Internal energy — AQA GCSE Combined Science
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Internal energy explained
Every system is made of particles, and those particles store energy.
Read the full explanation
The energy stored by the particles of a system is called its internal energy. The particles have kinetic energy because they move or vibrate, and potential energy because of their positions and the forces between them. Heating a system transfers energy to its particles, increasing their kinetic or potential energy, so internal energy rises. For example, heating 1 kg of water from 20 °C to 80 °C increases its internal energy; the particles vibrate and move faster. Cooling reduces internal energy. Internal energy is not the same as temperature: temperature measures the average kinetic energy of the particles, while internal energy is the total energy stored by all the particles. A larger mass at the same temperature has more internal energy because it contains more particles.
Internal energy is the total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.
Internal energy is the total energy stored by all the particles in a system. It is the sum of the kinetic energy of every particle, due to its movement or vibration, and the potential energy of every particle, due to its position and the forces between particles. For example, in a gas, fast-moving particles have high kinetic energy, and particles far apart have high potential energy. Heating transfers energy to the particles, increasing their kinetic or potential energy and so increasing internal energy. Temperature is different: it measures the average kinetic energy of the particles, not the total. A larger mass at the same temperature has more internal energy because there are more particles. Changes of state involve energy transfers that alter potential energy while temperature stays constant.
Heating changes the energy stored within the system by increasing the energy of the particles that make up the system. This either raises the temperature of the system or produces a change of state.
Internal energy is the total energy stored in a system, made up of the kinetic energy of its particles and the potential energy linked to their positions and bonds. Heating transfers energy to the system, so the particles gain energy. That extra energy can be shared out in two ways. If the particles vibrate or move faster, their average kinetic energy rises and the temperature increases. If the energy instead breaks or loosens bonds between particles, the arrangement changes and a change of state occurs, such as melting or boiling, while the temperature stays constant. For example, heating ice warms it to 0 °C, then melting absorbs energy without a temperature rise until all the ice has melted.
Your focus
- Describe internal energy as energy stored by the particles of a system.
- Explain how heating or cooling changes the internal energy of a system.
- Distinguish between internal energy and temperature using particle ideas.
Show all 9 objectives
- State that internal energy is the total kinetic and potential energy of all particles in a system.
- Explain how kinetic and potential energy of particles contribute to internal energy.
- Compare internal energy and temperature for systems with different masses.
- Define internal energy as the total kinetic and potential energy of the particles in a system.
- Explain how heating can either raise temperature or cause a change of state.
- Describe why temperature remains constant during a change of state.
Internal energy exam tips
Marking Points
- Internal energy is energy stored by the particles that make up a system.
- Particles have kinetic energy due to movement or vibration and potential energy due to their positions and the forces between them.
- Heating a system transfers energy to its particles, increasing internal energy; cooling decreases it.
- Internal energy is a total energy store, whereas temperature relates to the average kinetic energy of the particles.
- For the same material and temperature, a larger mass has more internal energy because it contains more particles.
- Internal energy is the total kinetic energy plus the total potential energy of all particles in a system.
- Kinetic energy comes from the movement or vibration of particles.
- Potential energy comes from the positions of particles and the forces between them.
- Heating a system increases its internal energy by transferring energy to its particles.
- Temperature is related to average kinetic energy, while internal energy is the total for all particles.
- Internal energy is the sum of the kinetic and potential energies of all the particles in a system.
- Heating transfers energy to the particles, increasing the energy stored in the system.
- An increase in the average kinetic energy of the particles raises the temperature of the system.
- Energy used to overcome forces between particles can change the state without changing temperature.
- During a change of state, energy is transferred but the temperature remains constant until the change is complete.
- A temperature rise and a change of state are alternative outcomes of the same energy transfer by heating.
Examiner Tips
- 💡Define internal energy in terms of the particles of the system, not just 'heat inside'.
- 💡When comparing internal energies, state both temperature and mass because both affect the total.
- 💡Use the idea of energy transfer by heating to explain changes in internal energy, and link particle motion to kinetic energy.
- 💡Use the phrase 'total kinetic energy and potential energy of all the particles' when defining internal energy.
- 💡Link changes in state to changes in potential energy of particles while temperature remains constant.
- 💡When comparing two systems, refer to both the number of particles and their average kinetic energy.
- 💡Use the phrase 'energy stored within the system' when defining internal energy, and link it to both kinetic and potential energy of particles.
- 💡When describing a change of state, state clearly that temperature is constant and explain where the transferred energy goes.
- 💡Sketch or describe a heating curve to show a flat section during melting or boiling, labelling the temperature as constant.
Common Mistakes
- Saying internal energy is the same as temperature. Correction: temperature is a measure of average kinetic energy of particles, while internal energy is the total kinetic plus potential energy of all particles.
- Thinking internal energy only includes kinetic energy. Correction: it also includes potential energy from particle positions and intermolecular forces.
- Believing a substance at a lower temperature always has less internal energy than one at a higher temperature. Correction: mass matters too, so a large cool object can store more internal energy than a small hot object.
- Writing that internal energy is only kinetic energy. Correction: it is the total kinetic energy plus the total potential energy of all particles.
- Confusing internal energy with temperature. Correction: temperature is the average kinetic energy per particle, whereas internal energy is the total energy of all particles.
- Ignoring mass when comparing internal energies. Correction: a larger mass contains more particles, so at the same temperature it has more internal energy.
- Saying that heating always raises the temperature: correct this by noting that during a change of state the energy goes into breaking bonds, so temperature stays constant.
- Confusing temperature with internal energy: correct this by explaining that temperature depends on average particle kinetic energy, while internal energy includes potential energy too.
- Thinking particles gain energy only by moving faster: correct this by stating that energy can also increase the potential energy of particles as bonds are overcome.