70. Which example mainly illustrates heat transfer by convection?
[SSC Selection Post, July 2025]
(A) Hands warmed by radiation from a fire
(B) Energy passing through empty space
(C) Heat moving along a solid metal rod
(D) Water circulating while a vessel is heated on a stove
Answer: (D) Water circulating while a vessel is heated on a stove
Explanation: Convection moves thermal energy through bulk motion of a fluid. Water warmed near the bottom becomes less dense and rises, while cooler water sinks to replace it. That circulation distributes heat. A metal rod mainly transfers heat through conduction.
69. Which instrument is used to measure temperature?
[SSC Selection Post, August 2025]
(A) Barometer
(B) Thermometer
(C) Speedometer
(D) Altimeter
Answer: (B) Thermometer
Explanation: A thermometer measures temperature through a calibrated physical response such as thermal expansion or electrical resistance. A barometer measures atmospheric pressure, a speedometer measures speed and an altimeter estimates altitude. Temperature reflects the thermal state of a body.
68. The transformation of liquid water in seas and rivers into atmospheric water vapour is known as what?
[SSC GD Constable, February 2025]
(A) Evaporation
(B) Precipitation
(C) Condensation
(D) Transpiration
Answer: (A) Evaporation
Explanation: Evaporation transfers water from the liquid state into gaseous form through escape of energetic surface molecules. Solar energy drives much of the process in nature. Condensation reverses it, precipitation brings water to Earth, and transpiration releases moisture from plants.
67. For an ideal air-standard Otto cycle, which two quantities govern thermal efficiency?
[RRB Junior Engineer Mechanical, June 2025]
(A) Engine displacement and rotational speed
(B) Heat supplied and absolute pressure alone
(C) Working-fluid mass and temperature alone
(D) Compression ratio and specific-heat ratio
Answer: (D) Compression ratio and specific-heat ratio
Explanation: Otto-cycle efficiency is 1 − 1/r^(γ−1), where r is compression ratio and γ is the ratio of specific heats. Higher compression generally improves ideal efficiency. The relation is derived for a simplified constant-specific-heat, internally reversible engine cycle.
66. Choose the descending order of thermal conductivity for brass, iron, water and steam.
[RRB Junior Engineer CMA, April 2025]
(A) Brass > Steam > Iron > Water
(B) Brass > Iron > Water > Steam
(C) Iron > Water > Steam > Brass
(D) Steam > Iron > Water > Brass
Answer: (B) Brass > Iron > Water > Steam
Explanation: Brass and iron conduct heat far more effectively than water or steam because metals have mobile conduction electrons. Ordinary liquid water conducts better than water vapour. Brass generally surpasses iron, giving brass, iron, water and steam in decreasing order.
65. Which statement about a substance at its critical point is correct?
[SSC JE Mechanical, December 2025]
(A) Liquid and vapour are sharply separated
(B) Vaporisation requires infinite energy
(C) The saturated-liquid and saturated-vapour densities become equal
(D) The substance must freeze
Answer: (C) The saturated-liquid and saturated-vapour densities become equal
Explanation: At the critical temperature and pressure, the usual boundary separating liquid and gas terminates. Saturated liquid and vapour approach the same density and other thermodynamic properties. Above that point, a supercritical fluid cannot be classified by an ordinary boiling transition.
64. What is the main purpose of thermal overload protection in a transformer?
[RRB Junior Engineer Electrical, April 2025]
(A) To detect overvoltage only
(B) To prevent all short circuits
(C) To monitor and limit excessive temperature rise
(D) To measure frequency exclusively
Answer: (C) To monitor and limit excessive temperature rise
Explanation: Excessive temperature accelerates breakdown of transformer winding insulation. Thermal protection responds to sustained overload or overheating before damaging temperatures persist. It complements electrical fault protection but does not substitute for instantaneous short-circuit and earth-fault detection.
63. On what principle does a thermal protective relay operate?
[RRB JE CBT-II Electronics Engineering, June 2025]
(A) Circuit impedance exceeding a threshold
(B) Voltage difference across protected equipment
(C) Phase-angle difference between line currents
(D) Heating due to current flow until a preset temperature is reached
Answer: (D) Heating due to current flow until a preset temperature is reached
Explanation: Thermal relays respond to temperature increase caused by current and resistance, frequently using a bimetallic strip or sensing element. When sustained overload generates sufficient heat, the relay trips. This protects motors and other electrical equipment from overheating.
62. Which description best defines heat in thermodynamics?
[UPSC NDA-II GAT, September 2024]
(A) Any change in one energy form
(B) Conversion between mass and energy
(C) Energy transferred because of temperature difference
(D) Volume change due to rising temperature
Answer: (C) Energy transferred because of temperature difference
Explanation: Heat is energy in transit between systems because their temperatures differ. It flows spontaneously from hotter to cooler surroundings. A body possesses internal energy, not stored “heat”; heat and work are modes of energy transfer across a system boundary.
61. At which absolute temperature does pure liquid water have its maximum density?
[UPSC NDA-II GAT, September 2024]
(A) 299 K
(B) 277 K
(C) 285 K
(D) 373 K
Answer: (B) 277 K
Explanation: Water reaches maximum density at approximately 4°C, equal to 277 K. Below this point, formation of more open hydrogen-bonded structures causes expansion. This anomaly helps lakes freeze from the surface while denser water remains below.
