Secondary 3 Physics Thermal Physics Quiz

Secondary 3 Physics Quiz - Thermal Physics — Answer Key

Total Marks: 50

Section A: Kinetic Particle Model of Matter (Questions 1–5)

1. (a) Brownian motion [1 mark]

(b) The smoke particles are constantly being bombarded by fast-moving, invisible air molecules from all directions. [1 mark] Because the collisions are uneven at any instant, there is a net force in a random direction, causing the smoke particles to move in a random, zigzag path. [1 mark]

2. (a) The average speed of the air particles decreases. [1 mark] Temperature is a measure of the average kinetic energy of particles. When the temperature drops, the particles lose kinetic energy and therefore move more slowly. [1 mark]

(b) The pressure decreases. [1 mark] The slower-moving particles collide with the walls of the can less frequently and with less force per collision. Since pressure is caused by particle collisions with the walls, the pressure decreases. [1 mark]

3.

[1 mark for each correct entry; total 3 marks]

4. The statement is incorrect because gas particles themselves do not expand; the size of individual particles remains the same. [1 mark] When a gas is heated in a sealed container, the particles gain kinetic energy and move faster. They collide with the container walls more frequently and with greater force per collision. This increases the pressure. [1 mark]

Section B: Thermal Processes (Questions 5–10)

5. (a) Conduction [1 mark]

(b) At the heated end, metal atoms vibrate more vigorously. [1 mark] These vibrations are passed along to neighbouring atoms through the lattice structure, transferring energy along the rod. [1 mark] In metals, free electrons also move rapidly through the structure, colliding with atoms and transferring kinetic energy, which makes metals particularly good conductors. [1 mark]

6. (a) Convection [1 mark]

(b) The heater warms the air directly above it. [1 mark] This warm air expands, becomes less dense than the surrounding cooler air, and rises. [1 mark] As the warm air rises, cooler air moves in to replace it near the floor, is heated in turn, and also rises. This sets up a convection current that circulates warm air throughout the room. [1 mark]

7. (a) The matt black can will cool down faster. [1 mark] Matt black surfaces are better emitters of thermal radiation than shiny white surfaces, so the black can radiates heat energy away more rapidly. [1 mark]

(b) Radiation [1 mark]

8. (a) The vacuum contains no particles, so conduction and convection cannot occur across it. This prevents heat transfer by these two processes between the inner and outer walls. [1 mark]

(b) The silvered surfaces are poor emitters and poor absorbers of thermal radiation. The inner silvered surface reflects radiated heat back into the contents (reducing heat loss from hot liquids), and the outer silvered surface reflects external radiation away (reducing heat gain by cold liquids). [1 mark]

9. (a) Land has a lower specific heat capacity than water, so it heats up more quickly when absorbing the same amount of solar radiation. [1 mark]

(b) The air above the warmer land heats up, expands, and rises, creating a region of low pressure over the land. [1 mark] Cooler, denser air from above the sea moves in towards the land to replace the rising warm air, creating a sea breeze. [1 mark]

Section C: Thermal Properties of Matter (Questions 10–15)

10. (a) Q = mcΔθ Q = 1.5 × 4200 × (100 − 25) [1 mark] Q = 1.5 × 4200 × 75 Q = 472,500 J [1 mark]

(b) P = E / t, so t = E / P t = 472,500 / 2200 [1 mark] t = 214.8 s ≈ 215 s (or 3 min 35 s) [1 mark]

11. Q = mcΔθ 5000 = 0.50 × c × (44 − 22) [1 mark] 5000 = 0.50 × c × 22 [1 mark] c = 5000 / (0.50 × 22) = 5000 / 11 = 454.5 J/(kg·°C) ≈ 455 J/(kg·°C) [1 mark]

12. (a) Q = mL_f Q = 0.20 × 3.34 × 10⁵ [1 mark] Q = 66,800 J [1 mark]

(b) Q = mcΔθ Q = 0.20 × 4200 × (15 − 0) [1 mark] Q = 0.20 × 4200 × 15 = 12,600 J [1 mark]

(c) Total energy = 66,800 + 12,600 = 79,400 J [1 mark]

13. (a) The temperature remains constant at 100 °C (the boiling point). [1 mark]

(b) During boiling, the thermal energy supplied is used to overcome the attractive forces between water particles, allowing them to escape as gas (steam). [1 mark] This energy goes into increasing the potential energy of the particles (latent heat of vaporisation), not their kinetic energy, so the temperature does not rise. [1 mark]

14. On a warm day, water particles in the cloth have higher average kinetic energy, so more particles have enough energy to escape the liquid surface. [1 mark] On a windy day, water vapour above the cloth is constantly removed, maintaining a low concentration of water vapour near the surface, which increases the rate of evaporation. [1 mark] Both factors increase the rate at which the fastest-moving particles escape, so the cloth dries faster. [1 mark]

15. (a) Q = mcΔθ Q = 0.30 × 4200 × (28 − 20) [1 mark] Q = 0.30 × 4200 × 8 = 10,080 J [1 mark]

(b) Heat lost by copper = Heat gained by water = 10,080 J Q = mcΔθ for copper 10,080 = 0.40 × c_copper × (95 − 28) [1 mark] 10,080 = 0.40 × c_copper × 67 c_copper = 10,080 / (0.40 × 67) = 10,080 / 26.8 = 376.1 J/(kg·°C) ≈ 376 J/(kg·°C) [1 mark]

Section D: Integrated Thermal Physics (Questions 16–20)

16. (a) Black surfaces are good absorbers of thermal radiation, so the pipes absorb more energy from the Sun and heat the water more effectively. [1 mark]

(b) The glass cover traps infrared radiation inside the box (the greenhouse effect). Short-wavelength solar radiation passes through the glass, but the longer-wavelength infrared radiation re-emitted by the warm pipes cannot escape easily, reducing heat loss. [1 mark]

17. The metal base of the saucepan is a good thermal conductor, so heat from the stove is quickly transferred to the food. [1 mark] The plastic handle is a poor thermal conductor (a good insulator), so heat does not travel quickly to the handle, keeping it cool enough to hold safely. [1 mark]

18. When sweat evaporates from the skin, the fastest-moving water particles escape from the liquid surface. [1 mark] This removes thermal energy from the remaining liquid (and the skin), causing the body to cool down. The energy used for evaporation is the latent heat of vaporisation, taken from the body. [1 mark]

19. (a) The water in the metal cup will cool faster. [1 mark]

(b) Metal is a good thermal conductor, so heat is conducted quickly through the cup walls and lost to the surroundings. Polystyrene foam is a poor conductor (a good insulator), so heat loss is much slower. [1 mark]

20. The fabric traps a layer of air around the plants. Air is a poor thermal conductor, so it reduces heat loss by conduction from the plants to the cold surroundings. [1 mark] The fabric also reduces heat loss by convection (by blocking air currents) and by radiation (by reflecting some infrared radiation back towards the plants). [1 mark]

END OF ANSWER KEY