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Secondary 4 Pure Physics Thermal Physics Quiz

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Questions

Secondary 4 Pure Physics Quiz - Thermal Physics

Name: __________________________
Class: __________________________
Date: __________________________
Score: ________ / 40

Duration: 45 minutes
Total Marks: 40

Instructions:

Answer all questions.
Write your answers in the spaces provided.
Show all working clearly. Marks may be awarded for correct working even if the final answer is incorrect.
Take the acceleration of free fall, $g = 10 \text{ m/s}^2$ .
Specific heat capacity of water = $4200 \text{ J}/(\text{kg}^\circ\text{C})$ .
Specific latent heat of fusion of ice = $334,000 \text{ J/kg}$ .
Specific latent heat of vaporisation of water = $2,260,000 \text{ J/kg}$ .

Section A: Multiple Choice & Short Definitions (Questions 1-5)

1. Which of the following statements best describes the arrangement and motion of particles in a liquid?
[1]
A. Particles are closely packed in a regular pattern and vibrate about fixed positions.
B. Particles are far apart and move randomly at high speeds.
C. Particles are closely packed but can slide past one another.
D. Particles are far apart and vibrate about fixed positions.

Answer: ______

2. A metal rod is heated at one end. The other end eventually becomes hot. What is the primary mechanism of heat transfer through the metal rod?
[1]
A. Convection only
B. Radiation only
C. Conduction via lattice vibrations and free electron diffusion
D. Conduction via lattice vibrations only

Answer: ______

3. Why does a double-glazed window reduce heat loss from a house more effectively than a single-glazed window of the same total thickness?
[1]
A. Glass is a better insulator than air.
B. The air gap prevents conduction and convection.
C. The air gap reflects all thermal radiation.
D. The two layers of glass create a vacuum.

Answer: ______

4. Two identical metal cans, L and M, are filled with equal masses of hot water at the same initial temperature. Can L has a dull black surface, and Can M has a shiny silver surface. Which can cools down faster and why?
[1]
A. Can L, because black surfaces are better emitters of infrared radiation.
B. Can M, because silver surfaces are better emitters of infrared radiation.
C. Can L, because black surfaces are better absorbers of infrared radiation.
D. Can M, because silver surfaces are better reflectors of infrared radiation.

Answer: ______

5. Define specific heat capacity.
[2]

Section B: Kinetic Model & Heat Transfer Mechanisms (Questions 6-10)

6. Explain, in terms of particle motion, why the pressure of a fixed mass of gas increases when its temperature is increased at constant volume.
[2]

7. State one difference between boiling and evaporation.
[2]

8. A student investigates the cooling of hot water. She records the temperature of the water every minute for 10 minutes. The room temperature is $25^\circ\text{C}$ .

(a) Sketch a graph of temperature ( $\theta$ ) against time ( $t$ ) for the cooling water. Label the initial temperature $\theta_0$ and the room temperature.
[2]

(b) Explain why the rate of cooling decreases as the water temperature approaches room temperature.
[2]

9. Figure 9.1 shows a vacuum flask used to keep liquids hot.

Figure 9.1

(a) Explain how the vacuum between the double walls reduces heat loss.
[2]

(b) Explain how the silvered surfaces on the walls reduce heat loss.
[2]

10. Why is the stopper of a vacuum flask usually made of plastic or cork?
[1]

Section C: Calculations & Energy Changes (Questions 11-15)

11. A block of copper of mass $2.0 \text{ kg}$ is heated using an electric heater. The heater supplies energy at a constant rate of $200 \text{ W}$ . The specific heat capacity of copper is $385 \text{ J}/(\text{kg}^\circ\text{C})$ .

(a) Calculate the time taken to raise the temperature of the copper block from $20^\circ\text{C}$ to $50^\circ\text{C}$ , assuming no energy is lost to the surroundings.
[3]

(b) In reality, the time taken is longer than calculated in (a). Explain why.
[1]

12. An ice cube of mass $0.05 \text{ kg}$ at $0^\circ\text{C}$ is placed into a beaker containing $0.20 \text{ kg}$ of water at $20^\circ\text{C}$ .

(a) Calculate the thermal energy required to melt the ice cube completely at $0^\circ\text{C}$ .
[2]

(b) Calculate the thermal energy lost by the warm water if its temperature drops to $0^\circ\text{C}$ .
[2]

13. A gas is contained in a cylinder fitted with a movable piston. The gas is compressed slowly so that its temperature remains constant.

(a) State what happens to the pressure of the gas.
[1]

(b) Explain this change in pressure in terms of the kinetic particle model.
[3]

14. A student performs an experiment to determine the specific latent heat of vaporisation of water. She uses an electric heater immersed in boiling water.

