3.6.2.1
The first law of thermodynamics is conservation of energy applied to thermal systems
Thermal Energy Transfer — AQA A-Level Physics
Key Definition
First law of thermodynamics — The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it.
- This is the principle of conservation of energyThe capacity to do work. Measured in joules (J).Energy cannot be created or destroyed, only transferred from one form to another. The total energyThe capacity to do work. Measured in joules (J). of a closed system remains constant. stated for thermal systems.
- When a gas is compressed (volume decreases), work is done on the gas. Internal energyThe capacity to do work. Measured in joules (J).The sum of the randomly distributed kinetic and potential energies of all the molecules in a system. increases.
- When a gas expands (volume increases), the gas does work on its surroundings. Internal energyThe sum of the randomly distributed kinetic and potential energies of all the molecules in a system. decreases.
- If the system is insulated (no heat transfer), all the work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). times potential difference). goes into changing internal energyThe sum of the randomly distributed kinetic and potential energies of all the molecules in a system..
Gas in a piston
- Piston pushed in: gas compressed, work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). times potential difference). on gas, particles gain kinetic energyThe energy an object possesses due to its motion., temperature rises.
- Piston moves out: gas expands, gas does work on piston, particles lose kinetic energyThe energy an object possesses due to its motion., temperature falls.
Examiner Tips and Tricks
- You do not need an equation for the first law at A-level.
- But you must be able to explain qualitatively whether internal energy increases or decreases, and link this to work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). times potential difference). and heat transfer.