AQA GCSE Combined Science (8464), Higher tier · Physics › Particle model of matter › Internal energy and energy transfers
Practise Internal energy. 12 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State that heating increases the particles' energyHeating a system transfers energy to its particles, so the energy they store increases.
4
Define internal energyThe total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.
5
State the two possible effects of heatingHeating either raises the temperature of the system or produces a change of state.
5
Link temperature to the particles' kinetic energyA higher temperature means the particles have a higher average kinetic energy: they move or vibrate faster.
6
Explain how temperature differs from internal energyA large mass at a low temperature can have more internal energy than a small mass at a high temperature, because it has many more particles.
7
Explain energy changes during a change of stateThe energy supplied increases the potential energy of the particles, so internal energy rises but the temperature does not.
Notes
What internal energy is
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.
Heating a system
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.
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.
Cheatsheet
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
How to answer each type of question
Define internal energy
1 to 2 marks4
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 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 marks5
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 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).
Compare the internal energy of two objects
2 marks6
Compare the temperatures: the same temperature means the same average kinetic energy per particle.
Compare the masses: more mass means more particles, so more total energy.
Example. Beaker A contains 200 g of water at 60 °C. Beaker B contains 800 g of water at 60 °C. Explain which beaker of water has the greater internal energy.
Show the model answer
Beaker B (1). The water in both beakers is at the same temperature, but B contains four times as many particles, so the total kinetic and potential energy of all its particles is greater (1).
Explain why the temperature is constant during a change of state
3 marks7
Say that energy is still being transferred, so the internal energy increases.
Say that the energy overcomes the forces between the particles, increasing their potential energy.
Say that the kinetic energy of the particles does not change, so the temperature stays the same.
Example. Some ice at 0 °C is heated. The ice melts, but its temperature stays at 0 °C until all the ice has melted. Explain, in terms of internal energy, why the temperature does not change while the ice is melting.
Show the model answer
Energy is still being transferred to the ice, so its internal energy increases (1). This energy overcomes the forces between the particles, increasing their potential energy (1). The kinetic energy of the particles does not increase, so the temperature stays the same (1).
Shortcuts and 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.
Where marks are lost
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.
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'.
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)
Question 3Hard6 marks
Water boils at 100 °C. While water is boiling, its temperature does not change, even though energy is still being transferred to it.
(a) Compare the internal energy of 1 kg of water at 100 °C with the internal energy of 1 kg of steam at 100 °C. Explain your answer in terms of the energy of the particles.[4]
(b) Burns caused by steam at 100 °C are usually much more serious than burns caused by the same mass of water at 100 °C. Suggest why.[2]
Show the answer and mark scheme
(a)Answer: The steam has more internal energy. Both are at 100 °C, so the particles have the same average kinetic energy, but the particles in the steam have much more potential energy because energy was supplied to pull them apart.
the steam has more internal energy
the (average) kinetic energy of the particles is the same (because the temperature is the same)
the (total) potential energy of the particles in the steam is greater
because energy was transferred to separate the particles / to overcome the forces (bonds) between them
(b)Answer: Steam first condenses on the skin, transferring its large latent heat, and the hot water formed then cools as well, so far more energy is transferred to the skin.
when steam condenses on the skin it transfers a large amount of energy (latent heat) to the skin without its temperature falling
the condensed water then cools as well, so the steam transfers much more energy to the skin than the same mass of water