AQA GCSE Physics Foundation (8463), Foundation tier · Particle model of matter › Internal energy and energy transfers
Practise Internal energy. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Internal energy is the energy stored by the particles that make up a system. You need to define it, explain how heating changes it, and explain why heating either raises the temperature or changes the state. Most questions are 1 to 3 mark definitions and particle explanations, often leading into specific heat capacity or latent heat calculations.
Key facts
Internal energy = total kinetic energy + potential energy of all the particles in a system
Kinetic energy of the particles: linked to temperature
Potential energy of the particles: due to the forces between them; depends on their separation
Heating → internal energy increases → temperature rises OR state changes
During a change of state: internal energy increases, temperature stays constant
Temperature: average kinetic energy of the particles. Internal energy: total energy of all the particles
Notes
What internal energy is
diagram
Energy is stored inside a system by the particles (atoms and molecules) that make it up. This is its internal energy.
Internal energy = the total kinetic energy and potential energy of all the particles in the system.
Kinetic energy: the particles are always moving (vibrating in a solid, moving around in a liquid or gas). Faster particles have more kinetic energy.
Potential energy: energy stored because of the forces between the particles. It depends on how far apart the particles are. For the same substance, the particles have more potential energy as a liquid than as a solid, and more again as a gas.
Internal energy = kinetic energy + potential energy of all the particles.
Heating a system
diagram
Heating transfers energy to a system. This increases the energy of its particles, so the internal energy of the system increases.
The energy does one of two things. It raises the temperature (the kinetic energy of the particles increases), or it produces a change of state (the potential energy of the particles increases as the forces between them are overcome).
While a substance is changing state, its temperature stays constant even though energy is still being supplied.
Heating raises the internal energy all the time; during melting the temperature stays constant.
Cooling does the opposite: energy is transferred away from the system, so its internal energy decreases.
Temperature is not the same as internal energy
Temperature is linked to the average kinetic energy of the particles.
Internal energy is the total energy of all the particles, so it also depends on how many particles there are (the mass) and on the state.
Example: a swimming pool at 25 °C has far more internal energy than a cup of tea at 80 °C, because it contains vastly more particles.
How to answer each type of question
Define internal energy
1 to 2 marksGrade 4
Say 'the total kinetic energy and potential energy'.
Say 'of all the particles (atoms and molecules) in the system'.
Example. What is meant by the internal energy of a system?
Show the model answerHide the model answer
The total kinetic energy and potential energy (1) of all the particles (atoms and molecules) that make up the system (1).
Describe and explain the effect of heating
2 to 3 marksGrade 5
Say what happens to the internal energy (it increases).
Say which energy of the particles changes: kinetic energy if the temperature rises, potential energy if the state changes.
Finish with the result: the temperature rises, or the substance changes state.
Example. A pan of water at 20 °C is heated on a hob. Describe what happens to the internal energy of the water, and explain how this affects the water particles and the temperature.
Show the model answerHide the model answer
The internal energy of the water increases (1). The water particles gain kinetic energy / move faster on average (1), so the temperature of the water rises (1).
Don’t lose marks
Defining internal energy as 'heat' or 'the temperature'. It is the total kinetic and potential energy of all the particles.
Leaving potential energy (or kinetic energy) out of the definition.
Saying the temperature rises during melting or boiling. It stays constant while the state changes.
Saying that a hotter object always has more internal energy. Internal energy also depends on the mass (number of particles) and the state.
More tips
Memory tricks
Internal energy = kinetic + potential energy of ALL the particles. The words 'total' and 'all' earn the marks.
Temperature rising → kinetic energy of the particles increasing. State changing → potential energy of the particles increasing.
Temperature is an average per particle; internal energy is a total for all the particles. A warm swimming pool beats a hot cup of tea.
Exam technique
Definitions are marked strictly: learn the full wording, including 'of all the particles'.
In 'explain' questions, go step by step: energy transferred → internal energy increases → which energy of the particles changes → what happens to the temperature or the state.
When the question says 'in terms of particles', write about the particles (atoms or molecules), not just 'the substance'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that heating increases the particles' energyHeating a system transfers energy to its particles, so the energy they store increases.
Grade 4
Define internal energyThe total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.
Grade 5
State the two possible effects of heatingHeating either raises the temperature of the system or produces a change of state.
Grade 5
Link temperature to the particles' kinetic energyA higher temperature means the particles have a higher average kinetic energy: they move or vibrate faster.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Heating a system increases the energy of the particles in the system. Give the two possible effects of heating a system.
Its temperature rises, or it changes state.
Complete the sentence. The internal energy of a system is the total ................ energy and ................ energy of all the particles that make up the system.
kinetic; potential
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Which statement describes the internal energy of a system? Tick (✓) one box.[1]
The total kinetic energy of all the particles in the system
The total kinetic energy and potential energy of all the particles in the system
The total potential energy of all the particles in the system
The average kinetic energy of the particles in the system
(b) Heating a system increases the energy of the particles in the system. Give the two possible effects of heating a system.[2]
(c) A saucepan of water is heated on a cooker. What happens to the internal energy of the water?[1]
Show the answer and mark scheme
(a)Answer: The total kinetic energy and potential energy of all the particles in the system
(b)Answer: Its temperature rises, or it changes state.
the temperature of the system increases
the system changes state (e.g. melts or boils)
(c)Answer: It increases.
it increases
Question 2Medium5 marks
A block of ice at −10 °C was taken out of a freezer and put in a warm room. The ice warmed up to 0 °C and then melted.
(a) Describe what happens to the particles in the ice as its temperature increases from −10 °C to 0 °C.[2]
(b) While the ice is melting, its temperature stays at 0 °C even though energy is still being transferred to it. Explain what happens to the internal energy of the ice while it melts.[3]
Show the answer and mark scheme
(a)Answer: The particles vibrate more vigorously about their fixed positions: their average kinetic energy, and so the internal energy of the ice, increases.
the particles vibrate faster / with a larger amplitude (about their fixed positions)
the (average) kinetic energy of the particles increases
the internal energy of the ice increases
(b)Answer: The internal energy increases. The energy increases the potential energy of the particles as the bonds holding them in place are broken, but their kinetic energy stays the same, so the temperature stays at 0 °C.
the internal energy (of the ice) increases
the (average) kinetic energy of the particles does not change because the temperature is constant
the potential energy of the particles increases (as the forces / bonds holding the particles in place are overcome)