100. An insulated vessel initially has 250 g water at 20°C. Steam at 100°C heats it to 25°C and the water mass becomes 252 g. If the specific heat of water is 1 cal/g°C, what is the steam’s latent heat?
[UPSC NDA-II GAT, September 2026]
(A) 530 cal/g
(B) 540 cal/g
(C) 550 cal/g
(D) 560 cal/g
Answer: (C) 550 cal/g
Explanation: The original water gains 250 × 5 = 1,250 calories. Two grams of steam condense and their resulting water cools from 100°C to 25°C. Therefore 2(L + 75) = 1,250, giving L = 550 cal/g.
99. Choose the correct order of processes in the natural water cycle.
[RRB Group D, August 2026]
(A) Precipitation → runoff → evaporation
(B) Evaporation → precipitation → condensation
(C) Evaporation → condensation → precipitation
(D) Condensation → precipitation → evaporation
Answer: (C) Evaporation → condensation → precipitation
Explanation: Sunlight causes liquid water to evaporate. Rising moist air cools until water vapour condenses into droplets, forming clouds. Droplets later fall as precipitation. These phase transitions exchange latent heat and play a central role in Earth’s hydrological and energy cycles.
98. Arrange these stages of the water cycle: 1. Condensation; 2. Evaporation; 3. Precipitation; 4. Collection in water bodies.
[RRB Group D, August 2026]
(A) 3 → 2 → 1 → 4
(B) 1 → 2 → 3 → 4
(C) 2 → 3 → 1 → 4
(D) 2 → 1 → 3 → 4
Answer: (D) 2 → 1 → 3 → 4
Explanation: Water absorbs solar energy and evaporates first. Cooling converts vapour into condensed droplets; precipitation follows when droplets grow heavy enough. Water subsequently collects in rivers, lakes and oceans. The repeated evaporation-condensation sequence involves absorption and release of latent heat.
97. Which substance has the lowest coefficient of thermal conductivity?
[UPSC CDS-I General Knowledge, April 2026]
(A) Hydrogen gas (H₂)
(B) Liquid water (H₂O)
(C) Copper (Cu)
(D) Aluminium (Al)
Answer: (A) Hydrogen gas (H₂)
Explanation: Hydrogen is a gas, with thermal conductivity much lower than liquid water or the listed metals under ordinary conditions. Copper and aluminium contain mobile conduction electrons and transfer heat efficiently. Thermal conductivity quantifies energy transferred through a material per temperature gradient.
96. According to Joule’s heating law, for fixed resistance and time, heat produced in a resistor is proportional to what?
[RRB NTPC Graduate CBT-I, March 2026]
(A) Square of electric current
(B) Square of resistance
(C) Inverse square of current
(D) Inverse square of resistance
Answer: (A) Square of electric current
Explanation: Joule’s law gives H = I²Rt, where I is current, R is resistance and t is time. Holding resistance and duration constant, doubling current produces four times the heat. This principle underlies electric heaters, irons and protective fuses.
95. Why are cotton garments preferred during hot summer weather?
[SSC MTS 2025 paper, February 2026]
(A) They stop sweat from evaporating
(B) They increase body temperature by trapping heat
(C) They absorb sweat and facilitate evaporation
(D) They cannot absorb moisture
Answer: (C) They absorb sweat and facilitate evaporation
Explanation: Cotton fibres readily absorb moisture and spread sweat over a larger surface. Evaporation then requires latent heat, which is taken partly from the skin. The associated energy loss produces a cooling effect, making cotton clothing comfortable in hot weather.
94. Why does a paper cup containing water usually not ignite when heated carefully?
[SSC MTS 2025 paper, February 2026]
(A) Water evaporation immediately extinguishes every flame
(B) Water absorbs heat and holds paper below its ignition temperature
(C) Wet paper becomes permanently fireproof
(D) Steam automatically puts out the fire
Answer: (B) Water absorbs heat and holds paper below its ignition temperature
Explanation: Water has substantial heat capacity and absorbs the supplied energy. While water remains in contact with the paper, its temperature stays relatively low, often near its boiling point. Thus, the paper may not reach the temperature required for ignition.
93. At normal atmospheric pressure, what is the temperature at which a solid begins to turn into a liquid called?
[SSC MTS 2025 paper, February 2026]
(A) Boiling point
(B) Melting point
(C) Absolute point
(D) Vaporisation point
Answer: (B) Melting point
Explanation: The melting point is the temperature at which solid and liquid phases can coexist in equilibrium at a specified pressure. Added heat during melting mainly overcomes particle attractions rather than raising temperature. Different substances possess different characteristic melting points.
92. People sprinkle water on a sun-heated roof mainly to take advantage of which effect?
[SSC MTS 2025 paper, February 2026]
(A) Heating of the roof by incoming water
(B) Increase in the surface temperature
(C) Large latent heat of vaporisation of water
(D) Increased slipperiness of the surface
Answer: (C) Large latent heat of vaporisation of water
Explanation: Water absorbs substantial energy while evaporating from a hot surface. Its latent heat of vaporisation is drawn from the roof and nearby air, lowering their temperature. This cooling mechanism also operates when sweat evaporates and when water is stored in porous earthen pots.
