Internal energy — AQA GCSE Physics
Test yourself on Internal energy with AQA GCSE practice questions.
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Internal energy explained
Every substance stores energy in the particles it is made of, and that store is called internal energy.
Read the full explanation
It is not a new kind of energy hiding inside matter; it is the total of the energy the particles have because they are moving and because of the way they are arranged. Heating a system raises its internal energy, and the store is shared over every particle, so the amount depends on how much substance there is as well as on how hot it is. When a question asks which store increases as water is heated, the accepted answers are the thermal or internal store, or the kinetic energy of the water particles, while the word heat on its own is ignored. Cooling lowers the internal energy by transferring energy to the surroundings.
Internal energy is the total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.
Internal energy has two parts. The kinetic part comes from the motion of the particles, vibrating in a solid or moving around in a liquid or a gas, and it is this part that temperature measures, so raising the temperature raises the mean kinetic energy of the particles. The potential part comes from the separations of the particles and the forces between them, so pulling particles further apart against their mutual attraction increases it. Internal energy is the total of both parts for every particle in the system. Separating them is what makes changes of state make sense: when liquid nitrogen boils at constant temperature, the mean kinetic energy does not change but the potential energy rises sharply, so internal energy increases.
Heating changes the energy stored within the system by increasing the energy of the particles that make up the system.
Heating transfers energy into a system, and that energy ends up with the particles. Which part of their energy rises depends on what the substance is doing. While the temperature is rising, the extra energy goes to the kinetic store, so the particles vibrate or move faster and their mean kinetic energy increases. While the substance is changing state, the temperature holds constant and the extra energy goes to the potential store instead, weakening the attractions that hold the particles close together, so their potential energy increases while their kinetic energy stays the same. Either way the internal energy of the system increases, because the energy is not lost, only stored differently.
This either raises the temperature of the system or produces a change of state.
Energy supplied by heating does one of two things, never both at once. It raises the temperature, in which case the energy goes to the kinetic store of the particles and the rise depends on the mass, the specific heat capacity and the energy supplied. Or it produces a change of state, in which case the temperature stays constant while the energy goes to the potential store, separating the particles against the forces holding them together. A heating curve shows this directly, because the sloping sections are temperature rises and the flat sections at the melting point and the boiling point are changes of state. Which section you are in tells you which equation to use: specific heat capacity on the slopes, specific latent heat on the flats.
Your focus
- Name the energy store that increases when a substance is heated, in wording a mark scheme accepts.
- Explain where internal energy is stored in a system and why the amount of substance matters.
- Describe what happens to the internal energy of a cooling system and where that energy goes.
Show all 12 objectives
- Decide, for water being heated and for ice melting at 0 °C, whether the kinetic part, the potential part or both increase, and say what happens to the total.
- Explain how the internal energy of nitrogen changes as it boils, treating the kinetic and potential parts separately.
- Link a rise in temperature to a rise in the mean kinetic energy of the particles.
- Predict what happens to the kinetic and potential energies of the particles as a substance melts at constant temperature.
- Explain where the energy goes when a substance is heated and its temperature rises.
- Describe the change in internal energy of a system during heating, naming both contributions.
- Identify from a heating curve whether the energy supplied is raising the temperature or changing the state.
- Explain why the temperature stays constant while a substance melts although energy is still supplied.
- Choose between the specific heat capacity and specific latent heat equations for a given part of a heating curve.
Internal energy exam tips
Marking Points
- one mark for naming the store as the internal, or thermal, energy store
- one mark for the energy being stored by the particles that make up the system
- one mark for heating increasing the internal energy of the system
- one mark for the internal energy depending on the amount of substance as well as on its temperature
- one mark for internal energy as the total of the kinetic and potential energies of all the particles
- one mark for the kinetic part being linked to the temperature of the system
- one mark for the potential part being linked to the separation of the particles
- one mark for a change of state raising the potential energy while the kinetic energy stays the same
- one mark for heating increasing the energy of the particles in the system
- one mark for the mean kinetic energy of the particles increasing while the temperature rises
- one mark for the potential energy of the particles increasing during a change of state
- one mark for the kinetic energy of the particles staying the same while a substance melts at constant temperature
- one mark for the energy either raising the temperature or causing a change of state
- one mark for the temperature remaining constant during a change of state
- one mark for identifying a flat section of a heating curve as melting or boiling
- one mark for selecting the specific heat capacity equation for a sloping section and latent heat for a flat one
Examiner Tips
- 💡Use 'internal energy store' or 'thermal store' and avoid the word heat as a noun.
- 💡Mention the particles explicitly, because the store belongs to them and not to the container.
- 💡If a mass is given, expect it to matter, since internal energy is not the same as temperature.
- 💡Split any internal energy answer into a kinetic sentence and a potential sentence.
- 💡If the temperature is constant, say that the kinetic part is constant before mentioning the total.
- 💡Watch for dependent marks, because the conclusion about internal energy usually needs both earlier points.
- 💡Check the temperature first, because constant temperature means potential energy and rising temperature means kinetic energy.
- 💡In tick-box questions with a row for kinetic and a row for potential, give one tick per row only.
- 💡Write 'mean kinetic energy' rather than 'speed' when describing a whole system of particles.
- 💡Label the sloping and flat parts of any heating curve before attempting the calculation.
- 💡Ask which store is changing, kinetic for a slope and potential for a plateau, then pick the equation.
- 💡State that the temperature is constant during a change of state, because it is often worth a mark on its own.
Common Mistakes
- answering 'heat' when asked to name the store, which examiners ignore
- treating internal energy as something separate from the particles, such as a fluid inside the substance
- assuming two objects at the same temperature must hold the same internal energy whatever their masses
- saying internal energy is destroyed when a substance cools rather than transferred to the surroundings
- saying internal energy is only the kinetic energy of the particles
- claiming the kinetic energy of the particles rises during boiling even though the temperature is constant
- confusing the potential energy of the particles with the gravitational potential energy of the whole object
- giving the change for a single particle when the question asks about the total for the system
- saying the particles move faster while a substance melts, even though the temperature is constant
- stating that the potential energy of the particles decreases as a solid melts
- claiming the particles expand or gain mass when they are heated
- treating a constant temperature as evidence that no energy is being transferred in
- reading a flat section of a heating curve as a period when no energy is being supplied
- using the specific heat capacity equation across a change of state
- saying the temperature rises during melting because energy is still being supplied
- confusing the melting plateau with the boiling plateau on a labelled graph