Edexcel A-Level Physics A2 5.3 Thermodynamics Questions

Practise Edexcel A-Level Physics thermodynamics through heating and latent heat, core practicals, internal energy, ideal gases and molecular kinetic theory.

Exam points

calculate heating or latent-heat transfers using E=mcΔθ, E=mL and power-time data
use pV=NkT or pV/T constants with kelvin temperatures to model ideal gases
link molecular kinetic energy, r.m.s. speed and pressure changes to particle motion

Covered syllabus points

5.3.125 - Heating and latent heat equations 5.3.126 - Core Practical 12 - thermistor calibration 5.3.127 - Core Practical 13 - specific latent heat 5.3.128 - Internal energy 5.3.129 - Absolute zero and molecular kinetic energy 5.3.130 - Ideal gas equation 5.3.131 - Core Practical 14 - gas pressure and volume 5.3.132 - Molecular kinetic theory equation

[Maximum number: 3]

The water in a swimming pool at the Olympic Games should be at a temperature of $28.0^{\circ}\mathrm{C}$ .

A pool contains $2750\,\mathrm{m^3}$ of water at $16.5^{\circ}\mathrm{C}$ .

Calculate the increase in internal energy $\Delta E$ of the water when it is heated to $28.0^{\circ}\mathrm{C}$ .

Density of water $=998\,\mathrm{kg\,m^{-3}}$

Specific heat capacity of water $=4190\,\mathrm{J\,kg^{-1}\,K^{-1}}$

Use $\rho=m/V$ , so $m=\rho V=(998)(2750)=2.74\times10^6\,\mathrm{kg}$ . Then

$\Delta E=mc\Delta T=(2.74\times10^6)(4190)(28.0-16.5)=1.32\times10^{11}\,\mathrm{J}.$

[Maximum number: 6]

The diagram shows some equipment that could be used to calibrate a thermistor.

Figure for Question 2 — Edexcel A-Level Physics A2

Describe a method a student could use to obtain a calibration curve for the thermistor from $0^{\circ} \mathrm{C}$ to $100^{\circ} \mathrm{C}$ .

You should identify any extra equipment that would be needed.

[Maximum number: 1]

In an experiment to determine the specific latent heat of vaporisation of water $L_{\mathrm{V}}$ , a student used an electrical heater to boil water in a beaker.

The experimental data gave $L_{\mathrm{V}}=2.20 \mathrm{MJ} \mathrm{kg}^{-1}$ . The textbook value of $L_{\mathrm{V}}$ is $2.26 \mathrm{MJ} \mathrm{kg}^{-1}$ .
Which of the following could be an explanation for this difference in the values of $L_{\mathrm{V}}$ ?

Some energy is transferred to the surroundings.

The heater power was underestimated.

The student did not stir the water.

The heater is inefficient.

[Maximum number: 1]

Ice at $0^{\circ} \mathrm{C}$ melts into water at $0^{\circ} \mathrm{C}$ .
Which row of the table gives the changes in mean molecular kinetic energy and mean molecular potential energy as the ice melts?

Mean molecular
kinetic energy

Mean molecular
potential energy

no change

increases

no change

increases

[Maximum number: 6]

According to the Football Association (FA) rules, the football used in a professional match should have a circumference of between 68.0 cm and 70.0 cm . The pressure of the air in the football must be between 60 kPa and 110 kPa above atmospheric pressure.

The pressure inside a football decreases as the temperature of the air inside falls. The volume of the ball remains constant.

Explain why, in terms of the motion of the molecules.

Indicative content:

- As the temperature of the air decreases, the mean kinetic energy of the molecules decreases.
- Therefore the root mean square speed decreases;
equivalently, since $E_k=p^2/(2m)$ , the average momentum of the molecules decreases.
- The change of momentum of a molecule during a collision with the container wall decreases.
- The rate of collisions with the wall also decreases.
- Hence the rate of change of momentum, and therefore the force on the wall, decreases.
- The pressure exerted by the gas decreases because p=F/A.

Award according to the official levels-based structure and reasoning grids shown below.

Figure for Answer to Question 5 — Edexcel A-Level Physics A2

[Maximum number: 6]

A football was inflated with air at a temperature of $16.0^{\circ} \mathrm{C}$ . When inflated, the circumference of the football was 68.5 cm and it contained $2.50 \times 10^{23}$ molecules of air.

Deduce whether this football met the FA rules.
atmospheric pressure =105 kPa

Use $C=2\pi r$ , $V=4\pi r^3/3$ , convert $16.0^\circ\mathrm{C}$ to 289 K, and use pV=NkT. This gives absolute pressure $1.84\times10^5\,\mathrm{Pa}$ and excess pressure $7.9\times10^4\,\mathrm{Pa}=79\,\mathrm{kPa}$ . The circumference is $68.5\,\mathrm{cm}$ and the excess pressure is within 60–110 kPa, so the ball meets the rules.

[Maximum number: 4]

A student investigated the relationship between the pressure and volume of a fixed mass of gas at constant temperature.

He used his data to plot the following graph.

Figure for Question 7 — Edexcel A-Level Physics A2

The student stated that his graph demonstrated that the pressure exerted by the gas was inversely proportional to the volume occupied by the gas.

Assess the validity of the student's statement.

[Maximum number: 3]

The spectrum of light emitted by the star Chi Lupi provides evidence of mercury atoms in the outer layers of the star. The light emitted from the star is compared with light emitted from a mercury lamp on Earth.

The lamp contains 1.65 x 10^19 mercury atoms in a volume of 1.50 x 10^-5 m^3.

The pressure of the mercury vapour is 4.25 x 10^4 Pa.
Calculate the mean kinetic energy of the mercury atoms.

Mean kinetic energy of mercury atoms =

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Covered syllabus points

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8