91. What produces water droplets on the outside of a glass containing ice-cold water?
[SSC MTS 2025 paper, February 2026]
(A) Evaporation of the water inside the glass
(B) Condensation of water vapour present in surrounding air
(C) Absorption of moisture through solid glass
(D) Heating of the air next to the glass
Answer: (B) Condensation of water vapour present in surrounding air
Explanation: Warm ambient air contains water vapour. At the cold glass surface, nearby air cools, possibly below its dew point, causing vapour to condense into droplets. The moisture therefore originates outside the glass, not from water penetrating the vessel.
90. Why do droplets appear on the outside of a cold drinking glass on a warm day?
[RRB Group D, February 2026]
(A) Freezing
(B) Sublimation
(C) Condensation
(D) Evaporation
Answer: (C) Condensation
Explanation: The glass cools adjacent humid air. When this air reaches its dew point, gaseous water changes into liquid droplets on the outer surface. Condensation releases latent heat. The glass does not leak; the deposited moisture comes from atmospheric water vapour.
89. Evaporation of a liquid can occur at which temperature?
[RRB Group D, February 2026]
(A) Only below its freezing point
(B) Only at 100°C
(C) Only at the boiling point
(D) At any temperature where the liquid exists
Answer: (D) At any temperature where the liquid exists
Explanation: Evaporation occurs at a liquid’s exposed surface whenever some molecules possess enough energy to escape. Unlike boiling, it does not require the entire liquid to reach its boiling point. Higher temperature generally accelerates the process by increasing molecular kinetic energy.
88. Which change does NOT increase the rate of evaporation?
[RRB Group D, February 2026]
(A) Increasing exposed surface area
(B) Increasing wind speed
(C) Decreasing temperature
(D) Decreasing humidity
Answer: (C) Decreasing temperature
Explanation: Lowering temperature reduces the average kinetic energy of liquid molecules and generally slows their escape from the surface. A larger surface, stronger airflow and drier air normally increase evaporation. The precise rate depends on the liquid and surrounding conditions.
87. Which change normally increases evaporation by exposing more liquid molecules to air?
[RRB Group D, February 2026]
(A) Reducing temperature
(B) Increasing atmospheric humidity
(C) Reducing wind speed
(D) Increasing the liquid’s surface area
Answer: (D) Increasing the liquid’s surface area
Explanation: Evaporation is a surface process. A wider exposed area allows more molecules to escape into the atmosphere during a given interval. This explains why wet clothes spread on a line dry faster than when they remain bundled together.
86. What should be done to evaporate water more quickly?
[RRB Group D, January 2026]
(A) Cover the container
(B) Use a deep, narrow vessel
(C) Keep it in a cool, humid room
(D) Raise its temperature and use a fan
Answer: (D) Raise its temperature and use a fan
Explanation: Warmer water has more energetic molecules, so more can overcome surface attraction. A fan replaces moisture-rich air with drier moving air and prevents vapour accumulation above the surface. Both changes promote more rapid evaporation compared with a covered container.
85. Why does skin feel cooler when sweat evaporates?
[RRB Group D, January 2026]
(A) Sweat crystallises on skin
(B) Sweat freezes before vaporising
(C) Evaporating sweat absorbs heat from the skin
(D) Evaporating sweat releases heat to the skin
Answer: (C) Evaporating sweat absorbs heat from the skin
Explanation: Vaporisation requires latent heat. Sweat obtains this energy from its surroundings, including the body surface, so the skin loses heat and feels cooler. In humid weather evaporation slows, which is why sweating becomes a less effective cooling mechanism.
84. What is the likely sequence when a typical solid is continuously heated?
[RRB Group D, January 2026]
(A) It expands and immediately becomes gas
(B) It always sublimates directly
(C) It continually contracts
(D) It may melt and later vaporise
Answer: (D) It may melt and later vaporise
Explanation: Heating generally raises particle kinetic energy until the solid reaches its melting point. Added energy then changes the phase into liquid. Continued heating can bring the liquid to its boiling point, where it changes into vapour; exceptions include subliming substances.
83. Ice has latent heat of fusion 3.34 × 10⁵ J/kg. How much heat melts 0.20 kg of ice at 0°C without raising its temperature?
[Delhi Police Head Constable AWO/TPO 2025 paper, January 2026]
(A) 6.68 × 10³ J
(B) 1.67 × 10⁴ J
(C) 3.34 × 10⁴ J
(D) 6.68 × 10⁴ J
Answer: (D) 6.68 × 10⁴ J
Explanation: Use Q = mL because the ice changes phase at constant temperature. Multiplying 0.20 kg by 3.34 × 10⁵ J/kg gives 6.68 × 10⁴ J. This energy disrupts the ice crystal structure rather than heating the meltwater.
82. Which heat-transfer mechanism works without any material medium, including through a vacuum?
[Delhi Police Head Constable AWO/TPO 2025 paper, January 2026]
(A) Only conduction
(B) Only convection
(C) Both conduction and convection
(D) Radiation
Answer: (D) Radiation
Explanation: Thermal radiation consists of electromagnetic waves and can travel through empty space. Conduction involves microscopic energy transfer through matter, while convection needs movement of a liquid or gas. Consequently, solar heat reaches Earth across the near-vacuum of space through radiation.
