The powers dissipated by them are in the ratio a) 1 : 2 b) 1 : 1 c) 2 : 1 d) 4 : 1 50. A 40 W tube light is in parallel with a room heater on a main line. What happens when light is switched off? a) The heater output remains the same b) The heater output is larger c) The heater output is smaller d) None of the above 51. battery supplied 150 W and 196 W power to two resistors of 6 Ω and 4 Ω when they are connected separately to its. The internal resistance of the battery is

a) 0.5 Ω b) 1 Ω c) 1.5 Ω d) 2 Ω 53. Two identical heaters, each marked 1000 W, 250 V are placed are placed in series and then connected to a 250 V supply. Their combined rate of heating is a) 2000 W b) 250 W c) 1000 W d) 500 W 54. An electric kettle boils 1 kg of water in time 𝑡1 and another kettle boils the same amount of water in time 𝑡2 . When they are joined in series, the time 𝑡 required to boil 1 kg of water is a) 𝒕𝟏 + 𝒕𝟐

b)

𝑡 1 𝑡2 𝑡1 +𝑡2

c)

2𝑡1 𝑡2 𝑡1 +𝑡2

d)

𝑡1+𝑡2 2

56. A voltage drop of 10 V develops across a 1 kΩ resistors. The power dissipated in the resistor is a) 1000 W b) 100 W c) 1 W d) 0.1 W 57. A 24 V battery of internal resistance 4 Ω is connected to a variable resistor. At what value of current drawn from the battery is the heat produced in the resistor maximum? a) 1 A b) 1.5 A c) 3 A d) 2.5 A 58. A cell sends current through a resistance 𝑅1 for time 𝑡. Next the same cell sends current through another resistance 𝑅2 for the same time 𝑡. If the same heat is developed in both the cases, then internal resistance of the coil is a)

𝑅1 + 𝑅2 2

b)

𝑅1 − 𝑅2 2

c) √𝑹𝟏 𝑹𝟐

d)

√𝑅1 𝑅2 2

60. You are given resistance wire of length 50 cm and a battery of negligible internal resistance. In which case the maximum amount of heat is generated? a) when the wire is connected directly across the battery b) when the wire is divided into two equal parts and both parts are connected in parallel across the battery c) when the wire is divided into four equal parts and all the four parts are connected in parallel across the battery d) when only one-half of the wire is connected across the battery 61. Two electrical appliances are connected in parallel to a constant voltage supply. If current in one appliance is less than the second appliance by 1%, then power of the first appliance will be less by a) 1% b) 2% c) 4% d) 5% 62. If 𝑅1 and 𝑅2 are respectively the filament resistances of 200 W bulb and 100 W bulb designed to operate at the same voltage, then, b) 𝑅2 = 4 𝑅1 a) 𝑅1 = 2 𝑅2 c) 𝑅1 = 4 𝑅2 d) 𝑹𝟐 = 𝟐 𝑹𝟏 63. A generator produces 100 k W of power at a potential difference of 10 k V. The power is transmitted through cables of total resistance 5 Ω. What is power loss in cables? a) 1000 W b) 500 W c) 250 W d) 50 W

64. A constant voltage is applied across the end of a conductor. The heat produced is a) directly proportional to length b) inversely proportional to length c) directly proportional to square of length d) none of the above 65. An electric kettle of 2000W contains 1 kg of water at 20⁰C. Calculate the time required to heat water to its boiling point. The water equivalent of kettle is 0.5 kg and J = 4.2 joules/cal. a) 4.2 m b) 4.2 sec c) 2.1 mm d) 2.1 sec 66. Atomic power station of Tarapore has a generating capacity of 200 MW. The energy generated in a day by this station is a) 1.6 × 103J b) 2300 J c) 16 × 107 J d) 𝟏𝟕𝟐𝟖 × 𝟏𝟎𝟏𝟎 J 67. A resistance coil of 60 Ω is immersed in 42 kg of water. What is the rise of temperature of water per minute if a steady current of 7A is made to flow through the coil? a) 0.5⁰C b) 10⁰C c) 1⁰C d) 10.15⁰C 68. An immersion heater is rated 418 W. It should heat a litre of water from 10⁰C to 30⁰C in nearly a) 100 s b) 200 s c) 144 s d) 400 s 69. Two bulbs of equal wattage, one having carbon filament and the other having tungsten filament, are connected in series to the mains, then, a) both bulbs glow equally b) tungsten filament glows more c) carbon filament glows less d) carbon filament glows more 70. Two wires A and B with lengths in the ratio of 3 : 1, diameters in the ratio od 1 : 2 with specific resistances in the ratio of 1 : 20 are joined in parallel across a cell of emf 2 V and of negligible internal resistance. The ratio of rates of heat production (𝐻𝐴 /𝐻𝐵 )is a) 3 : 5 b) 5 : 3 c) 4 : 9 d) 6 : 24 71. Of the two bulbs in a house, one glows brighter than the other. Which of the two has a larger resistance? a) the dim bulb b) the bright bulb c) both have same resistance d) brightness does not depend upon resistance 72. A dc supply voltage has an internal resistance of r ohm. A resistive load R is connected across the terminals of the supply. The power developed in R is maximum when the value of R is a) √𝑟 ohm b) 𝑟 2 ohm c) r ohm d) none of the above 73. Two identical heater coils are connected in parallel across the mains. If one of the coil breaks, the other coil

