A Level H1 Physics Thermal Physics Quiz

A-Level Physics H1 Quiz - Thermal Physics (Answer Key)

1. B
Reasoning: For an ideal gas, there are no intermolecular forces, so potential energy is zero. Internal energy is solely the sum of random kinetic energies, which is proportional to absolute temperature ($U \propto T$).

2. B
Reasoning: Specific latent heat of fusion is the energy per unit mass to change state from solid to liquid at constant temperature.

3. B
Reasoning: At constant pressure, if volume increases, temperature must increase ($V \propto T$). Average KE depends only on temperature. Density decreases (volume up), Internal Energy increases (Temp up), Number density decreases. Wait, let's re-evaluate. Correction: If gas expands at constant pressure, $V/T = constant$. If $V$ increases, $T$ increases. A: Density $\rho = m/V$. $V$ increases, so $\rho$ decreases. B: Avg KE $\propto T$. $T$ increases, so Avg KE increases. C: Internal Energy $U \propto T$. $T$ increases, so $U$ increases. D: Number per unit volume $N/V$. $V$ increases, so $N/V$ decreases. Re-reading Question 3: "Which of the following quantities remains constant?" Actually, none of the above remain constant if it expands. Let's look at the options again. Usually, these questions ask about isothermal expansion or adiabatic. If it expands at constant pressure, $T$ rises. Let's check the options again. A. Density changes. B. Avg KE changes. C. Internal Energy changes. D. Number density changes. Self-Correction for Generation: The question likely intended an isothermal expansion or asked what changes. However, based on standard templates, let's assume the question meant Isothermal Expansion. If Isothermal: A. Density decreases. B. Avg KE constant (T constant). C. Internal Energy constant. D. Number density decreases. Both B and C would be correct. Alternative Interpretation: Perhaps the question is "Which quantity is not constant?" No, that's ambiguous. Let's adjust the answer key to reflect a standard "Isothermal" scenario which is common in exams, or correct the question logic. Actually, looking at Template 3 (Equilibrium/Momentum), let's stick to the generated question text. If the question says "Constant Pressure", then $T$ changes. Let's assume the question meant: "A fixed mass of ideal gas expands isothermally." If Isothermal: A. Density decreases. B. Avg KE constant. C. Internal Energy constant. D. Number density decreases. This creates a double answer (B and C). Let's look at Option C again. For an ideal gas, $U$ depends only on $T$. So if $T$ is constant, $U$ is constant. Let's look at Option B. Avg KE depends only on $T$. So if $T$ is constant, Avg KE is constant. Fix: In many A-Level contexts, they distinguish between microscopic (KE) and macroscopic (U). But they are linked. Let's change the Question 3 in the key to reflect Isothermal expansion and accept B as the primary molecular answer, or C as the macroscopic. However, since I cannot change the generated MD, I must provide the answer for the text as written. Text as written: "Expands at constant pressure." If $P$ is constant and $V$ increases, $T$ increases. Therefore: A. Density decreases. B. Avg KE increases. C. Internal Energy increases. D. Number density decreases. There is no correct option for "remains constant" in the provided text for Constant Pressure expansion. Assumption: The question intended Isothermal expansion. I will provide the answer for Isothermal expansion as it is the standard exam pattern, and note the discrepancy. Answer: B (Assuming Isothermal context which is standard for "what remains constant" questions involving KE). Note: If strictly Constant Pressure, none remain constant. Given the constraints, B is the intended concept for Isothermal.

4. Thermal equilibrium exists when two bodies in thermal contact have the same temperature and there is no net flow of thermal energy between them.

5.

• Temperature is a measure of the average kinetic energy of the molecules. [1]
• As temperature increases, the average speed/kinetic energy of molecules increases.
• Molecules collide with the walls more frequently and with greater momentum change per collision. [1]
• Since Pressure = Force/Area and Force is rate of change of momentum, the pressure increases.

6.

• Energy loss to the surroundings (air/container) during heating.
• Or, the heater itself absorbs some energy (heat capacity of heater not accounted for).
• This means more energy/time is required to raise the temperature, leading to a calculated $c$ that is higher than actual if losses are attributed to the water. Wait, if $Q = mc\Delta T$, and we measure $Q_{supplied} > Q_{absorbed}$, then calculated $c = Q_{supplied} / (m\Delta T)$ will be higher than the true value. Yes.

7.

• Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1 K (without change of state).
• Specific latent heat is the energy required to change the state of 1 kg of a substance at constant temperature.

8. Zero.
Reasoning: Internal energy is a state function. For a complete cycle, the system returns to its initial state, so $\Delta U = 0$.

9. $\Delta U = Q + W$
Where $\Delta U$ is the change in internal energy, $Q$ is the thermal energy supplied to the system, and $W$ is the work done on the system.
(Note: If using $W$ as work done by the gas, then $\Delta U = Q - W$. Both are accepted if defined clearly.)

10.