81. One kilojoule of heat energy is equal to how many joules?
[SSC MTS 2025 paper, February 2026]
(A) 10
(B) 100
(C) 1,000
(D) 10,000
Answer: (C) 1,000
Explanation: The SI prefix kilo means one thousand. A kilojoule is therefore 10³ joules. The joule is the SI unit for heat, work and energy, whereas the watt measures energy transfer per second and is a unit of power.
80. At the critical point of a pure substance, what happens to saturated liquid and saturated vapour?
[SSC JE Mechanical, December 2025]
(A) Latent heat becomes infinite
(B) Latent heat remains finite and nonzero
(C) Enthalpy becomes zero
(D) Their properties become identical
Answer: (D) Their properties become identical
Explanation: At the critical point, the distinction between saturated liquid and its vapour disappears. Their densities and other properties converge, and the latent heat of vaporisation approaches zero. Beyond the critical point there is no ordinary liquid-vapour phase boundary.
79. For adiabatic compression, how much heat crosses the system boundary?
[SSC JE Mechanical, December 2025]
(A) Maximum heat transfer
(B) Minimum positive heat transfer
(C) Zero heat transfer
(D) Very high heat transfer
Answer: (C) Zero heat transfer
Explanation: An adiabatic process is defined by zero heat exchange with the surroundings, Q = 0. Temperature may still rise during compression because work done on the gas increases its internal energy. Adiabatic does not mean constant temperature or zero work.
78. The energy needed to change one kilogram of a solid into liquid at its melting point is called the ____ heat of fusion.
[SSC CHSL Tier-I, November 2025]
(A) Latent
(B) Boiling
(C) Melting
(D) Absolute
Answer: (A) Latent
Explanation: Specific latent heat of fusion measures the energy needed to melt unit mass at constant temperature. During melting, heat changes molecular arrangement rather than producing a temperature rise. For an arbitrary mass, the required energy equals mass multiplied by specific latent heat.
77. What is the direct change from gas to solid without forming an intermediate liquid called?
[SSC CHSL Tier-I, November 2025]
(A) Solidification
(B) Deposition
(C) Freezing
(D) Evaporation
Answer: (B) Deposition
Explanation: Deposition converts a vapour straight into a solid without passing through the liquid phase. Frost forming from atmospheric water vapour is an example. The reverse process, solid changing directly to gas, is sublimation. Both processes exchange latent heat.
76. Which examples demonstrate cooling by evaporation:
I. Acetone applied to the palm;
II. Water sprinkled on a hot roof?
[SSC CHSL Tier-I, November 2025]
(A) Only I
(B) Only II
(C) Both I and II
(D) Neither I nor II
Answer: (C) Both I and II
Explanation: Acetone rapidly evaporates, extracting latent heat from the skin. Water evaporating from a sun-heated roof similarly removes thermal energy from that surface. In both examples a liquid changes into vapour while drawing heat from its surroundings.
75. Why can steam at 100°C cause worse burns than liquid water at the same temperature?
[SSC CGL Tier-I, September 2025]
(A) Steam is hotter than boiling water
(B) Steam contains more latent heat of vaporisation
(C) Steam has a higher specific heat capacity
(D) Steam molecules are moving faster
Answer: (B) Steam contains more latent heat of vaporisation
Explanation: Steam contains latent heat acquired during vaporisation. When it touches cooler skin, some steam condenses and releases this extra energy in addition to sensible heat. Consequently, steam at the same measured temperature can transfer substantially more heat than boiling water.
74. Why are the bases of many cooking utensils made from copper?
[SSC CGL Tier-I, September 2025]
(A) Copper is a strong thermal insulator
(B) Copper melts at very low temperatures
(C) Copper conducts heat well, helping distribute it evenly
(D) Copper must chemically react with food
Answer: (C) Copper conducts heat well, helping distribute it evenly
Explanation: Copper has high thermal conductivity because electrons transport energy efficiently through the metal. A copper layer spreads burner heat across the base and reduces local hotspots. The good conductor therefore promotes rapid, more uniform heating of the cooking vessel.
73. Which statement best describes a usual urban heat-island cause?
[SSC CGL Tier-I, September 2025]
(A) High humidity alone
(B) Extensive tree cover
(C) Dense concrete surfaces, limited vegetation and waste heat
(D) High elevation above sea level
Answer: (C) Dense concrete surfaces, limited vegetation and waste heat
Explanation: Buildings and paved surfaces absorb and store solar energy, while limited vegetation reduces cooling by transpiration. Traffic, air conditioners and other activities release additional heat. Together, these effects can make heavily built-up urban areas warmer than nearby rural surroundings.
72. Regarding pressure cookers, which statements are correct?
I. They lower pressure and lower boiling point;
II. Increased pressure raises water’s boiling point;
III. Higher-temperature steam helps food cook faster.
[SSC Stenographer, August 2025]
(A) Only II and III
(B) All three
(C) Only I and II
(D) Only I and III
Answer: (A) Only II and III
Explanation: A pressure cooker traps steam, raising the internal pressure. Water can then remain liquid above its ordinary 100°C boiling point, so cooking occurs at a higher temperature. Statement I reverses the pressure effect; statements II and III express the operating principle.
