Chapter 3 — Thermal Physics: Multiple Choice Questions
Instructions: Each question carries 1 mark. Choose the best answer.
1. The SI unit of temperature is ______.
Answer
(a) Celsius (°C) (b) Fahrenheit (°F) (c) Kelvin (K) (d) Joule (J)
✅ Answer: (c) Kelvin (K)
ExplanationThe SI unit of temperature is the Kelvin (K). It is the absolute temperature scale where 0 K is absolute zero (the lowest possible temperature). Conversion: K = °C + 273. Celsius and Fahrenheit are common scales but not SI units.
2. The value of absolute zero in Celsius is ______.
Answer
(a) 0°C (b) −100°C (c) −273°C (d) −373°C
✅ Answer: (c) −273°C
ExplanationAbsolute zero is 0 K = −273°C (more precisely, −273.15°C). At absolute zero, all molecular motion theoretically stops. No temperature lower than this is possible.
3. The specific heat capacity of water is ______.
Answer
(a) 2100 J kg⁻¹ K⁻¹ (b) 4200 J kg⁻¹ K⁻¹ (c) 840 J kg⁻¹ K⁻¹ (d) 1000 J kg⁻¹ K⁻¹
✅ Answer: (b) 4200 J kg⁻¹ K⁻¹
ExplanationWater has a very high specific heat capacity of 4200 J kg⁻¹ K⁻¹ (or 4.2 kJ kg⁻¹ K⁻¹). This means it absorbs a lot of heat for a small temperature rise, which is why it is used as a coolant and why coastal areas have moderate climates.
4. Heat flows from a hot body to a cold body until ______.
Answer
(a) both bodies have the same volume (b) both bodies have the same density (c) both bodies reach thermal equilibrium (d) both bodies have the same mass
✅ Answer: (c) both bodies reach thermal equilibrium
ExplanationHeat naturally flows from a region of higher temperature to lower temperature. This continues until both bodies reach the same temperature — called thermal equilibrium. At this point, there is no net heat flow.
5. The formula for heat gained or lost by a body is ______.
Answer
(a) Q = mT (b) Q = mcT (c) Q = mc/T (d) Q = m/cT
✅ Answer: (b) Q = mcT
ExplanationQ = mcΔT, where Q = heat energy (J), m = mass (kg), c = specific heat capacity (J kg⁻¹ K⁻¹), and ΔT = change in temperature (K or °C). This is the fundamental calorimetry equation.
6. The process of heat transfer without any medium is ______.
Answer
(a) Conduction (b) Convection (c) Radiation (d) Evaporation
✅ Answer: (c) Radiation
ExplanationRadiation does not need any medium — it can travel through vacuum. The Sun's heat reaches Earth through radiation across empty space. Conduction requires a solid medium; convection requires a fluid (liquid or gas).
7. Which of the following is a good conductor of heat?
Answer
(a) Wood (b) Plastic (c) Copper (d) Rubber
✅ Answer: (c) Copper
ExplanationCopper is an excellent conductor of heat (and electricity) because of its free electrons. Metals in general are good conductors. Wood, plastic, and rubber are poor conductors (insulators) because they have few free electrons.
8. The latent heat of fusion is the heat required to ______.
Answer
(a) convert solid to gas (b) convert solid to liquid at constant temperature (c) convert liquid to gas (d) raise the temperature of a solid
✅ Answer: (b) convert solid to liquid at constant temperature
ExplanationLatent heat of fusion is the heat required to change 1 kg of solid to liquid at its melting point (constant temperature). The latent heat of fusion of ice = 336,000 J/kg (336 kJ/kg). Temperature does not change during this phase change.
9. When a substance changes from liquid to gas at its boiling point, the heat absorbed is called ______.
Answer
(a) Latent heat of fusion (b) Specific heat (c) Latent heat of vaporisation (d) Heat of radiation
✅ Answer: (c) Latent heat of vaporisation
ExplanationLatent heat of vaporisation is the heat required to change 1 kg of liquid to vapour at its boiling point (constant temperature). For water: L_v = 2,260,000 J/kg (2260 kJ/kg). This is why steam burns are more severe than hot water burns.
10. The greenhouse effect is mainly due to ______.
Answer
(a) Conduction of heat from Earth's surface (b) Reflection of sunlight by clouds (c) Trapping of heat radiation by CO₂ and other gases (d) Increase in solar radiation
✅ Answer: (c) Trapping of heat radiation by CO₂ and other gases
ExplanationGreenhouse gases like CO₂, CH₄, and water vapour absorb infrared radiation emitted by Earth's surface and re-radiate it back, trapping heat in the atmosphere. This natural process keeps Earth warm, but excess CO₂ from burning fossil fuels is causing global warming.
Quick Revision — Key Thermal Physics Concepts
| Concept | Key Formula / Value |
|---|
| Temperature conversion | K = °C + 273 |
| Absolute zero | 0 K = −273°C |
| Specific heat of water | 4200 J kg⁻¹ K⁻¹ |
| Heat formula | Q = mcΔT |
| Latent heat of fusion (ice) | L_f = 336,000 J/kg |
| Latent heat of vaporisation (water) | L_v = 2,260,000 J/kg |
| Conduction | Through solids via molecular vibration |
| Convection | Through fluids via bulk movement |
| Radiation | Through vacuum / no medium needed |
Common MCQ Traps
- Heat vs Temperature: Heat is energy (J); Temperature is hotness (K or °C). Don't confuse them.
- Absolute zero = 0 K = −273°C (not −273 K or 0°C)
- Latent heat: Temperature does NOT change during phase change
- Water's high specific heat: This is why water heats up and cools down slowly
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