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IBDP Physics- B.4 Thermodynamics- IB Style Questions For HL Paper 1A -FA 2025

IBDP Physics HL Paper 1 -FA 2025- All Chapters

Question 

A substance changes from a liquid into a solid without a change in temperature.

What is true about the internal energy of the substance and the total intermolecular potential energy of the substance when this phase change occurs?

 Internal energy of the substanceTotal intermolecular potential energy of the substance
(A)decreasedecrease
(B)no changedecrease
(C)decreaseno change
(D)no changeno change
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{A}} \)

The internal energy of a substance can be written as

\(U=E_{\mathrm{k}}+E_{\mathrm{p}}\)

where \(E_{\mathrm{k}}\) is the total kinetic energy of the particles and \(E_{\mathrm{p}}\) is the total intermolecular potential energy.

During a change of state, the temperature remains constant. Therefore, the average kinetic energy of the particles remains constant.

However, when a liquid changes into a solid, the particles become more strongly bound and their intermolecular separation decreases. The intermolecular potential energy decreases.

Since the kinetic energy remains constant while the potential energy decreases, the internal energy also decreases.

Therefore:

\(\text{internal energy: decrease}\)

\(\text{intermolecular potential energy: decrease}\)

Hence, the correct answer is \( \boxed{\mathrm{A}} \).

Question 

The Carnot efficiency of a heat engine is \(0.4\) when the cold reservoir temperature is \(T_{\mathrm{c}}\). The hot reservoir is held at a constant temperature. What is the Carnot efficiency when the cold reservoir temperature is \(\dfrac{T_{\mathrm{c}}}{2}\)?

(A) \(0.2\)
(B) \(0.3\)
(C) \(0.7\)
(D) \(0.8\)
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{C}} \)

The efficiency of a Carnot engine is

\(\eta=1-\dfrac{T_{\mathrm{c}}}{T_{\mathrm{h}}}\)

Initially,

\(0.4=1-\dfrac{T_{\mathrm{c}}}{T_{\mathrm{h}}}\)

Therefore,

\(\dfrac{T_{\mathrm{c}}}{T_{\mathrm{h}}}=0.6\)

The new cold reservoir temperature is \(\dfrac{T_{\mathrm{c}}}{2}\), while \(T_{\mathrm{h}}\) remains constant. Hence, the new efficiency is

\(\eta’=1-\dfrac{T_{\mathrm{c}}/2}{T_{\mathrm{h}}}\)

\(\eta’=1-\dfrac{1}{2}\left(\dfrac{T_{\mathrm{c}}}{T_{\mathrm{h}}}\right)\)

\(\eta’=1-\dfrac{1}{2}(0.6)\)

\(\eta’=0.7\)

Hence, the correct answer is \( \boxed{\mathrm{C}} \).

Question

A fixed mass of an ideal gas expands slowly at constant temperature in a container.

Three statements about the gas molecules during the expansion are:

I. They collide with the walls of the container at a reduced rate.
II. They travel further on average between each collision.
III. Their average kinetic energy decreases as the gas expands.
Which statements are correct?
(A) I and II only
(B) I and III only
(C) II and III only
(D) I, II and III
▶️ Answer/Explanation
Detailed solution

During a slow expansion at constant temperature, the volume of the gas increases while the temperature remains unchanged.

I. As the volume increases, the number density of molecules decreases. This results in fewer collisions per unit time with the walls of the container. Hence, statement I is correct.

II. With increased volume and reduced density, the average distance travelled by a molecule between successive collisions (mean free path) increases. Hence, statement II is correct.

III. The average kinetic energy of gas molecules depends only on the absolute temperature. Since the temperature is constant, the average kinetic energy remains unchanged and does not decrease. Hence, statement III is incorrect.

Therefore, the correct statements are I and II only.

✅ Answer: (A)

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