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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

An ideal gas is confined in a cylinder by a piston. When the piston rapidly compresses the gas, the average speed of the gas molecules increases. This occurs because the gas molecules…
 
 
 
 
 
 
(A) have a smaller volume available in which they can move
(B) receive thermal energy transferred from outside the cylinder
(C) receive energy from the piston as they collide with it
(D) make more collisions every second with each other
▶️ Answer/Explanation
Detailed solution

During rapid compression, the process is approximately adiabatic with negligible heat transfer (B is incorrect).
The increased molecular speed (temperature) comes from work done on the gas. As the piston moves inward, gas molecules colliding with it rebound with higher speed due to the piston’s motion, increasing their kinetic energy.
(A) and (D) describe effects of compression but don’t directly explain the temperature increase.
✅ Answer: (C)

Question

A balloon of volume \(V\) contains \(10\,\text{mg}\) of an ideal gas at a pressure \(P\). An additional mass of the gas is added without changing the temperature of the balloon. This causes the volume to increase to \(2V\) and the pressure to increase to \(3P\).
What is the mass of gas added to the balloon?
(A) \(5\,\text{mg}\)
(B) \(15\,\text{mg}\)
(C) \(50\,\text{mg}\)
(D) \(60\,\text{mg}\)
▶️ Answer/Explanation
Detailed solution

Since the temperature remains constant, the ideal gas equation \(PV = nRT\) applies to both situations.

Initial state:
\( PV = n_1 RT \), where \(n_1\) corresponds to a mass of \(10\,\text{mg}\).

Final state:
\( (3P)(2V) = n_2 RT \), where \(n_2\) corresponds to a mass of \((10 + m)\,\text{mg}\).

Taking the ratio of the final to initial states:
\( \dfrac{3P \cdot 2V}{PV} = \dfrac{n_2}{n_1} \).
\( 6 = \dfrac{10 + m}{10} \).

Solving:
\( 60 = 10 + m \)
\( m = 50\,\text{mg} \).

✅ Answer: (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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