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Current Electricity MCQs — Physics (page 3)

270 multiple-choice questions on Current Electricity from Physics, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

51. An LDR potential divider can control:

  1. automatic lighting that turns on at dusk
  2. a microphone
  3. a fuse
  4. the speed of a fan by sound

Answer: A) automatic lighting that turns on at dusk

Explanation: Light sensor. SLO P-09-E-25 · Understanding

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52. Adding another resistor in parallel makes the total current from the supply:

  1. increase
  2. decrease
  3. become zero
  4. stay the same

Answer: A) increase

Explanation: Total resistance falls. SLO P-09-E-21 · Application

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53. In a parallel circuit, the branch with the smallest resistance carries:

  1. the largest current
  2. no current
  3. the smallest current
  4. the same current as the others

Answer: A) the largest current

Explanation: I = V/R. SLO P-09-E-21 · Application

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54. A source of e.m.f. 1.5 V drives 0.3 A for 600 s. The energy it supplies is:

  1. 0.45 J
  2. 540 J
  3. 270 J
  4. 135 J

Answer: C) 270 J

Explanation: W = mathcalEIt = 270 J. SLO P-09-E-15 · Application

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55. A 6 V source with internal resistance 1 Ω supplies a 5 Ω resistor. The efficiency of the power transfer is:

  1. 16.7%
  2. 50%
  3. 83.3%
  4. 100%

Answer: C) 83.3%

Explanation: Efficiency = I^2RI^2(R + r) = RR + r = 83.3%. SLO P-09-E-26 · Understanding

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56. A battery of e.m.f. 12 V has a terminal p.d. of 10.8 V when delivering 3 A. Its internal resistance is:

  1. 3.6 Ω
  2. 0.4 Ω
  3. 0.8 Ω
  4. 0.2 Ω

Answer: B) 0.4 Ω

Explanation: mathcalE = V + Ir Rightarrow r = 12 - 10.83 = 0.4 Ω. SLO P-09-E-17 · Understanding

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57. A copper wire has n = 8.5 10^28 m^-3, cross-section 2 10^-6 m^2 and drift velocity 5 10^-5 m/s. The current is (e = 1.6 10^-19 C):

  1. 1.36 A
  2. 2.72 A
  3. 13.6 A
  4. 0.136 A

Answer: A) 1.36 A

Explanation: I = nAvq = 8.5 10^28 2 10^-6 5 10^-5 1.6 10^-19 = 1.36 A. SLO P-09-E-10 · Application

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58. 0.6 J of energy is transferred when 0.05 C of charge passes between two points. The p.d. is:

  1. 0.03 V
  2. 6 V
  3. 12 V
  4. 24 V

Answer: C) 12 V

Explanation: V = WQ = 0.60.05 = 12 V. SLO P-09-E-11 · Application

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59. A standard cell of e.m.f. 1.08 V balances at 54 cm on a potentiometer wire. Another cell balances at 75 cm. Its e.m.f. is:

  1. 0.75 V
  2. 3 V
  3. 1.5 V
  4. 0.778 V

Answer: C) 1.5 V

Explanation: mathcalE2mathcalE1 = l2l1 Rightarrow mathcalE2 = 1.08 7554 = 1.5 V. SLO P-09-E-23 · Application

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60. A metre bridge balances at 40 cm with a 6 Ω standard resistor in the right gap. The unknown resistor in the left gap is:

  1. 4 Ω
  2. 9 Ω
  3. 2.4 Ω
  4. 15 Ω

Answer: A) 4 Ω

Explanation: X6 = 4060. SLO P-09-E-24 · Understanding

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61. At the balance point of a potentiometer, the galvanometer shows:

  1. a negative current
  2. maximum current
  3. the e.m.f.
  4. zero current

Answer: D) zero current

Explanation: Null deflection. SLO P-09-E-23 · Application

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62. Maximum power is delivered to an external resistor when its resistance equals:

  1. twice the internal resistance
  2. the internal resistance of the source
  3. zero
  4. infinity

Answer: B) the internal resistance of the source

Explanation: Maximum power transfer theorem. SLO P-09-E-17 · Understanding

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63. In the circuit above, the current in the 6 Ω resistor is:

  1. 2 A
  2. 43 A
  3. 1 A
  4. 23 A

Answer: D) 23 A

Explanation: P.d. across the pair = 2 2 = 4 V; I = 4/6. SLO P-09-E-22 · Application

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64. Which device has an e.m.f. rather than just a p.d.?

