Internal energy and absolute zeroEdexcel International A Level Physics: Revision notes
Section 1
What is internal energy?
The molecules of any substance are in constant random motion and exert forces on each other. The internal energy of a substance is the sum of the random distribution of the kinetic energy and the potential energy of its molecules.
- Kinetic energy: from the random motion of the molecules (vibration, and translation in gases and liquids).
- Potential energy: from the forces between the molecules. It is larger when the molecules are further apart against attractive forces.
The kinetic energy of a whole object moving through space is not part of its internal energy.
Do not describe internal energy as 'heat' or as only the kinetic energy of the molecules.
Section 2
Changing the internal energy
Supplying energy by heating or doing work on a substance increases its internal energy. Where it goes depends on the situation.
- Temperature rising: the mean kinetic energy of the molecules increases.
- Change of state at constant temperature: the mean kinetic energy is constant. The energy increases the potential energy as molecules separate and forces are overcome.
- Ideal gas: there are no forces between molecules, so there is no potential energy. The internal energy is just the total kinetic energy.
Section 3
Heating curve in terms of molecules
When a solid is heated to a gas, the temperature–time record has sloping and flat parts.
- Sloping parts: temperature rises, so the mean kinetic energy rises.
- Flat part at melting: potential energy rises as the molecules move apart slightly.
- Flat part at boiling: potential energy rises greatly as the molecules separate completely.
The internal energy rises throughout. The rise is greater in boiling than in melting.
Section 4
Absolute zero and the kelvin scale
Absolute zero is the lowest possible temperature, 0 K or −273 °C, where the molecules have minimum internal energy. The mean kinetic energy of the molecules is zero there.
Temperatures in kelvin are found from
A temperature interval of 1 K is the same size as 1 °C.
The molecules still have potential energy at absolute zero, so the internal energy is a minimum rather than zero.
Section 5
Temperature and mean kinetic energy
The mean kinetic energy of the molecules is proportional to the absolute temperature in kelvin:
mean kinetic energy ∝ T
So doubling the kelvin temperature doubles the mean kinetic energy.
Worked example: gas heated from 27 °C to 327 °C. T goes from 300 K to 600 K, so the mean kinetic energy doubles.
The same gas heated from 27 °C to 54 °C does not double: T goes from 300 K to 327 K, an increase of 9%.
Always convert to kelvin before taking a ratio of temperatures.
That's the notes covered.
Carry on to the next subtopic.
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
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).