Q20. What physical phenomenon is responsible for the operation of a basic electric heater?
[RRB Group D — 10 October 2018]
(A) Magnetic attraction
(B) Chemical reaction
(C) Heating effect of current
(D) Photoelectric emission
Answer: (C) Heating effect of current
Explanation: A heater contains a resistive element that converts electrical energy to internal energy through interactions between moving electrons and the atomic lattice. The heat developed depends on current squared, element resistance, and operating time.
Q19. For a uniform wire made of one material, resistance is inversely proportional to which quantity?
[RRB Group D — 4 October 2018]
(A) Its length
(B) Its cross-sectional area
(C) Its resistivity
(D) Applied voltage only
Answer: (B) Its cross-sectional area
Explanation: The resistance of a uniform wire is R = ρL/A. With material and length unchanged, increasing cross-sectional area gives charges more conducting pathways and reduces resistance. Doubling the area halves resistance if temperature remains fixed.
Q18. What is the usual commercial unit of electrical energy?
[RRB Group D — 20 September 2018]
(A) Ampere
(B) Watt
(C) Kilowatt-hour
(D) Ohm
Answer: (C) Kilowatt-hour
Explanation: Electricity meters record energy in kilowatt-hours, often called units. Multiply appliance power in kilowatts by the number of operating hours to obtain consumption. Watts measure power, whereas kilowatt-hours measure the total amount of energy delivered.
Q17. A wire carrying an electric current exhibits which magnetic behaviour?
[SSC MTS — 10 October 2017]
(A) It acts as a magnet
(B) It loses all magnetic effects
(C) It becomes perfectly insulating
(D) It stops interacting with a compass
Answer: (A) It acts as a magnet
Explanation: The movement of charge through a conductor creates a magnetic field around it. A compass placed near the wire may deflect. The strength of the field increases with current, a principle exploited by electromagnets.
Q16. Current in a coil changes from +2 A to −2 A in 0.05 s, inducing 8 V. What is its self-inductance?
[RPSC Senior Teacher Grade II (Science) — 1 July 2017]
(A) 0.05 H
(B) 0.1 H
(C) 0.2 H
(D) 0.8 H
Answer: (B) 0.1 H
Explanation: The magnitude of induced voltage is L|ΔI/Δt|. Here the total current change is 4 A, occurring in 0.05 second. Consequently L = 8 × 0.05/4 = 0.1 henry. The voltage opposes the change in current.
Q15. What shunt resistance converts a 0.1 mA meter of internal resistance 100 Ω into an ammeter capable of 100 mA?
[CGPSC Assistant Engineer (Electrical) — 2017]
(A) 0.1 Ω
(B) 1 Ω
(C) 10 Ω
(D) 100 Ω
(E) 1000 Ω
Answer: (A) 0.1 Ω
Explanation: At full scale the meter carries 0.1 mA while the remainder, 99.9 mA, bypasses it through the shunt. Equating their voltage drops gives Rsh = (0.0001 × 100)/0.0999 ≈ 0.1001 Ω, approximately 0.1 Ω.
Q14. Three equal resistors each of resistance R are first connected in parallel and then in series. What is the series-to-parallel resistance ratio?
[UPSC NDA — 2016-II]
(A) 1:3
(B) 3:1
(C) 9:1
(D) 1:9
Answer: (C) 9:1
Explanation: In series three identical resistors have total resistance 3R. In parallel their equivalent resistance is R/3. Dividing series by parallel resistance gives (3R)/(R/3) = 9. The comparison demonstrates the large impact of circuit arrangement.
Q13. What is the protective purpose of a fuse in a household electric circuit?
[UPSC NDA — 18 September 2016]
(A) To prevent damage from excessive current
(B) To boost electric power
(C) To permanently join two live wires
(D) To measure the resistance of appliances
Answer: (A) To prevent damage from excessive current
Explanation: The fuse acts as a sacrificial safety link. Too much current raises its temperature by resistive heating until it melts, disconnecting power. It can help protect against overloads and short circuits but is not a voltage regulator or measuring instrument.
Q12. When does the resistivity of a uniform conductor change even if only its geometric shape changes?
[UPSC NDA — 2016-II]
(A) It always changes with wire length
(B) It changes with cross-section only
(C) It does not change solely because of the dimensions
(D) It becomes zero upon bending
Answer: (C) It does not change solely because of the dimensions
Explanation: Resistivity is an intrinsic property of the material at a given temperature. Changing length or area modifies resistance through R = ρL/A but leaves ρ unchanged. Only conditions such as composition and temperature alter the material’s resistivity.
Q11. What device converts chemical energy into electrical energy?
[RPSC RAS Prelims — 2015 (re-examination of 2013)]
(A) Electric motor
(B) Generator
(C) Moving-coil meter
(D) Battery
Answer: (D) Battery
Explanation: A battery contains electrochemical cells in which spontaneous redox reactions establish a potential difference between terminals. When connected to a circuit, chemical energy is converted to electrical energy. Generators instead convert mechanical energy, while motors consume electrical energy.
