What causes the kinetic part of internal energy?

The random motion of the molecules.

What causes the potential part of internal energy?

The forces between the molecules.

Where does energy go during melting at constant temperature?

Into the potential energy of the molecules; the mean kinetic energy stays constant.

What is absolute zero?

0 K (−273 °C), the lowest possible temperature, where the molecules have minimum internal energy.

How is mean kinetic energy related to temperature?

It is proportional to the absolute temperature in kelvin.

Why does an ideal gas have no potential energy?

There are no forces between its molecules.

Is the internal energy zero at absolute zero?

No. The kinetic energy is zero but the potential energy remains, so the internal energy is a minimum.

Why is the kinetic energy of a moving object not internal energy?

Internal energy covers the random motion of the molecules, not the motion of the whole object.

Why is more energy needed for boiling than for melting (same mass)?

The potential energy rises more because the molecules separate completely.

Internal energy and absolute zeroEdexcel International A Level Physics: Flashcards

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What is internal energy?

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What is internal energy?: The sum of the random distribution of kinetic and potential energies of the molecules.
What causes the kinetic part of internal energy?: The random motion of the molecules.
What causes the potential part of internal energy?: The forces between the molecules.
Where does energy go during melting at constant temperature?: Into the potential energy of the molecules; the mean kinetic energy stays constant.
What is absolute zero?: 0 K (−273 °C), the lowest possible temperature, where the molecules have minimum internal energy.
How do you convert °C to K?: Add 273.
How is mean kinetic energy related to temperature?: It is proportional to the absolute temperature in kelvin.
What happens to the mean kinetic energy when the kelvin temperature doubles?: It doubles.
Why does an ideal gas have no potential energy?: There are no forces between its molecules.
Is the internal energy zero at absolute zero?: No. The kinetic energy is zero but the potential energy remains, so the internal energy is a minimum.
Why is the kinetic energy of a moving object not internal energy?: Internal energy covers the random motion of the molecules, not the motion of the whole object.
Why is more energy needed for boiling than for melting (same mass)?: The potential energy rises more because the molecules separate completely.

Exam questions on Internal energy and absolute zero

A 0.20 kg block of ice at 0 °C is placed in a warm room. It absorbs energy from the air, melts completely and forms water that is still at 0 °C. A physics class is asked to describe what has changed for the molecules in the sample.
Explain why the temperature of the sample does not change while the ice is melting, although it is absorbing energy.2 marks
A cryogenics laboratory stores liquid nitrogen, which boils at 77 K, and uses it to cool samples. Technicians compare the behaviour of nitrogen molecules at the temperature of the cold store with their behaviour at room temperature, 300 K.
A technician says that absolute zero is the temperature at which the molecules have no energy at all. Explain what is meant by absolute zero in terms of the molecules, correcting the statement if necessary.2 marks
A sealed, rigid flask contains a fixed mass of gas that behaves as an ideal gas. At 27 °C the internal energy of the gas is 1.5 kJ. The flask is then heated until the gas is at 327 °C.
Calculate the ratio of the mean kinetic energy of the molecules at 327 °C to that at 27 °C.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).