Mass of water boiled away in 5 minutes = $0.012 \text{ kg}$
Power of heater = $100 \text{ W}$

(a) Calculate the energy supplied by the heater in 5 minutes.
[2]

(b) Calculate the specific latent heat of vaporisation of water obtained from this experiment.
[2]

15. The accepted value for the specific latent heat of vaporisation of water is $2,260,000 \text{ J/kg}$ . The student’s calculated value in Question 14 is significantly higher. Suggest one reason for this discrepancy.
[1]

Section D: Phase Changes & Data Analysis (Questions 16-20)

16. Figure 16.1 shows a cooling curve for naphthalene.

Figure 16.1

Identify the state of naphthalene in region BC (the horizontal plateau).
[1]

17. Explain why the temperature remains constant in region BC of the cooling curve even though energy is being lost to the surroundings.
[2]

18. Describe the change in the internal energy of the naphthalene in region BC.
[2]

19. A solar water heater panel is painted black. Explain why this color is chosen for the panel's surface.
[2]

20. Explain why convection currents cannot form in solids.
[2]

Answers

Secondary 4 Pure Physics Quiz - Thermal Physics (Answer Key)

1. C
[1]
Reasoning: Liquids have particles close together (incompressible) but with enough energy to slide past each other (flow).

2. C
[1]
Reasoning: Metals conduct heat via lattice vibrations and, more importantly, the diffusion of free electrons.

3. B
[1]
Reasoning: Air is a poor conductor. The narrow gap prevents convection currents from forming effectively.

4. A
[1]
Reasoning: Dull black surfaces are better emitters of thermal radiation than shiny surfaces.

5. The amount of thermal energy required to raise the temperature of 1 kg [1] of a substance by 1°C (or 1 K) [1].

As temperature increases, the average kinetic energy of the gas particles increases [1].
Particles collide with the walls more frequently and with greater force [1].
Since Pressure = Force/Area, the pressure increases.

7. Any one of the following:

Boiling occurs at a fixed temperature (boiling point); evaporation occurs at any temperature [1].
Boiling occurs throughout the liquid; evaporation occurs only at the surface [1].
Boiling involves bubble formation; evaporation does not [1]. (Award 2 marks for a clear, complete distinction)

8. (a) Graph requirements:

Y-axis: Temperature ( $\theta$ ), X-axis: Time ( $t$ ) [1].
Curve starts high, decreases with decreasing gradient, and asymptotically approaches $25^\circ\text{C}$ (room temp) [1].

(b)

The rate of heat loss depends on the temperature difference between the water and the surroundings [1].
As the water cools, the temperature difference decreases, so the rate of energy loss decreases [1].

9. (a)

Vacuum contains no particles [1].
Therefore, heat cannot be transferred by conduction or convection [1].

(b)

Silvered surfaces are poor emitters of infrared radiation [1].
This reduces heat loss via radiation [1].

10.

Plastic/cork are poor conductors (good insulators) [1], reducing heat loss by conduction through the stopper.

11. (a)

Energy required ( $Q$ ) = $mc\Delta\theta$
$Q = 2.0 \times 385 \times (50 - 20)$
$Q = 2.0 \times 385 \times 30 = 23,100 \text{ J}$ [1]
Power ( $P$ ) = Energy / Time $\rightarrow$ Time ( $t$ ) = Energy / Power
$t = 23,100 / 200$ [1]
$t = 115.5 \text{ s}$ [1]

(b)

Energy is lost to the surroundings / heated container [1].

12. (a)

$Q = mL_f$
$Q = 0.05 \times 334,000$
$Q = 16,700 \text{ J}$ [2]

(b)

$Q = mc\Delta\theta$
$Q = 0.20 \times 4200 \times (20 - 0)$
$Q = 0.20 \times 4200 \times 20$
$Q = 16,800 \text{ J}$ [2]

(c)

Yes, all the ice will melt [1].
The energy available from the water cooling to $0^\circ\text{C}$ ( $16,800 \text{ J}$ ) is greater than the energy required to melt the ice ( $16,700 \text{ J}$ ) [1].

13. (a) Pressure increases [1].

(b)

Volume decreases, so particles are confined to a smaller space [1].
The frequency of collisions with the container walls increases [1].
Since temperature is constant, the force per collision is unchanged, but the total force per unit area (pressure) increases due to more frequent collisions [1].

14. (a)

Energy ( $E$ ) = Power $\times$ Time
Time = $5 \times 60 = 300 \text{ s}$
$E = 100 \times 300 = 30,000 \text{ J}$ [2]

(b)

$E = mL_v \rightarrow L_v = E / m$
$L_v = 30,000 / 0.012$
$L_v = 2,500,000 \text{ J/kg}$ [2]

15.

Heat is lost to the surroundings / container absorbs heat [1].
Reasoning: The calculated $L_v$ uses the total energy supplied by the heater. If some energy is lost to the surroundings or heating the apparatus, the energy attributed to vaporising the mass is artificially high, leading to a higher calculated value.

16. Solid and liquid (mixture) [1].

17.

Energy is being released to the surroundings as the substance changes state from liquid to solid (formation of bonds) [1].
Therefore, the average kinetic energy of the particles does not change, so temperature remains constant [1].

18.

Internal energy decreases [1].
Potential energy of the particles decreases as they move closer together into a more ordered solid structure, while kinetic energy remains constant [1].

19.

Black surfaces are good absorbers of infrared radiation / thermal energy [1].
This allows the panel to absorb maximum heat from the sun to warm the water [1].

20.

Particles in solids are held in fixed positions by strong forces [1].
They cannot move freely to carry thermal energy from one place to another (which is required for convection) [1].