  1. a light bulb
  2. a heater
  3. a battery
  4. a resistor

Answer: C) a battery

Explanation: It supplies energy. SLO P-09-E-16 · Understanding

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65. The sensitivity of a null method depends mainly on:

  1. the colour of the wire
  2. the length of connecting leads
  3. how small a current the galvanometer can detect
  4. the size of the wire

Answer: C) how small a current the galvanometer can detect

Explanation: Sensitive detector gives precise balance. SLO P-09-E-24 · Understanding

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66. For the same current, a thinner wire has electrons drifting:

  1. faster
  2. at the same speed
  3. backwards
  4. slower

Answer: A) faster

Explanation: v = I/(nAq). SLO P-09-E-10 · Application

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67. At a junction, currents of 10 A, 4 A, 1 A flow in. Currents of 7 A, 2 A flow out, together with one unknown current I. I is:

  1. 6 A
  2. 12 A
  3. 60 A
  4. 3 A

Answer: A) 6 A

Explanation: sum Iin = sum Iout Rightarrow I = 15 - 9 = 6 A. SLO P-09-E-18 · Knowledge

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68. 1500 J of energy is transferred when 125 C of charge passes between two points. The p.d. is:

  1. 24 V
  2. 1.88 10^5 V
  3. 12 V
  4. 6 V

Answer: C) 12 V

Explanation: V = WQ = 1500125 = 12 V. SLO P-09-E-11 · Application

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69. A source of e.m.f. 12 V and internal resistance 1 Ω drives a 5 Ω load. The power delivered to the load is:

  1. 20 W
  2. 40 W
  3. 200 W
  4. 10 W

Answer: A) 20 W

Explanation: I = 126 = 2 A; P = I^2R = 20 W. SLO P-09-E-26 · Understanding

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70. The combined resistance of resistors in series is:

  1. larger than the largest individual resistance
  2. equal to the average
  3. zero
  4. smaller than the smallest

Answer: A) larger than the largest individual resistance

Explanation: They add. SLO P-09-E-20 · Application

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71. When a battery is being charged, the p.d. across it is:

  1. equal to zero
  2. equal to its e.m.f. always
  3. less than its e.m.f.
  4. greater than its e.m.f.

Answer: D) greater than its e.m.f.

Explanation: Extra energy is needed to drive current backwards through its internal resistance. SLO P-09-E-16 · Understanding

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72. Carbon fibres in concrete allow bridge inspectors to check reliability because:

  1. the fibres glow when cracked
  2. the fibres conduct electricity, and cracks change their resistance
  3. they are magnetic
  4. they make the concrete transparent

Answer: B) the fibres conduct electricity, and cracks change their resistance

Explanation: Electrical monitoring. SLO P-09-E-27 · Understanding

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73. Carbon fibres are chosen for this use because they are:

  1. transparent
  2. insulating and weak
  3. magnetic
  4. strong, light and electrically conducting

Answer: D) strong, light and electrically conducting

Explanation: Dual role: reinforcement and sensing. SLO P-09-E-27 · Understanding

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74. Null methods are accurate because:

  1. the galvanometer is large
  2. at balance no current flows, so the galvanometer's own resistance and calibration don't matter
  3. large currents are used
  4. they need no wires

Answer: B) at balance no current flows, so the galvanometer's own resistance and calibration don't matter

Explanation: Only detects zero. SLO P-09-E-24 · Understanding

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75. A solar cell's e.m.f. comes from converting:

  1. nuclear energy
  2. chemical energy
  3. kinetic energy
  4. light energy into electrical energy

Answer: D) light energy into electrical energy

Explanation: Photovoltaic effect. SLO P-09-E-15 · Application

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