a) develops lower temperature b) develops same temperature as before c) develops higher temperature d) will also not be heated 74. The power rating of a motor drawing a current of 40 A at 400 V is a) 16000 W b) 400 W c) 10 W d) 0.1 W 75. The current in a 960 W, 240 V heater operated at 200 V is a) 0.25 A b) 3/10 A c) 2.5 A d) 10/3 A 76. One heater coil boils certain amount of water in 10 minutes and another heater coil in 15 minutes. In how much time the same water will be boiled if the two heaters are put in series? a) 12.5 min b) 25 min c) 2.5 min d) none of the above 77. One heater coil boils certain amount of water in 10 minutes and another heater coil in 15 minutes. In how much time the same amount water will be boiled if the two heaters are put in parallel? a) 25 min b) 2.5 min c) 6 min d) none of the above 78. Two lamps of/ 100 W and 200 W rated for 220 V are placed in series and a 440 V applied across them. Then a) only 100 W lamp will fuse b) only 200 W lamp will fuse c) both the maps will fuse d) no lamp will fuse 79. A coil of wire of resistance 50 Ω is embedded in a block of ice. What is the amount of ice melted in 1 s if a potential difference of 210 V is applied across the coil? a) 2.625 g b) 3.25 kg c) 4.12 kg d) none of the above 81. When a certain motor is operated for 30 minutes, it consumes 0.75 kWh of energy. During that time, its total energy loss is 3 × 105 J. How many joules of work does it perform in 30 minutes? a) 𝟐𝟒 × 𝟏𝟎𝟓J

b) 27 × 105 J

c) 30 × 105 J d) 32 × 105 J 82. The total power supplied to an engine the drives an electric generator is 40.25 kW. The generator delivers 15 A to a load of 100 Ω. The efficiency of the system is a) 0.493 b) 0.682 c) 0.789 d) 0.559 83. A certain motor draws 17 A from a 240 V source. If the motor is 91.5% efficient, the horsepower rating of the motor is a) 3 hp b) 6 hp c) 5 hp d) 10 hp 84. A certain amplifier develops 80 W of output power. A dc power supply furnishes the amplifier 3 A at 40 V. The power

supply draws 1.6 A form 120 V source. The efficiency f the amplifier is a) 0.525 b) 0.666 c) 0.825 d) 0.752 85. A certain amplifier develops 80 W of output power. A dc power supply furnishes the amplifier 3 A at 40 V. The power supply draws 1.6 A form 120 V source, the efficiency of power supply? a) 0.5 b) 0.75 c) 0.85 d) 0.625 86. A certain amplifier develops 80 W of output power. A dc power supply furnishes the amplifier 3 A at 40 V. The power supply draws 1.6 A form 120 V source, the overall efficiency is a) 0.416 b) 0.938 c) 0.832 d) 0.732 87. What maximum voltage can be connected across a 0.5 W, 270 Ω resistor without exceeding the rated power dissipation? a) 22.6 V b) 23.2 V c) 11.62 V d) 5.83 V 88. What is the maximum safe current flow in a 47 Ω, 2 W resistor? a) 0.42 A b) 0.21 A c) 0.63 A d) none of the above 89. How long would it take 0.5 MΩ resistance to dissipate 1000 joules of heat if the current in the resistance is 20 μA? a) 𝟓 × 𝟏𝟎𝟔 s b) 2.5 × 106 s c) 7.5 × 106 s d) 5.5 × 106 s 90. Two resistors of resistance 8 Ω and 12 Ω are connected in parallel. The power dissipated in 8 Ω resistor is 9 W. The power dissipated is 12 Ω resistor is a) 12 W b) 24 W c) 6 W d) 16 W 91. A coil develops heat of 800 cals/sec. When 20 V is applied across it. The resistance of the coil (1 cal = 4.2 J) is a) 0.4 Ω b) 0.24 Ω c) 0.36 Ω d) 0.12 Ω 92. An electric motor operating at 100 V dc draws current of 10 A. The resistance of the windings of the motor if its efficiency is 25% is a) 5 Ω b) 7.5 Ω c) 15 Ω d) 25 Ω 93. An electric kettle working at 220 V heats up 2 litres of water form 20⁰C to 100⁰C in 2minutes. The current drawn by the kettle is a) 25.45 A b) 12.35 A c) 8.75 A d) 12.85 A 94. A 500 W heater is designed to operate on a 220 V line. If the line voltage drops to 200 V, then percentage drop in the heat output is a) 11.25% b) 17.36% c) 8.75% d) 16.35% 95. A 5⁰C rise in temperature is observed in a conductor by passing a current. When the current is doubled, the rise in temperature will be a) 10⁰C b) 12⁰C c) 16⁰C d) 20⁰C...