Heat and its Transmission — GK Questions with Answers

1. Which is the correct relation between K and °C?
   1. K = °C − 273
   2. K = °C + 273
   3. K = °C × 2
   4. K = °C ÷ 2
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   Answer: (B) K = °C + 273

   The Kelvin scale starts from absolute zero (−273°C), so K = °C + 273 (exactly 273.15).

2. At what temperature do Celsius and Fahrenheit thermometers show the same reading?
   1. 0°
   2. −273°
   3. 100°
   4. −40°
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   Answer: (D) −40°

   Putting C = F gives C = 1.8C + 32 → C = −40. So −40°C = −40°F.

3. Normal human body temperature, 98.6°F, is how much in Celsius?
   1. 37°C
   2. 36°C
   3. 38°C
   4. 39°C
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   Answer: (A) 37°C

   C = (98.6 − 32) × 5/9 = 37°C.

4. The temperature of a very hot body like the Sun is measured with a —
   1. mercury thermometer
   2. clinical thermometer
   3. pyrometer
   4. gas thermometer
   Show answer

   Answer: (C) pyrometer

   A pyrometer measures temperature from the radiation of a body and need not touch it — right for very high temperatures.

5. Why are alcohol thermometers used instead of mercury in very cold regions?
   1. alcohol is cheaper
   2. alcohol is shiny
   3. mercury freezes at −39°C
   4. mercury is not poisonous
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   Answer: (C) mercury freezes at −39°C

   The freezing point of mercury is about −39°C, while alcohol stays liquid down to about −115°C.

6. The ratio of the linear, superficial and volume expansion coefficients of a solid is —
   1. 1 : 2 : 3
   2. 1 : 1 : 1
   3. 3 : 2 : 1
   4. 1 : 3 : 2
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   Answer: (A) 1 : 2 : 3

   β = 2α and γ = 3α, so α : β : γ = 1 : 2 : 3.

7. Why is a small gap left between railway rails?
   1. for drainage
   2. to save the rail
   3. to reduce noise
   4. for thermal expansion
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   Answer: (D) for thermal expansion

   Rails expand in summer; without a gap they could bend.

8. At what temperature is the density of water greatest?
   1. 0°C
   2. 4°C
   3. 25°C
   4. 100°C
   Show answer

   Answer: (B) 4°C

   Anomalous expansion of water: it contracts from 0 to 4°C and is densest at 4°C.

9. Fish stay alive even when the surface of a lake freezes in winter because —
   1. water at the bottom stays liquid at about 4°C
   2. the water gets warm
   3. fish do not feel cold
   4. ice is a good conductor of heat
   Show answer

   Answer: (A) water at the bottom stays liquid at about 4°C

   Because of the anomalous expansion of water, 4°C water stays at the bottom and the ice on top protects it like an insulator.

10. Why is water used as the coolant in a car radiator?
    1. its boiling point is low
    2. it is light
    3. its specific heat is very high
    4. it conducts electricity
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    Answer: (C) its specific heat is very high

    Water absorbs a lot of heat while warming up only a little.

11. 100 g of water at 80°C is mixed with 200 g of water at 20°C. Final temperature?
    1. 50°C
    2. 40°C
    3. 30°C
    4. 60°C
    Show answer

    Answer: (B) 40°C

    T = (100 × 80 + 200 × 20)/300 = 40°C.

12. Why is a burn from steam worse than one from boiling water at the same temperature?
    1. steam is hotter
    2. steam is poisonous
    3. steam moves faster
    4. steam carries latent heat
    Show answer

    Answer: (D) steam carries latent heat

    Steam at 100°C gives out about 540 extra calories per gram while turning into water.

13. How much heat turns 10 g of ice at 0°C into water at 0°C?
    1. 80 calories
    2. 5400 calories
    3. 540 calories
    4. 800 calories
    Show answer

    Answer: (D) 800 calories

    Q = mL = 10 × 80 = 800 calories. The temperature does not change, only the state.

14. Why does cooking take longer on mountains?
    1. there is less oxygen
    2. at low pressure water boils below 100°C
    3. the air is cold
    4. vessels cool quickly
    Show answer

    Answer: (B) at low pressure water boils below 100°C

    Lower pressure lowers the boiling point, so water boils at a lower temperature and food cooks slowly.

15. On raising pressure, the melting point of ice —
    1. rises
    2. does not change
    3. falls
    4. first rises then falls
    Show answer

    Answer: (C) falls

    Ice contracts on melting, so raising the pressure lowers its melting point — this is regelation.

16. Mixing salt with ice makes the freezing mixture —
    1. go below 0°C
    2. go above 0°C
    3. stay at 0°C
    4. reach 100°C
    Show answer

    Answer: (A) go below 0°C

    Salt lowers the freezing point; an ice–salt mixture cools to about −20°C.

17. When temperature falls (with the same vapour), relative humidity —
    1. falls
    2. rises
    3. does not change
    4. becomes zero
    Show answer

    Answer: (B) rises

    Cold air can hold less vapour, so the same vapour is now nearer saturation — RH rises.

