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IBDP Physics- B.3 Gas laws- IB Style Questions For SL Paper 1A -FA 2025

IBDP Physics SL 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 

A rigid vessel of volume \(V\) contains \(N\) molecules of an ideal monatomic gas. The average kinetic energy of the molecules is \(E_k\). What is the pressure in the vessel?

(A) \(\dfrac{2N}{3VE_k}\)
(B) \(\dfrac{2NE_k}{3V}\)
(C) \(\dfrac{3N}{2VE_k}\)
(D) \(\dfrac{3NE_k}{2V}\)
▶️ Answer/Explanation

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

For an ideal gas, the kinetic theory relationship between pressure and the total translational kinetic energy is

\(PV=\dfrac{2}{3}E_{\mathrm{total}}\)

The average kinetic energy of one molecule is \(E_k\), so for \(N\) molecules the total kinetic energy is

\(E_{\mathrm{total}}=NE_k\)

Therefore,

\(PV=\dfrac{2}{3}NE_k\)

Rearranging for pressure gives

\(P=\dfrac{2NE_k}{3V}\)

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

Question

An ideal gas is held in a cylinder by a piston as shown in picture below. The piston compresses the gas rapidly.
Which one of below is correct .
The average speed of the gas molecules increases 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

1. Analyze Process:
Rapid compression implies an adiabatic process (no time for heat transfer, so Q=0). The work is done on the gas.

2. Microscopic View:
When a molecule collides with a moving wall (the piston moving inward), it rebounds with a higher speed than it struck with (similar to a tennis ball hit by a racket). This gain in kinetic energy increases the average speed (and temperature) of the gas.
✅ Answer: (C)

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