20. Which process transfers heat by actual bulk motion of matter?
[RRB ALP Electrician, January 2019]
(A) Convection
(B) Radiation
(C) Vaporisation
(D) Conduction
Answer: (A) Convection
Explanation: Convection transfers thermal energy when a fluid such as air or water moves from one region to another. Warm material often rises because of reduced density. Conduction does not require bulk flow, and electromagnetic radiation requires no matter at all.
19. Heat energy travelling in straight lines at the speed of light is an example of which process?
[UPSC NDA-II GAT, 2019]
(A) Conduction
(B) Convection
(C) Radiation
(D) Diffusion of molecules
Answer: (C) Radiation
Explanation: Radiation consists of electromagnetic waves and propagates through a vacuum at light speed. Direct sunlight illustrates radiative heat transfer. Conduction needs matter for microscopic interactions, whereas convection requires bulk motion of fluids, so neither explains propagation through space.
18. A sunny day records 113°F. Approximately what temperature is this in kelvin?
[UPSC NDA-II GAT, 2019]
(A) 273 K
(B) 298 K
(C) 318 K
(D) 373 K
Answer: (C) 318 K
Explanation: Convert Fahrenheit to Celsius using C = (F − 32) × 5/9. For 113°F, C = 45°C. Adding 273.15 gives 318.15 K, usually rounded to 318 K. Kelvin temperatures are measured from absolute zero.
17. The terms pyrolysis and plasma gasification are most commonly associated with which technology?
[UPSC Civil Services Prelims, 2019]
(A) Rare-earth extraction
(B) Natural-gas drilling
(C) Hydrogen-car fuel cells
(D) Waste-to-energy processing
Answer: (D) Waste-to-energy processing
Explanation: Pyrolysis decomposes substances with heat under oxygen-limited conditions. Plasma gasification uses extremely hot ionised gas to convert waste into synthesis gas and solid residues. Both approaches employ thermal transformations of matter for waste treatment and potential energy recovery.
16. Which appliance does NOT primarily use the heating effect of electric current?
[RRB Group D, October 2018]
(A) Electric heater
(B) Incandescent bulb filament
(C) Electric iron
(D) Microwave oven
Answer: (D) Microwave oven
Explanation: An electric heater and iron produce heat through resistive elements, while an incandescent filament becomes white-hot. A microwave oven heats food mainly through interaction of microwave electromagnetic fields with polar molecules, rather than direct resistive heating inside the food.
15. Which scientist’s law describes heating generated in a conductor by electric current?
[RRB Group D, October 2018]
(A) Joule
(B) Ohm
(C) Faraday
(D) Newton
Answer: (A) Joule
Explanation: Joule’s heating law states that thermal energy generated in a resistor is H = I²Rt. It depends on current squared, resistance and duration. The law explains the operation of heaters and why overloaded wires may become dangerously hot.
14. Which equation gives electrical heating over time t for a conductor of resistance R carrying current I?
[RRB Group D, September 2018]
(A) H = I²R
(B) H = IRt
(C) H = I²t
(D) H = I²Rt
Answer: (D) H = I²Rt
Explanation: Electrical power dissipated by a resistor is P = I²R. Multiplying power by elapsed time t gives energy H = I²Rt. The expression assumes resistance and current remain constant during the interval; joules are the resulting SI energy unit.
13. Which pair is comparatively poor at conducting heat?
[RRB Group D, October 2018]
(A) Copper and aluminium
(B) Copper and iron
(C) Gold and silver
(D) Lead and mercury
Answer: (D) Lead and mercury
Explanation: Lead and mercury conduct heat less efficiently than copper, aluminium, gold or silver. Metals differ in thermal conductivity because of their electronic structures and scattering behaviour. Silver and copper, in particular, are among the best familiar metallic conductors.
12. Dulong and Petit’s law predicts that at sufficiently high temperatures, molar heat capacity of a classical crystalline solid varies as T raised to which power?
[RPSC Second Grade Science, November 2018]
(A) 0
(B) 1
(C) 2
(D) 3
Answer: (A) 0
Explanation: Dulong and Petit’s classical limit gives approximately 3R for molar heat capacity of many crystalline solids. Because it is nearly independent of temperature in that regime, it varies as T⁰. The approximation fails notably at very low temperatures.
11. The statement that heat cannot spontaneously flow from a colder body to a hotter body is an expression of which law?
[UPSC NDA-II GAT, 2017]
(A) Zeroth law
(B) First law
(C) Second law
(D) Third law
Answer: (C) Second law
Explanation: The Clausius formulation of the second law prohibits spontaneous heat transfer from a low-temperature body to a higher-temperature one. Refrigerators accomplish such transfer only by consuming external work. The first law alone establishes energy conservation, not transfer direction.