18. If the wet-bulb and dry-bulb readings of a hygrometer are equal, the relative humidity is —
    1. 0%
    2. 50%
    3. cannot say
    4. 100%
    Show answer

    Answer: (D) 100%

    If the air is saturated, no evaporation happens from the wet bulb and both readings stay equal — RH 100%.

19. Which is the best conductor of heat?
    1. diamond
    2. silver
    3. copper
    4. aluminium
    Show answer

    Answer: (A) diamond

    Diamond has the highest thermal conductivity, though it is an electrical insulator. Among metals silver is the best.

20. Why do two thin blankets keep us warmer than one thick blanket?
    1. they weigh more
    2. thin blankets absorb radiation
    3. the air layer between them is an insulator
    4. convection happens in thin blankets
    Show answer

    Answer: (C) the air layer between them is an insulator

    The air trapped between two blankets is a very good insulator of heat.

21. In which direction does the wind blow at the seashore during the day?
    1. from land to sea
    2. no wind blows
    3. from sea to land
    4. from top to bottom
    Show answer

    Answer: (C) from sea to land

    By day the land heats up fast, the air above it rises, and cool sea air moves towards the land — a sea breeze.

22. What does radiation not need in order to travel?
    1. a medium
    2. temperature
    3. time
    4. energy
    Show answer

    Answer: (A) a medium

    Radiation travels as electromagnetic waves and passes even through vacuum.

23. By Stefan’s law, if the absolute temperature of a black body is doubled, the energy given out becomes —
    1. double
    2. four times
    3. eight times
    4. sixteen times
    Show answer

    Answer: (D) sixteen times

    E ∝ T⁴, so 2⁴ = 16 times.

24. By Wien’s law, as temperature rises the wavelength of the strongest radiation —
    1. increases
    2. decreases
    3. does not change
    4. becomes zero
    Show answer

    Answer: (B) decreases

    λ_max × T = constant, so λ_max gets shorter as temperature rises — a hot body’s colour shifts from red towards blue.

25. In a thermos flask, what does the vacuum stop?
    1. conduction and convection
    2. radiation only
    3. convection only
    4. conduction only
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    Answer: (A) conduction and convection

    Vacuum has no particles, so both conduction and convection stop; the silvering reflects radiation.

26. Which radiation gets trapped in the greenhouse effect?
    1. ultraviolet
    2. visible light
    3. infrared
    4. X-rays
    Show answer

    Answer: (C) infrared

    Long-wavelength infrared from the ground cannot get out through greenhouse gases or glass.

27. A thermometer is based on which law of thermodynamics?
    1. first
    2. zeroth
    3. second
    4. third
    Show answer

    Answer: (B) zeroth

    The zeroth law gives thermal equilibrium and the idea of temperature; the thermometer is the "third body".

28. A gas absorbs 800 J of heat and does 300 J of work. Change in its internal energy?
    1. 1100 J
    2. 800 J
    3. 300 J
    4. 500 J
    Show answer

    Answer: (D) 500 J

    First law: ΔU = ΔQ − ΔW = 800 − 300 = 500 J.

29. In an adiabatic process —
    1. temperature stays constant
    2. pressure stays constant
    3. volume stays constant
    4. no heat is exchanged
    Show answer

    Answer: (D) no heat is exchanged

    In an adiabatic process Q = 0; the temperature can change — a gas cools on expansion and heats up on compression.

30. In an isochoric process the work done by the gas is —
    1. maximum
    2. zero
    3. negative
    4. infinite
    Show answer

    Answer: (B) zero

    If the volume does not change, W = PΔV = 0; all the heat raises the internal energy.

31. A Carnot engine works between 600 K and 300 K. Efficiency?
    1. 25%
    2. 75%
    3. 50%
    4. 100%
    Show answer

    Answer: (C) 50%

    η = 1 − 300/600 = 0.5 = 50%.

32. If the door of a running fridge is left open in a closed room, the room temperature will —
    1. rise
    2. fall
    3. stay the same
    4. first fall then return
    Show answer

    Answer: (A) rise

    The fridge takes Q₂ from the room and gives back Q₂ + W; overall the room gains heat equal to W.

33. In natural processes the entropy of the universe —
    1. decreases
    2. increases
    3. stays constant
    4. is zero
    Show answer

    Answer: (B) increases

    Second law: the entropy of an isolated system rises in irreversible processes.

34. At constant temperature, halving the volume of a gas makes its pressure —
    1. half
    2. the same
    3. four times
    4. double
    Show answer

    Answer: (D) double

    Boyle’s law: PV = constant, so halving V doubles P.

35. The rms speed of a gas at 27°C is v. At 927°C the speed will be —
    1. 2v
    2. v
    3. 4v
    4. √2 v
    Show answer

    Answer: (A) 2v

    v ∝ √T; from 300 K to 1200 K is 4 times the temperature → speed doubles.

36. The value of γ = Cₚ/Cᵥ for a monatomic gas is —
    1. 1.4
    2. 1.33
    3. 1.67
    4. 1.29
    Show answer

    Answer: (C) 1.67

    A monatomic gas has f = 3, so γ = 1 + 2/3 = 5/3 ≈ 1.67.

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