71. Boiling and evaporation are respectively which types of phenomena?
[SSC GD Constable, February 2025]
(A) Bulk and bulk
(B) Surface and bulk
(C) Surface and surface
(D) Bulk and surface
Answer: (D) Bulk and surface
Explanation: Boiling produces vapour bubbles throughout a liquid once its vapour pressure matches the surrounding pressure. Evaporation instead occurs at the exposed surface and can happen below boiling point. The distinction also explains why boiling has a characteristic temperature at fixed pressure.
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.
60. Which situation most directly follows Newton’s law of cooling?
[UPSC NDA-II GAT, September 2024]
(A) Melting of ice at constant temperature
(B) Cooling of hot coffee in a room
(C) Boiling water at constant pressure
(D) Expansion of a heated metal rod
Answer: (B) Cooling of hot coffee in a room
Explanation: Newton’s law of cooling relates the rate of heat loss approximately to the temperature difference between an object and its surroundings, under suitable conditions. A hot cup of coffee therefore cools quickly initially, then more slowly as it approaches room temperature.
59. Which example represents convection of heat?
[UPSC CDS-II General Knowledge, September 2024]
(A) Microwave heating of food
(B) Water circulating while boiling in a pot
(C) Feeling solar warmth outdoors
(D) Heating the far end of a brass rod
Answer: (B) Water circulating while boiling in a pot
Explanation: In boiling water, warmer regions become less dense and rise, while cooler water descends. This bulk fluid movement transfers energy by convection. Heat spreading along a stationary metal rod is conduction; warmth from sunlight is received by radiation.
58. Which statement about water is incorrect?
[UPSC NDA-I GAT, April 2024]
(A) Water has relatively high specific heat
(B) The latent heat of fusion of water is very low
(C) Liquid water is denser than ice
(D) Pure water is a poor electrical conductor
Answer: (B) The latent heat of fusion of water is very low
Explanation: Water has unusually high specific heat and significant latent heat of fusion. These properties moderate changes of temperature and allow ice to absorb considerable energy while melting. Ice is less dense than water, so it generally floats on the surface.
57. At what temperature do Celsius and Fahrenheit scales show the same numerical reading?
[SSC MTS, October 2024]
(A) 40°
(B) 32°
(C) −40°
(D) −32°
Answer: (C) −40°
Explanation: The conversion is F = 9C/5 + 32. Setting Fahrenheit and Celsius readings equal yields C = −40. Thus −40°C and −40°F represent the same temperature; the scales have different zero points and degree sizes.
56. What is the name given to −273.15°C?
[SSC MTS, October 2024]
(A) Absolute zero
(B) Standard temperature
(C) Zero-point energy
(D) Freezing point of water
Answer: (A) Absolute zero
Explanation: Absolute zero is 0 kelvin, equivalent to −273.15°C. It represents the lower limit of thermodynamic temperature. Quantum mechanical systems retain zero-point motion, so absolute zero should not be interpreted as every atom becoming completely motionless.
55. What is the direct transition from a solid to a gas called?
[SSC MTS, November 2024]
(A) Sublimation
(B) Fusion
(C) Evaporation
(D) Condensation
Answer: (A) Sublimation
Explanation: Sublimation bypasses the liquid phase as solid particles escape into the gaseous state. Camphor, dry ice and iodine provide familiar examples under suitable conditions. The reverse process, vapour depositing directly as solid, is called deposition.
54. Identify the incorrect statement concerning evaporation and boiling.
[SSC MTS, October 2024]
(A) Boiling is a bulk phenomenon
(B) Both boiling and evaporation take place only throughout the liquid
(C) Evaporation is a surface phenomenon
(D) Evaporation can cause cooling
Answer: (B) Both boiling and evaporation take place only throughout the liquid
Explanation: Boiling forms vapour bubbles throughout a liquid; evaporation occurs only from its exposed surface. Evaporation can proceed below the boiling point and removes energetic molecules, producing cooling. Therefore describing both mechanisms as exclusively bulk processes is incorrect.
53. Why is water stored in an earthen pot often cooler? Identify the incorrect explanation.
[SSC CGL Tier-I, September 2024]
(A) Water evaporates from its outer surface
(B) The pot contains fine pores
(C) Atmospheric water vapour enters the pot and provides the cooling
(D) Some water seeps through pores and evaporates
Answer: (C) Atmospheric water vapour enters the pot and provides the cooling
Explanation: Tiny pores allow a small amount of water to seep to the pot’s outside. As that moisture evaporates, it absorbs latent heat from the remaining water and vessel. The cooling does not depend on outside vapour entering the pot.
52. What is horizontal heat transfer associated with the movement of air called?
[SSC Selection Post, June 2024]
(A) Conduction
(B) Advection
(C) Variation
(D) Convection only
Answer: (B) Advection
Explanation: Advection transports heat horizontally with moving air or water. Convection commonly describes vertical circulation driven by density differences, while conduction transfers energy through microscopic collisions. In the atmosphere, warm and cold air advection strongly influence local temperature changes.
51. Which process is the reverse of solidification?
[SSC CPO, June 2024]
(A) Fusion or melting
(B) Vaporisation
(C) Sublimation
(D) Condensation
Answer: (A) Fusion or melting
Explanation: Solidification changes liquid into solid and generally releases latent heat. Melting, also termed fusion, reverses this transition and requires an energy input at the melting temperature. Condensation and vaporisation concern the liquid-gas boundary rather than the solid-liquid boundary.