• The energy supplied is used to break intermolecular bonds (or overcome intermolecular forces). [1]
• It does not increase the kinetic energy of the molecules, so the temperature (which depends on average KE) remains constant. [1]

11. (a) $Q = mc\Delta \theta$
$Q = 0.50 \times 385 \times (100 - 20)$
$Q = 0.50 \times 385 \times 80$
$Q = 15,400 \text{ J}$ or $15.4 \text{ kJ}$ [2]

(b) $P = E/t \Rightarrow t = E/P$
$t = 15,400 / 50$
$t = 308 \text{ s}$ [2]

12. (a) Work done by gas $W = P\Delta V$
$\Delta V = (5.0 - 2.0) \times 10^{-3} = 3.0 \times 10^{-3} \text{ m}^3$
$W = 1.5 \times 10^5 \times 3.0 \times 10^{-3}$
$W = 450 \text{ J}$ [2]

(b) First Law: $\Delta U = Q - W_{by}$ (using work done by gas convention)
$Q = +800 \text{ J}$ (supplied)
$W_{by} = +450 \text{ J}$
$\Delta U = 800 - 450 = 350 \text{ J}$ [2]
The internal energy increases. [1]

13. (a) To ensure that all the energy supplied by the heater is used for vaporization of water, rather than heating up the apparatus or the water to boiling point initially. It ensures a steady state where heat loss to surroundings is constant/minimized relative to the boiling process. [2]

(b) Energy supplied $E = P \times t$
$t = 5.0 \times 60 = 300 \text{ s}$
$E = 600 \times 300 = 180,000 \text{ J}$
$E = mL_v \Rightarrow L_v = E/m$
$L_v = 180,000 / 0.075$
$L_v = 2,400,000 \text{ J kg}^{-1}$ or $2.4 \times 10^6 \text{ J kg}^{-1}$ [3]

(c) Heat loss to the surroundings. Some of the energy supplied escapes to the air/container instead of vaporizing the water. This means the calculated $L_v$ (based on total input energy) is higher than the actual value required just for phase change. [2]

14. (a) An ideal gas is a theoretical gas where:

1. Molecules have negligible volume compared to the container volume.
2. There are no intermolecular forces (except during elastic collisions).
3. Collisions are perfectly elastic.
4. Internal energy is purely kinetic.
   [Any 2 points] [2]

(b) $pV = nRT$
$n = \frac{pV}{RT}$
$n = \frac{1.2 \times 10^5 \times 0.020}{8.31 \times 300}$
$n = \frac{2400}{2493}$
$n \approx 0.96 \text{ mol}$ [3]

(c) Constant Volume: $\frac{p_1}{T_1} = \frac{p_2}{T_2}$
$p_2 = p_1 \times \frac{T_2}{T_1}$
$p_2 = 1.2 \times 10^5 \times \frac{450}{300}$
$p_2 = 1.2 \times 10^5 \times 1.5$
$p_2 = 1.8 \times 10^5 \text{ Pa}$ [2]

15. (a) Air is a poor conductor of heat (gas molecules are far apart, reducing collision frequency for energy transfer). [1] The trapped air prevents large-scale convection currents if the gap is small, but primarily conduction is low because gases have low thermal conductivity compared to solids. [1]

(b) Convection requires the bulk movement of fluid. In a narrow gap, the air is trapped, preventing the formation of convection currents (hot air rising and cold air sinking). [2]

(c) If the gap is too large, convection currents can establish themselves within the air layer, increasing heat transfer. If too small, conduction across the solid/gas interfaces might dominate or manufacturing difficulties arise. The optimal size minimizes both conduction and convection. [2]

16. (a)

1. Free electrons in the metal gain kinetic energy from the heat source. [1]
2. These free electrons diffuse rapidly through the metal lattice, colliding with other electrons and ions. [1]
3. Lattice ions vibrate more vigorously and pass energy to neighboring ions (phonon transfer), but electron diffusion is the dominant mechanism in metals. [1]

(b) Metals contain free delocalized electrons which can move freely and transfer energy rapidly. Wood is an insulator with no free electrons; energy transfer relies only on slow lattice vibrations. [2]

17. (a) Melting (fusion) or Boiling (vaporization). Given the context of "heating curve" and typical plateaus, it is a change of state. [1]

(b) Energy supplied during plateau $E = P \times t$
$t = 200 \text{ s}$
$E = 100 \times 200 = 20,000 \text{ J}$
$E = mL \Rightarrow L = E/m$
$L = 20,000 / 0.2$
$L = 100,000 \text{ J kg}^{-1}$ or $1.0 \times 10^5 \text{ J kg}^{-1}$ [3]

18. (a) A collision where kinetic energy is conserved (total KE before = total KE after). [1]

(b) Zero. Since speed is the same and mass is constant, $KE = \frac{1}{2}mv^2$ remains unchanged. [1]

19. (a) From the hotter body to the colder body. [1]

(b) When both bodies reach the same temperature. [1]

20. (a) Silvered surfaces are good reflectors of infrared radiation. They reflect thermal radiation back into the liquid (keeping it hot) or reflect external radiation away (keeping it cold), minimizing heat transfer by radiation. [2]

(b) A vacuum contains no matter (particles). Therefore, heat transfer by conduction and convection is prevented, as both require a medium. [2]