O Level Physics Thermal Physics Quiz

O-Level Physics Quiz - Thermal Physics (Answer Key)

Total Marks: 40
Topic: Thermal Physics (Syllabus 6091, Section III)

Section A Answers (1 mark each)

1. C
Teaching note: In a liquid, particles are close together with attractive forces but are not fixed in a lattice; they can slide past one another. A describes a solid; B describes a gas; D describes a solid.

2. B
Teaching note: Convection is transfer of thermal energy by bulk movement of fluid (liquid or gas). Conduction is through vibrations/electrons, radiation via EM waves.

3. A
Teaching note: Specific heat capacity unit is J kg⁻¹ °C⁻¹ (or J kg⁻¹ K⁻¹). Specific latent heat is J kg⁻¹.

4. B
Teaching note: At boiling, supplied energy breaks intermolecular bonds (latent heat), not raising kinetic energy, so temperature constant.

5. B
Teaching note: Metal conducts heat away from hand faster (higher thermal conductivity), feeling colder though same temp.

Section B Answers (2–3 marks each)

6. [2]
Any one correct difference, e.g.:

• Evaporation occurs at surface and below boiling point; boiling occurs throughout liquid at fixed temperature.
• Boiling needs external heating to boiling point; evaporation happens at any temperature.
  (Mark: 1 for each clear point, max 2)

7. [2]
Woollen blanket traps air (1) which is a poor conductor / insulator (1), reducing heat loss from body by conduction/convection.

8. [3]
Formula: $Q = mc\Delta T$
$m = 2.0$ kg, $c = 900$ J kg⁻¹ °C⁻¹, $\Delta T = 45 - 20 = 25$ °C
$Q = 2.0 \times 900 \times 25 = 45\,000$ J
Marks: 1 for formula, 1 for substitution, 1 for answer.

9. [2]
Particle at bottom is heated, expands, becomes less dense (1), rises upward at centre (1). (Cold water from sides sinks to replace it.)

10. [2]
Specific latent heat of fusion is the thermal energy required to change 1 kg of a substance from solid to liquid at its melting point without temperature change. (1 for per kg, 1 for solid→liquid at constant temp)

11. [2]
$Q = mL = 0.50 \times 3.34 \times 10^5 = 1.67 \times 10^5$ J.
Marks: 1 formula/sub, 1 answer.

12. [2]
Any two: increase temperature, increase surface area, increase air flow / wind, reduce humidity. (1 each)

13. [3]
Gas particles far apart with weak forces (1), large empty space so particles pushed closer (1). Solid particles tightly packed in fixed positions with strong forces, no space to compress (1).

14. [2]
Silvered surfaces reflect radiation (1); vacuum between walls prevents conduction/convection (1). (Both reduce radiation loss via silvering)

15. [2]
Correct (1). Example: black radiator paint, solar heater absorber (1).

Section C Answers (4 marks each)

16. [4]
Let $m$ = mass hot water.
Heat lost by hot water = $m \times 4200 \times (80-35) = m \times 4200 \times 45$.
Heat gained by calorimeter = $1.5 \times 390 \times (35-15) = 1.5 \times 390 \times 20 = 11\,700$ J.
Heat gained by cold water = $0.80 \times 4200 \times 20 = 67\,200$ J.
Total gain = $78\,900$ J.
$m \times 189\,000 = 78\,900 \Rightarrow m = 0.417$ kg.
Marks: 1 calorimeter gain, 1 cold water gain, 1 hot water loss eqn, 1 solve + unit.

17. [4]
When heated, particles gain kinetic energy and vibrate more (1). Average separation increases (1). In solid, this expands the lattice (1). Macroscopic expansion is sum of particle spacing increase (1).

18. [4]
Energy supplied: $P \times t = 60 \times (5 \times 60) = 18\,000$ J.
$\Delta T = 37 - 22 = 15$ °C.
$c = Q / (m\Delta T) = 18\,000 / (0.20 \times 15) = 6000$ J kg⁻¹ °C⁻¹.
Assumption: no heat loss to surroundings / all heater energy absorbed.
Marks: 1 energy, 1 ΔT, 1 calc, 1 assumption.

19. [4]
(a) 50 °C [1]
(b) Energy supplied used to break forces between particles (1), changes state solid→liquid (1), kinetic energy unchanged so temp constant (1).
Marks as shown.

20. [4]
Metals: free electrons transfer kinetic energy rapidly (2). Non-metals: only vibrations passed via neighbours, slower (1). Particle theory: both have vibrating particles but metal has mobile electrons (1).