What is the internal energy of an ideal gas?

The total kinetic energy of its molecules, since there are no intermolecular forces.

How is the kelvin temperature related to the molecules?

It is proportional to their mean kinetic energy.

Define absolute zero.

The temperature at which the molecules have the minimum possible internal energy (0 K).

Why must temperatures be in kelvin when comparing mean kinetic energies?

Mean kinetic energy is proportional to the kelvin temperature, not the Celsius temperature.

What happens to the molecules' energy when a solid melts?

Potential energy increases while mean kinetic energy stays constant.

Why does internal energy depend on the amount of substance?

It is a total over all the molecules, so more molecules give more internal energy.

Does a hot cup of tea always have more internal energy than a cool swimming pool?

No. The pool has far more molecules, so more total internal energy.

Which energies are random in the molecules of a solid?

Kinetic energy from vibration and potential energy from the forces between molecules.

Can the temperature fall below 0 K?

No. There is no lower mean kinetic energy than the minimum.

Internal energy and absolute zeroEdexcel A-Level Physics: Flashcards

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Define internal energy.

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Define internal energy.: The sum of the random distribution of kinetic and potential energies of all the molecules in a system.
What is the internal energy of an ideal gas?: The total kinetic energy of its molecules, since there are no intermolecular forces.
How is the kelvin temperature related to the molecules?: It is proportional to their mean kinetic energy.
How do you convert from °C to K?: Add 273 (T = θ + 273).
What is absolute zero in °C?: About −273 °C.
Define absolute zero.: The temperature at which the molecules have the minimum possible internal energy (0 K).
Why must temperatures be in kelvin when comparing mean kinetic energies?: Mean kinetic energy is proportional to the kelvin temperature, not the Celsius temperature.
What happens to the mean kinetic energy when the kelvin temperature doubles?: It doubles.
What happens to the molecules' energy when a solid melts?: Potential energy increases while mean kinetic energy stays constant.
Why does internal energy depend on the amount of substance?: It is a total over all the molecules, so more molecules give more internal energy.
Does a hot cup of tea always have more internal energy than a cool swimming pool?: No. The pool has far more molecules, so more total internal energy.
Which energies are random in the molecules of a solid?: Kinetic energy from vibration and potential energy from the forces between molecules.
Can the temperature fall below 0 K?: No. There is no lower mean kinetic energy than the minimum.

Exam questions on Internal energy and absolute zero

A fixed mass of an ideal gas is sealed in a rigid container and heated from 27 °C to 327 °C. Take 0 °C as 273 K.
The gas is cooled from 27 °C until the mean kinetic energy of its molecules is half its original value. Calculate the new temperature in °C.2 marks
A 0.20 kg block of ice at −20 °C is supplied with energy at a steady rate until it has all melted and the water has then warmed to 20 °C. Take 0 °C as 273 K.
Explain why the temperature of the ice stays at 0 °C while it is melting, even though energy is still being supplied.2 marks
In a physics demonstration, a sample of an ideal gas is cooled towards absolute zero. Take 0 °C as 273 K.
Explain what is meant by absolute zero, and why there cannot be a temperature below 0 K.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).