Q40. Two conducting wires of the same material have lengths L and 1.2L. If the longer wire has half the volume of the shorter wire, how does its electrical resistance compare?
[UPSC NDA II, General Ability, 13 September 2026]
A. 288% larger
B. 200% larger
C. 188% larger
D. 108% larger
Answer: C. 188% larger
Explanation: Resistance is ρL/A. Since volume equals area times length, the longer wire has area A/2.4 if the shorter has area A. Its resistance is then 1.2 × 2.4 = 2.88 times the original, or 188% larger.
Q39. Three wires X, Y and Z have equal lengths and conductivity ratio 1 : 2 : 3. Z has twice the cross-sectional area of Y and X has the same resistance as Z. What is the ratio R(Y) : R(X)?
[UPSC NDA II, General Ability, 13 September 2026]
A. 1 : 1
B. 3 : 1
C. 2 : 1
D. 1 : 2
Answer: B. 3 : 1
Explanation: For equal lengths, resistance varies inversely with conductivity multiplied by cross-sectional area: R = L/(σA). Let Y have area A; Z has 2A. Equality of X and Z resistance gives X area 6A. Therefore R(Y)/R(X) = 6/2 = 3.
Q38. Why are cooking utensils often made with a copper bottom?
[SSC CGL 2025, 13 September, Shift 1]
A. Copper is a good insulator, preventing heat loss.
B. Copper has a low melting point, making it easy to manufacture.
C. Copper is a good conductor of heat, ensuring even heating.
D. Copper reacts readily with food, enhancing flavour.
Answer: C. Copper is a good conductor of heat, ensuring even heating.
Explanation: Copper has high thermal conductivity, so energy from the flame spreads rapidly across the utensil base. This reduces hot spots and helps food cook more uniformly. Copper bottoms are frequently combined with stainless steel to improve durability and avoid direct food contact.
Q37. The thermal conductivity of lightweight concrete is:
[SSC JE Civil Engineering, 6 December 2025, Shift 1]
A. Lower than dense concrete
B. Equal to dense concrete
C. Higher than dense concrete
D. Negligible
Answer: A. Lower than dense concrete
Explanation: Lightweight concrete contains more pores and air spaces than dense concrete. Trapped air restricts heat transfer, reducing the effective thermal conductivity. This makes lightweight concrete useful where thermal insulation and reduced structural load are desirable, although its conductivity is not zero.
Q36. What is the most common material used for the inner conductor of a coaxial cable, and what is the primary reason for its use?
[SSC JE Electrical Engineering, 4 December 2025, Shift 2]
A. Silver, for its superior conductivity
B. Aluminium, for its light weight and low cost
C. Copper, for its high conductivity and low resistance
D. Steel, for its low resistance
Answer: C. Copper, for its high conductivity and low resistance
Explanation: Copper offers high electrical conductivity with reasonable cost, ductility and mechanical reliability. A copper inner conductor allows high-frequency signals to travel with comparatively low resistive loss. Silver conducts still better, but the additional expense makes it less suitable for most coaxial cables.
Q35. A heating element of a heater is made of a material with high resistivity and high melting point. Which material property is most crucial for its function as a heating element?
[SSC Stenographer 2025, 8 August, Shift 2]
A. High modulus of elasticity
B. High thermal conductivity
C. Low specific heat capacity
D. High resistivity
Answer: D. High resistivity
Explanation: High resistivity allows a suitably designed element to develop substantial electrical resistance and produce heat through Joule heating. A high melting point also lets it tolerate operating temperatures. Nichrome is commonly used because it combines these properties with oxidation resistance.
Q34. The S.I. unit of thermal conductivity is:
[UPPSC Assistant Engineer Mains, Mechanical, 29 September 2025]
A. J/(m²·K)
B. J/(m·K)
C. W/(m·K)
D. None of the above
Answer: C. W/(m·K)
Explanation: Fourier’s law gives heat-transfer rate as kAΔT/L. Rearranging, k equals heat-transfer rate multiplied by length, divided by area and temperature difference. The corresponding SI unit is watt per metre per kelvin, written W/(m·K).
Q33. Calorimeter is a small container made of a thin sheet of _______ with good thermal conductivity.
[SSC CGL 2024 Tier I, 17 September, Shift 1]
A. Silver
B. Platinum
C. Copper
D. Caesium
Answer: C. Copper
Explanation: A calorimeter commonly uses a thin copper vessel because copper transfers heat efficiently and quickly reaches a nearly uniform temperature. The thin wall also minimizes heat capacity of the container, helping measurements of heat exchange between substances remain more precise.
Q32. Which property of the material of a conductor determines its resistivity?
[RRB Technician Grade I, 19 December, Shift 2]
A. Melting point
B. Electrical conductivity
C. Temperature coefficient
D. Density
Answer: B. Electrical conductivity
Explanation: Resistivity and conductivity are reciprocal intrinsic electrical properties: resistivity ρ equals 1/σ, where σ represents electrical conductivity. A highly conducting material therefore has low resistivity. The value also depends on the material and conditions such as temperature.
Q31. Graphite is a good conductor of electricity due to:
[WBCS Mains (Science), 2023]
A. Strong electrostatic bonding
B. Free localized bonds
C. Free delocalized electrons
D. Tetrahedral arrangement of particles
Answer: C. Free delocalized electrons
Explanation: In graphite, each carbon atom forms three covalent bonds within a hexagonal layer. Its fourth valence electron contributes to a delocalized electronic system. These mobile electrons carry electric charge along the layers, unlike diamond where valence electrons are localized in bonds.