Q40. Why does the current suddenly become very large when a short circuit occurs?
[UPSC CDS — 10 April 2022]
(A) Circuit resistance becomes very small
(B) Battery charge becomes infinite
(C) Supply frequency becomes zero
(D) The wire becomes an insulator
Answer: (A) Circuit resistance becomes very small
Explanation: In a short circuit an unintended, low-resistance route bypasses the load. For a fixed supply voltage, Ohm’s law predicts a large current as resistance becomes very small. Fuses or circuit breakers are installed to interrupt such dangerous faults.
Q39. A 50 Ω wire is cut into five equal pieces, all connected in parallel. What is their combined resistance?
[UPSC NDA — 2022-I]
(A) 2 Ω
(B) 5 Ω
(C) 10 Ω
(D) 250 Ω
Answer: (A) 2 Ω
Explanation: Each equal piece has resistance 50/5 = 10 Ω. Placing five 10 Ω pieces in parallel gives 10/5 = 2 Ω. More generally, cutting a uniform wire into n pieces and paralleling them yields R/n².
Q38. Which intrinsic property of a material describes opposition to the flow of electric charge?
[UPPSC Polytechnic Lecturer (Electrical) — 22 November 2021]
(A) Conductivity
(B) Thermoelectricity
(C) Dielectric strength
(D) Resistivity
Answer: (D) Resistivity
Explanation: Resistivity quantifies how strongly a material opposes electric current, independently of the dimensions of any particular specimen. Its unit is ohm metre. Conductivity expresses the reciprocal tendency to carry current; the product of the two properties is one.
Q37. To change a moving-coil galvanometer into a DC ammeter, what should be connected across it?
[UPPSC Polytechnic Lecturer (Electrical) — 22 November 2021]
(A) High resistance in series
(B) Low shunt resistance in parallel
(C) Pure inductance in series
(D) Capacitor in series
Answer: (B) Low shunt resistance in parallel
Explanation: An ammeter must offer little resistance to the measured circuit. A low-resistance shunt in parallel with the galvanometer carries most of the current, protecting the delicate coil while allowing the instrument to measure a larger current.
Q36. Which device–principle pair is incorrectly matched: potentiometer–Wheatstone bridge, voltameter–chemical effect, induction coil–electromagnetic induction, transformer–mutual induction?
[UPPSC GIC Lecturer (Physics) — 2021]
(A) Potentiometer–Wheatstone bridge
(B) Voltameter–chemical effect
(C) Induction coil–electromagnetic induction
(D) Transformer–mutual induction
Answer: (A) Potentiometer–Wheatstone bridge
Explanation: A potentiometer uses the proportional relation between potential drop and length along a uniform current-carrying wire. It is not based on Wheatstone-bridge balance. The other listed principles correctly describe electrolysis-based charge measurement and electromagnetic induction devices.
Q35. A nichrome wire carrying current is shortened to half its length without changing its cross-section or applied voltage. How does the current change?
[UPSC CDS — 14 November 2021]
(A) It becomes one-fourth
(B) It becomes half
(C) It doubles
(D) It remains unchanged
Answer: (C) It doubles
Explanation: For an unchanged material and cross-sectional area, resistance is proportional to length. Halving the wire length therefore halves resistance. Since supply voltage is unchanged, Ohm’s law predicts twice the current, although resistive heating may change the final temperature.
Q34. Which welding method is especially extensively carried out by industrial robots?
[MPPSC Assistant Engineer (Mechanical) — 2021]
(A) Spot welding
(B) MIG welding
(C) TIG welding
(D) Thermit welding
Answer: (A) Spot welding
Explanation: Resistance spot welding passes a large electric current between electrodes pressing metal sheets together. Joule heating at the contact forms a weld. Industrial robots repeat the placement, pressure and current cycle rapidly and precisely, particularly in automobile assembly.
Q33. Which instrument is used to measure electric current?
[SSC MTS — 5 October 2021]
(A) Barometer
(B) Thermometer
(C) Ammeter
(D) Hygrometer
Answer: (C) Ammeter
Explanation: An ammeter measures the current flowing in a circuit and is connected in series with the part being tested. It has low internal resistance to minimize interference with the current. A voltmeter instead measures the potential difference across two points.
Q32. A refrigerator rated at 5 kW operates 10 hours daily for 30 days. At ₹4 per unit, what is the cost?
[UPSC NDA and Naval Academy — 2020 (Combined Examination)]
(A) ₹600
(B) ₹3,000
(C) ₹6,000
(D) ₹12,000
Answer: (C) ₹6,000
Explanation: Energy consumed is 5 kW × 10 h × 30 = 1,500 kWh. One electricity unit is one kilowatt-hour. At ₹4 per unit, the bill for this usage alone is 1,500 × 4 = ₹6,000.
Q31. Which metal is commonly used for the filament of an incandescent electric bulb?
[UPSC NDA and Naval Academy — 2020 (Combined Examination)]
(A) Copper
(B) Tungsten
(C) Lead
(D) Aluminium
Answer: (B) Tungsten
Explanation: Tungsten has an exceptionally high melting point, allowing the filament to glow intensely without immediately melting. Its electrical resistance converts energy into heat and light. The sealed bulb also limits oxidation that would otherwise destroy the hot filament.