50. Which thermodynamic law provides the basis for temperature measurement and thermal equilibrium?
[SSC JE Mechanical, June 2024]
(A) Zeroth law
(B) First law
(C) Second law
(D) Third law
Answer: (A) Zeroth law
Explanation: The zeroth law states that if two systems are each in thermal equilibrium with a third system, they are in equilibrium with each other. It makes temperature a consistent property and justifies using thermometers to compare thermal states.
49. According to the second law of thermodynamics, how are work and heat commonly classified?
[SSC JE Mechanical, June 2024]
(A) Work is high-grade energy and heat is low-grade energy
(B) Heat is high-grade and work is low-grade
(C) Both are always high-grade
(D) Both are always low-grade
Answer: (A) Work is high-grade energy and heat is low-grade energy
Explanation: Mechanical work can, in principle, be converted completely into heat. A heat engine, however, cannot convert all heat obtained from a single reservoir into useful net work. This limitation leads engineers to describe work as higher-grade energy than heat.
48. For a good electrical heating element, which are desirable?
I. Low resistivity;
II. High melting point;
III. Small temperature coefficient of resistance;
IV. High specific heat.
[SSC JE Electrical, June 2024]
(A) II, III and IV
(B) I, II and IV
(C) Only II and III
(D) I, II and III
Answer: (C) Only II and III
Explanation: Heating elements should resist melting during operation and maintain relatively stable electrical resistance as temperature changes. High resistivity is generally desirable; low resistivity is not. Excessively high heat capacity delays warming, so only statements II and III are preferred.
47. Who established the quantitative mechanical equivalent of heat through experiments?
[SSC CPO, June 2024]
(A) William Henry
(B) James Prescott Joule
(C) Hermann von Helmholtz
(D) John Dalton
Answer: (B) James Prescott Joule
Explanation: James Prescott Joule demonstrated that mechanical work could raise water temperature, linking work and heat quantitatively. His paddle-wheel experiments supported energy conservation. The SI unit of energy, the joule, honours his contribution to thermodynamics.
46. Which of these is NOT merely a physical change?
[SSC CGL Tier-I, September 2024]
(A) Heating iron until red-hot
(B) Conversion of milk into curd
(C) Evaporation of diesel
(D) Sublimation of ammonium chloride
Answer: (B) Conversion of milk into curd
Explanation: Curdling involves biochemical conversion of milk components and formation of new products, so it is treated as a chemical change. Heating, evaporation and sublimation can occur without changing chemical identity, although the substance’s temperature or physical state changes.
45. Which scientist introduced the term zeroth law of thermodynamics?
[SSC CGL Tier-I, July 2023]
(A) James Clerk Maxwell
(B) Max Planck
(C) Josiah Willard Gibbs
(D) Ralph H. Fowler
Answer: (D) Ralph H. Fowler
Explanation: Ralph H. Fowler formally introduced the expression zeroth law in the twentieth century. It describes transitivity of thermal equilibrium and underlies meaningful temperature comparison. Although named after the first three laws, it is logically more fundamental to thermometry.
44. A thermometer works on which thermodynamic law?
[SSC JE Mechanical, October 2023]
(A) Second law
(B) Joule’s law
(C) Third law
(D) Zeroth law
Answer: (D) Zeroth law
Explanation: A thermometer reaches thermal equilibrium with an object whose temperature is being measured. By the zeroth law, systems in equilibrium with the same thermometer have the same temperature. This lets a calibrated thermometer indicate the thermal state of another body.
43. Which law rules out a heat engine operating at 100 percent thermal efficiency between ordinary reservoirs?
[SSC JE Mechanical, December 2023]
(A) Third law
(B) First law
(C) Second law
(D) Zeroth law
Answer: (C) Second law
Explanation: The second law requires an engine operating cyclically between heat reservoirs to reject some heat rather than convert all absorbed heat into work. The first law addresses energy conservation; it does not by itself prohibit a perfectly efficient heat engine.
42. Which claim about heat and work in thermodynamics is incorrect?
[SSC JE Mechanical, December 2023]
(A) A system possesses heat and work as stored properties
(B) Heat and work are path-dependent quantities
(C) Both are associated with processes rather than states
(D) Heat and work cross the system boundary
Answer: (A) A system possesses heat and work as stored properties
Explanation: A system stores internal energy, not heat or work. Heat and work describe two ways energy crosses the system boundary while a process occurs. Their amounts depend on the path followed, whereas internal energy is a state function.
41. Which transition does NOT describe latent heat of vaporisation?
[SSC JE Mechanical, October 2023]
(A) Saturated liquid turning to vapour
(B) Ice melting into liquid water
(C) Water becoming steam at its boiling point
(D) Liquid becoming gas without temperature change
Answer: (B) Ice melting into liquid water
Explanation: Latent heat of vaporisation applies to the liquid-to-gas transition at the saturation temperature. Melting ice requires latent heat of fusion, which concerns a solid-liquid phase change. During either ideal phase transition, supplied energy does not raise the temperature.
