B.4 Thermodynamics Topic Practice

IB Physics syllabusPhysics SL/HLFirst assessment 2025

Practise thermodynamics through gas processes and cycles by combining energy accounting, p-V graph reading and entropy decisions.

Exam points

apply the first law with signs for `Q`, `W` and `Delta U` to decide whether energy enters or leaves the gas
use area under a `p-V` graph or `pDeltaV` to calculate work for a named stage or complete cycle
use `Delta S=Q/T` and the second law to judge entropy changes for a gas, phase change or surroundings

[Maximum number: 3]

A student is investigating the emf and internal resistance of a cell, using the circuit shown. The ammeter and the voltmeter are ideal.

The graph shows the variation of the voltmeter reading V with the ammeter reading I.

(a)

The student then replaces the variable resistor with an electric heating element in order to investigate the thermal output of the system. The heating element is used to raise the temperature of the water to the boiling point.

After the boiling point is reached, 12 g of the water are vaporized. The specific latent heat of vaporization of water is $2.3 \times 10^{6} \mathrm{Jkg}^{-1}$ .

[ 3 ]

(i)

Calculate the change in entropy of the vaporized water.

[ 2 ]

(ii)

Explain why this entropy change is positive.

[ 1 ]

[Maximum number: 5]

The pressure-volume diagram shows a 3-stage cyclic process of 0.20 moles of an ideal gas in a closed container. A-B is an isothermal process. C-A is an adiabatic process.

The following data are available:

\begin{aligned} \text { Pressure at } A & =2.0 \times 10^{6} \mathrm{~Pa} \\ \text { Volume at } A & =1.0 \times 10^{-3} \mathrm{~m}^{3} \\ \text { Pressure at } B & =2.0 \times 10^{5} \mathrm{~Pa} \\ \text { Volume at } B & =1.0 \times 10^{-2} \mathrm{~m}^{3} \end{aligned}

(a)

Show that the temperature at C is about 260 K .

[ 2 ]

(b)

Determine the work done by the gas in C-A.

[ 2 ]

(c)

Outline why this cycle can be used for a heat engine.

[ 1 ]

[Maximum number: 1]

A thermal energy of 7.0 J is removed from an ideal gas, and a work of 2.0 J is done by the gas. What is the change in the internal energy of the gas?

-9.0 J

-5.0 J

$+5.0 \mathrm{~J}$

$+9.0 \mathrm{~J}$

[Maximum number: 1]

The entropy of a system is S, and the number of possible microstates is $\Omega$ . What is the number of possible microstates for a system of entropy $\frac{S}{2}$ ?

$\frac{\Omega}{2}$

$\frac{\sqrt{\Omega}}{2}$

$\sqrt{\Omega}$

$\Omega^{2}$

[Maximum number: 1]

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

0.2

0.3

0.7

0.8

[Maximum number: 3]

The graph shows the cycle of a heat engine operating with an ideal monatomic gas. It consists of three stages: an isothermal $A \rightarrow B$ , an isobaric $B \rightarrow C$ and an adiabatic $C \rightarrow A$ . The temperature at A is 560 K .

(a) Show that the thermal energy transferred from the gas along $\mathrm{B} \rightarrow \mathrm{C}$ is about 700 J .

The work done along $\mathrm{A} \rightarrow \mathrm{B}$ is 840 J . Calculate the efficiency of the engine.

[Maximum number: 2]

A boat is moved from land to water by rolling it across a set of cylindrical airbags.

When fully inflated, an unloaded airbag has a diameter of 1.80 m and a length of 24.0 m . At a temperature of $15^{\circ} \mathrm{C}$ , an airbag can hold 4200 mol of gas.

Outline the change in internal energy of the gas in the airbag.

0 selected

Exam points

Question 2

Question 2(d)

Question 2(d)(i)

Question 2(d)(ii)

Question 3

Question 3(a)

Question 3(b)

Question 3(c)

Question 15

Question 15

Question 16

Question 5(b)

Question 8(a)(iv)