Internal energy
Thermal Physics - OCR A-Level Physics
Key Definition
Internal energy
The sum of the random kinetic energies and potential energies of all the molecules in a system.
The sum of the random kinetic energies and potential energies of all the molecules in a system.
- The kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. component is due to the random translational (and rotational/vibrational) motion of molecules.
- The potential energyThe capacity to do work. Measured in joules (J). component is due to intermolecular forces between molecules.
- Heating a substance increases its internal energyThe sum of the randomly distributed kinetic and potential energies of all the molecules in a system. either by raising temperature (increasing KE) or by changing state (increasing PE).
- For an ideal gasA theoretical gas composed of many randomly moving point particles that do not interact except during brief elastic collisions., there are no intermolecular forces, so internal energyThe sum of the randomly distributed kinetic and potential energies of all the molecules in a system. is purely kinetic.
- During a change of stateA physical change between solid, liquid, and gas (e.g. melting, boiling, condensing) at constant temperature., temperature remains constant because energy goes into breaking/forming intermolecular bonds (changing PE, not KE).
Common Mistake
MEDIUM
Wrong: Saying internal energyThe sum of the randomly distributed kinetic and potential energies of all the molecules in a system. is just the kinetic energyThe energy an object possesses due to its motion. of molecules.
Right: Internal energy is the SUM of random kinetic energies AND potential energies of all molecules. Only for an ideal gasA theoretical gas composed of many randomly moving point particles that do not interact except during brief elastic collisions. is internal energy purely kinetic (no intermolecular forces).
Right: Internal energy is the SUM of random kinetic energies AND potential energies of all molecules. Only for an ideal gasA theoretical gas composed of many randomly moving point particles that do not interact except during brief elastic collisions. is internal energy purely kinetic (no intermolecular forces).
Examiner Tips and Tricks
- When asked to define internal energy, you must say 'random' kinetic energies.
- Ordered kinetic energyThe energy an object possesses due to its motion. (e.g. wind blowing all molecules in one direction) does not count.