40. Calorimetry is primarily concerned with measuring what?
[SSC MTS, September 2023]
(A) Compressive strength
(B) Mechanical force
(C) Electrical current
(D) Heat exchanged in a process
Answer: (D) Heat exchanged in a process
Explanation: Calorimetry determines the amount of heat absorbed or released during physical or chemical processes. By recording temperature changes and knowing heat capacities, one can evaluate energy transfer. Carefully insulated calorimeters reduce unwanted exchange with the environment.
39. Chloroform boils near 334 K and methane near 111 K. Which stated boiling temperatures are correct?
[SSC CGL Tier-I, July 2023]
(A) Chloroform only
(B) Neither
(C) Methane only
(D) Both
Answer: (D) Both
Explanation: Chloroform has a normal boiling point near 61°C, which equals approximately 334 K. Methane boils near −162°C, approximately 111 K, at atmospheric pressure. The disparity reflects major differences in molecular interactions and volatility between the two compounds.
38. Who investigated carbon dioxide near its critical temperature and pressure in the nineteenth century?
[SSC CHSL Tier-I, August 2023]
(A) Thomas Andrews
(B) Giulio Natta
(C) John Dalton
(D) Marguerite Perey
Answer: (A) Thomas Andrews
Explanation: Thomas Andrews performed influential nineteenth-century experiments on carbon dioxide. He showed how the distinction between liquid and gas disappears near a critical point. His work established foundational ideas about phase behaviour, critical temperature and pressure in thermodynamics.
37. A substance absorbs 20 kJ of heat and warms by 10°C. Its specific heat is 400 J/kg°C. What is its mass?
[UPSC NDA-I GAT, April 2022]
(A) 0.1 kg
(B) 1 kg
(C) 10 kg
(D) 5 kg
Answer: (D) 5 kg
Explanation: The heat equation is Q = mcΔT. Converting 20 kJ to 20,000 J and substituting c = 400 J/kg°C and ΔT = 10°C gives m = 20,000/(400 × 10) = 5 kg.
36. Which conditions generally cause a liquid to evaporate faster?
[UPSC NDA-I GAT, April 2022]
(A) Higher temperature and greater exposed surface area
(B) Lower temperature and smaller exposed area
(C) Higher humidity and lower wind speed
(D) Higher humidity and smaller surface area
Answer: (A) Higher temperature and greater exposed surface area
Explanation: Heating increases the fraction of molecules energetic enough to escape a liquid surface. A broader surface permits more escaping molecules at once. Low humidity and moving air can further enhance evaporation by removing vapour from the region near the surface.
35. What is the latent heat of vaporisation associated with?
[UPSC NDA-I GAT, April 2022]
(A) Liquid becoming vapour with increasing temperature
(B) Liquid becoming vapour at constant temperature
(C) Vapour becoming liquid with temperature increase
(D) Vapour becoming liquid with temperature decrease only
Answer: (B) Liquid becoming vapour at constant temperature
Explanation: At the boiling temperature under fixed pressure, heat can convert a liquid into vapour without changing temperature. This energy is latent heat of vaporisation. It increases molecular separation and potential energy rather than raising mean molecular kinetic energy.
34. Which claim about sound speed in air is incorrect?
[UPSC NDA-II GAT, September 2022]
(A) Sound normally travels faster in steel than water
(B) Sound speed in air decreases as temperature increases
(C) Sound speed in air increases as temperature increases
(D) Sound travels faster in water than air
Answer: (B) Sound speed in air decreases as temperature increases
Explanation: For an ideal gas, sound speed is approximately proportional to the square root of absolute temperature. As air warms, the speed of sound therefore rises. Typical solids transmit sound faster than liquids, and liquids faster than gases, though exceptions exist.
33. Which pair consists of especially good conductors of heat?
[UPSC CDS-II General Knowledge, September 2022]
(A) Silver and copper
(B) Silver and lead
(C) Copper and mercury
(D) Lead and mercury
Answer: (A) Silver and copper
Explanation: Silver and copper have exceptionally high thermal conductivity among familiar metals. Their abundant mobile electrons rapidly carry thermal energy. Lead and mercury conduct less effectively, so the pair containing silver and copper best satisfies the question.
32. Earth’s atmosphere is heated most directly from below by which radiation?
[UPPSC State Services Prelims, June 2022]
(A) Long-wave radiation emitted by Earth
(B) Scattered solar radiation
(C) Reflected solar radiation
(D) Incoming short-wave solar radiation
Answer: (A) Long-wave radiation emitted by Earth
Explanation: Solar radiation warms Earth’s surface, which subsequently emits long-wave infrared energy. Greenhouse gases absorb and re-emit part of this outgoing radiation, heating the lower atmosphere. The atmosphere is relatively transparent to much incoming short-wave sunlight.
31. The term thermal transmittance is also known as what?
[MPPSC Assistant Engineer Mechanical, July 2022]
(A) Energy
(B) Mass transfer
(C) Radiation
(D) Overall heat-transfer coefficient
Answer: (D) Overall heat-transfer coefficient
Explanation: Thermal transmittance, commonly denoted U, measures steady heat flow through an assembly per unit area per unit temperature difference. Its SI unit is W/m²K. It combines effects of conduction and boundary convection, making it useful in insulation and heat-exchanger analysis.
