Q60. A lamp rated 80 W at 220 V is connected to a 110 V supply. Assuming constant resistance, its power becomes:
[UPSC NDA — 2023-I]
(A) 10 W
(B) 20 W
(C) 40 W
(D) 80 W
Answer: (B) 20 W
Explanation: Power at constant resistance obeys P = V²/R. Reducing the supply voltage to half its rated value gives one-quarter the original power: 80/4 = 20 W. In a real incandescent bulb, resistance varies with filament temperature.
Q59. A high-resistivity material is chosen for a lamp filament primarily to achieve which result?
[RRB Group D — 11 October 2022]
(A) Rapid heat generation
(B) Zero energy loss
(C) Elimination of current
(D) Cold operation
Answer: (A) Rapid heat generation
Explanation: A thin, relatively high-resistance filament releases electrical energy as heat, reaching temperatures at which it emits visible light. Tungsten is preferred because it also survives very high temperatures. The filament’s glow is an example of incandescence.
Q58. A lamp works at 220 V while drawing 2 A. What is its electrical power?
[RRB Group D — 7 October 2022]
(A) 110 W
(B) 220 W
(C) 440 W
(D) 880 W
Answer: (C) 440 W
Explanation: Electrical power at a device is the potential difference across it multiplied by its current. Substituting the values gives P = 220 × 2 = 440 watts. This is the instantaneous conversion rate under the stated operating conditions.
Q57. Two resistors have combined resistance 120 Ω in series and 30 Ω in parallel. Find their separate resistance values.
[RRB Group D — 7 October 2022, Shift 1]
(A) 70 Ω and 50 Ω
(B) 80 Ω and 40 Ω
(C) 90 Ω and 30 Ω
(D) 60 Ω and 60 Ω
Answer: (D) 60 Ω and 60 Ω
Explanation: Let the resistances be R₁ and R₂. Series connection gives R₁ + R₂ = 120 Ω; their parallel equivalent is R₁R₂/(R₁ + R₂) = 30 Ω. Therefore the product is 3,600, satisfied by 60 Ω each.
Q56. Which equation correctly gives Joule heat produced in a resistor during time t?
[RRB Group D — 29 September 2022]
(A) H = I²Rt
(B) H = IRt²
(C) H = V²It
(D) H = I²R/t
Answer: (A) H = I²Rt
Explanation: Joule’s heating law is H = I²Rt. For constant resistance and elapsed time, doubling the current causes four times as much heat to be generated. It explains both normal resistance heating and why overloads can damage electrical wiring.
Q55. A conductor obeys Ohm’s law. How is current determined from voltage V and resistance R?
[RRB Group D — 28 September 2022]
(A) I = VR
(B) I = V/R
(C) I = R/V
(D) I = V + R
Answer: (B) I = V/R
Explanation: Ohm’s law states V = IR when temperature and other physical conditions are constant. Rearranging for current gives I = V/R. At unchanged potential difference, raising the resistance lowers current in inverse proportion.
Q54. Two groups each have six identical resistors connected in parallel across 20 V. Group A draws 6 A and B draws 8 A. What are the individual resistance values?
[RRB Group D — 19 September 2022]
(A) 20 Ω and 15 Ω
(B) 15 Ω and 20 Ω
(C) 10 Ω and 12 Ω
(D) 6 Ω and 8 Ω
Answer: (A) 20 Ω and 15 Ω
Explanation: Six identical resistors of resistance r in parallel have equivalent resistance r/6. Hence r = 6V/I. For group A, r = 120/6 = 20 Ω; for B, r = 120/8 = 15 Ω.
Q53. Each of two networks uses twelve equal resistors in parallel across 30 V. They draw 10 A and 18 A. What are the individual resistor values?
[RRB Group D — 16 September 2022, Shift 3]
(A) 26 Ω and 25 Ω
(B) 36 Ω and 15 Ω
(C) 26 Ω and 20 Ω
(D) 36 Ω and 20 Ω
Answer: (D) 36 Ω and 20 Ω
Explanation: With twelve equal parallel elements, total resistance equals individual resistance divided by twelve. Therefore r = 12V/I. Network A gives 12 × 30/10 = 36 Ω; network B gives 12 × 30/18 = 20 Ω.
Q52. On which principle does an ordinary electrical transformer operate?
[RRB Group D — 14 September 2022]
(A) Mutual electromagnetic induction
(B) Chemical electrolysis
(C) Photoelectric emission
(D) Thermal expansion
Answer: (A) Mutual electromagnetic induction
Explanation: A changing current in the primary winding produces a changing magnetic flux through the transformer core. That changing flux induces an electromotive force in the secondary winding. For this reason transformers ordinarily require changing current, such as alternating current.
Q51. In a parallel arrangement, how does the equivalent resistance compare with the lowest individual resistance?
[RRB Group D — 13 September 2022]
(A) It is always greater
(B) It equals the highest resistance
(C) It is smaller than the smallest resistance
(D) It is the arithmetic sum
Answer: (C) It is smaller than the smallest resistance
Explanation: Connecting resistors in parallel provides multiple pathways for charge. Their conductances add, so the total resistance is less than the resistance of the easiest single path. This remains true for any finite positive branch resistances.