30. Why is the logarithmic mean temperature difference used in heat-exchanger calculations?
[MPPSC Assistant Engineer Mechanical, July 2022]
(A) Heat-flow rate varies along the exchanger
(B) Fluid temperatures change from place to place
(C) Both of the above
(D) Neither of the above
Answer: (C) Both of the above
Explanation: As two fluids travel through a heat exchanger, their temperatures and local temperature difference usually vary with position. The logarithmic mean gives a suitable effective temperature difference for integrated heat-transfer calculations. It is used with Q = UAΔT_lm.
29. Why is the freezer compartment traditionally positioned near the top of a refrigerator?
[WBCS Preliminary Examination, August 2021]
(A) It is easiest to access
(B) It lowers manufacturing cost
(C) Cooling occurs only through conduction
(D) It promotes convection currents
Answer: (D) It promotes convection currents
Explanation: Cold air near the freezer is denser and descends; warmer air elsewhere rises to replace it. Natural convection distributes cooling through the interior. This is particularly important in older direct-cool refrigerators without fans circulating the air mechanically.
28. What is the approximate boiling point of liquid nitrogen at atmospheric pressure?
[RRB NTPC CBT-I, February 2021]
(A) −27°C
(B) −132°C
(C) −273°C
(D) −196°C
Answer: (D) −196°C
Explanation: Nitrogen boils at about 77 K, or −196°C, at standard atmospheric pressure. Its low boiling point makes it useful for cryogenic cooling and preserving biological material. The value depends on pressure, as with the boiling point of any liquid.
27. What is the usual temperature range of a clinical thermometer?
[SSC MTS 2020 paper, October 2021]
(A) 35°C to 42°C
(B) 32°C to 38°C
(C) 38°C to 48°C
(D) 25°C to 32°C
Answer: (A) 35°C to 42°C
Explanation: Traditional clinical thermometers are calibrated over approximately 35°C to 42°C, the relevant range for human body temperature. Laboratory thermometers cover much wider ranges. Digital clinical instruments may have different technical limits, but standard school-textbook coverage uses 35–42°C.
26. Which device is NOT primarily based on Joule heating?
[RRB NTPC CBT-I, April 2021]
(A) Electric iron
(B) Electric plug and switch
(C) Electric kettle
(D) Electric fuse
Answer: (B) Electric plug and switch
Explanation: An iron and kettle intentionally heat a resistive element, while a fuse melts when excessive current generates heat. Plugs and switches mainly establish, interrupt or route electrical contact; Joule heating in them is unwanted energy loss rather than their intended operating principle.
25. Consider furnace oil:
1. It is a refinery product;
2. Industries can use it to generate power;
3. Burning it can emit sulphur compounds.
Which statements are correct?
[UPSC Civil Services Prelims, October 2021]
(A) 1 and 2 only
(B) 2 and 3 only
(C) 1 and 3 only
(D) 1, 2 and 3
Answer: (D) 1, 2 and 3
Explanation: Furnace oil is obtained during crude-oil refining and burned in industrial boilers and power-generating equipment. Its chemical energy becomes heat through combustion. Sulphur compounds in the fuel may produce sulphur dioxide, contributing to air pollution if emissions are uncontrolled.
24. What does the Kelvin–Planck statement of the second law forbid?
[MPPSC Assistant Engineer Mechanical, 2021]
(A) Heat flowing from cold to hot without work only
(B) A cyclic engine producing net work while taking heat from only one reservoir
(C) Conversion of work to heat
(D) Conversion of some heat into work
Answer: (B) A cyclic engine producing net work while taking heat from only one reservoir
Explanation: A cyclic heat engine cannot turn all heat from a single thermal reservoir into net useful work without another effect. A heat sink is necessary. This formulation of the second law explains why ideal thermal efficiency cannot equal 100 percent.
23. For a closed system undergoing a complete thermodynamic cycle, what is the cyclic integral of (δQ − δW)?
[SSC JE Mechanical, October 2020]
(A) Positive
(B) Negative
(C) Zero
(D) Impossible to determine
Answer: (C) Zero
Explanation: The first law gives dU = δQ − δW for a closed system. Internal energy U is a state function, so its total change over a complete cycle equals zero. Therefore the cyclic integral of heat input minus work output vanishes.
22. Why is the thermal efficiency of a real Otto cycle lower than that of an ideal constant-specific-heat Otto cycle?
[RPSC Lecturer Technical Education Mechanical, 2020]
(A) The specific-heat ratio increases sharply with temperature
(B) The specific-heat ratio decreases as temperature rises
(C) The specific heat always falls with rising temperature
(D) Specific heats never vary with temperature
Answer: (B) The specific-heat ratio decreases as temperature rises
Explanation: In an actual high-temperature working gas, heat capacities change with temperature and the ratio γ = Cp/Cv often decreases. This reduces efficiency relative to an air-standard Otto-cycle model with constant γ, along with other real losses.
21. Why does liquid kept in a vacuum thermos remain hot or cold for a long period?
[UPPSC State Services Prelims, 2019]
(A) Conduction is reduced only
(B) Convection and radiation are reduced only
(C) Conduction, convection and radiation are all reduced
(D) None of these heat transfers matter
Answer: (C) Conduction, convection and radiation are all reduced
Explanation: A vacuum between double walls greatly suppresses conduction and convection. Reflective silvered surfaces reduce radiative exchange, and an insulating stopper limits heat leakage at the mouth. Because all three modes are restricted, the contents change temperature slowly.
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.
10. What is the heat required to change liquid into vapour without temperature change called?
[UPSC NDA-I GAT, 2017]
(A) Specific heat capacity
(B) Latent heat of vaporisation
(C) Latent heat of fusion
(D) Thermal expansion
Answer: (B) Latent heat of vaporisation
Explanation: During vaporisation at the saturation temperature, absorbed heat overcomes intermolecular attraction and changes the phase rather than raising temperature. That energy is latent heat of vaporisation. Melting a solid instead involves latent heat of fusion.
9. Why does water boil at a lower temperature at high altitudes?
[UPSC NDA-II GAT, 2017]
(A) Atmospheric pressure is lower
(B) Specific heat of water becomes zero
(C) Gravity is absent
(D) Water loses its latent heat
Answer: (A) Atmospheric pressure is lower
Explanation: As altitude increases, external atmospheric pressure usually decreases. Water boils when its vapour pressure equals surrounding pressure, so this condition is reached at a lower temperature. Pressure cookers exploit the opposite effect by increasing pressure and raising boiling temperature.
8. By what mechanism does heat from the Sun reach Earth?
[WBCS Preliminary Examination, 2016]
(A) Conduction
(B) Convection
(C) Radiation
(D) None of the above
Answer: (C) Radiation
Explanation: Space between Sun and Earth is nearly a vacuum, so heat cannot cross it by fluid convection or ordinary conduction. The Sun emits electromagnetic radiation, including visible and infrared wavelengths, which transfers energy through space to Earth.
7. Which quantity is NOT a unit of heat?
[MPPSC State Services General Studies, 2016]
(A) Calorie
(B) Kilocalorie
(C) Kilojoule
(D) Watt
Answer: (D) Watt
Explanation: Heat is energy and can be expressed in calories, kilocalories or joules. The watt instead measures power, meaning energy transfer per second. One watt equals one joule per second, so it is not itself a unit of heat energy.
6. Why are inner surfaces of a vacuum flask silvered?
[UPSC NDA-II GAT, 2015]
(A) To stop convection only
(B) To stop conduction only
(C) To stop conduction and convection only
(D) To reduce heat transfer by radiation
Answer: (D) To reduce heat transfer by radiation
Explanation: Shiny metallic coatings reflect much of the thermal infrared radiation exchanged between flask walls. The vacuum suppresses conduction and convection separately. Together these design features reduce all major avenues of heat exchange and preserve the liquid’s temperature.
5. Which is the SI unit of thermal conductivity?
[UPSC NDA GAT, 2015]
(A) W m⁻¹ K⁻¹
(B) J kg⁻¹ K⁻¹
(C) W m⁻² K⁻¹
(D) J K⁻¹
Answer: (A) W m⁻¹ K⁻¹
Explanation: Fourier’s law relates heat flow through a material to conductivity, cross-sectional area and temperature gradient. Rearranging its SI units yields watts per metre per kelvin. Units of specific heat capacity and heat-transfer coefficient are different.
4. An isentropic thermodynamic process is ideally which of the following?
[CGPSC Assistant Engineer, April 2014]
(A) Adiabatic and irreversible
(B) Adiabatic and frictionless or reversible
(C) Reversible and isothermal only
(D) Any adiabatic process
Answer: (B) Adiabatic and frictionless or reversible
Explanation: An isentropic process has constant entropy. An internally reversible adiabatic process satisfies this condition because no heat enters and no entropy is produced. Friction and other irreversibilities generally create entropy, even when an actual process is adiabatic.
3. Where do conduction, convection and radiation all play major roles in heat transfer?
[UPPSC Assistant Engineer Mechanical, 2013]
(A) Insulated hot-water pipe only
(B) Refrigerator cooling coil only
(C) Boiler furnace
(D) Steam condenser only
Answer: (C) Boiler furnace
Explanation: A boiler furnace receives radiant energy from flames, transfers heat by convection from hot gases and moves energy through metal surfaces by conduction. Different heat-transfer modes cooperate throughout the equipment, making the furnace a clear combined example.
2. Which instrument is suited for measuring high temperatures through emitted thermal radiation?
[AIEEE Physics, 2002]
(A) Spectrometer
(B) Pyrometer
(C) Nanometer
(D) Photometer
Answer: (B) Pyrometer
Explanation: A radiation pyrometer estimates an object’s temperature from emitted thermal radiation, often without direct contact. It is useful for furnaces and other very hot surfaces where an ordinary contact thermometer may fail. Radiation intensity depends strongly on absolute temperature and emissivity.
1. An ideal gas is carried inside a lorry moving uniformly along a level road. With no other changes, what happens to its temperature?
[AIEEE Physics, 2002]
(A) It increases
(B) It decreases
(C) It remains unchanged
(D) It must first rise then fall
Answer: (C) It remains unchanged
Explanation: Uniform translational motion of the whole sealed gas container does not change random molecular kinetic energy relative to the container. Temperature measures the microscopic thermal state, not the lorry’s overall speed. Without work, heating or other changes, temperature stays the same.