OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(d) internal energy as the sum of the random distribution of kinetic and…
Full specification wording
Solid, liquid and gas: (d) internal energy as the sum of the random distribution of kinetic and potential energies associated with the molecules of a system
Internal energy is the total energy stored in a system due to the molecules it contains.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 4 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Internal energy is the total energy stored in a system due to the molecules it contains. It is the sum of two contributions: the kinetic energies associated with the random motion of the molecules, and the potential energies associated with the forces between them. Because the molecules have a distribution of speeds, their kinetic energies are randomly distributed, and the potential energy depends on their separation. For an ideal gas, the potential energy contribution is taken as negligible, so internal energy depends only on the random kinetic energy and therefore on temperature. For a real substance, changing state at constant temperature can change internal energy because the potential energy contribution changes even though the average kinetic energy does not. MCQ items often test the definition, the two contributions, or the effect of a change such as melting.
What earns the marks
- Internal energy is the sum of the kinetic and potential energies associated with the molecules of a system.
- The kinetic energy contribution arises from the random motion of the molecules.
- The potential energy contribution arises from the forces between the molecules and depends on their separation.
- The kinetic energies of the molecules are randomly distributed, so internal energy includes a distribution of values rather than a single molecular energy.
- For an ideal gas, the potential energy contribution is negligible, so internal energy depends on the random kinetic energy and hence on temperature.
- During a change of state at constant temperature, the potential energy contribution changes, so internal energy changes even though the average kinetic energy does not.
Common mistakes and how to put them right
Defining internal energy as only the kinetic energy of the molecules.
Put it right: It also includes the potential energy associated with intermolecular forces.
Confusing internal energy with temperature.
Put it right: Temperature relates to average kinetic energy, while internal energy includes both kinetic and potential contributions and depends on the amount of substance.
Saying internal energy is the sum of the kinetic and potential energies of one molecule.
Put it right: It is the total for all the molecules in the system.
Assuming internal energy cannot change during melting or boiling because temperature is constant.
Put it right: The potential energy contribution changes, so internal energy changes.
Exam tips
- Quote the definition in full, naming both kinetic and potential contributions and the random distribution.
- Distinguish clearly between temperature and internal energy when an option links them.
- For an ideal gas, state that the potential energy contribution is negligible before relating internal energy to temperature.
- When a change of state is involved, consider what happens to the potential energy contribution at constant temperature.
Try the lesson
The start of the Solid, liquid and gas: (d) internal energy as the sum of the random distribution of kinetic and potential energies associated with the molecules of a system lesson, made from checked teaching notes for this specification point.
Listen
Internal energy is the total energy stored in a system due to the molecules it contains. It is the sum of two contributions: the kinetic energies associated with the random motion of the molecules, and the potential energies associated with the forces between them. Because the molecules have a distribution of speeds, their kinetic energies are randomly distributed, and the potential energy depends on their separation. For an ideal gas, the potential energy contribution is taken as negligible, so internal energy depends only on the random kinetic energy and therefore on temperature. For a real substance, changing state at constant temperature can change internal energy because the potential energy contribution changes even though the average kinetic energy does not. MCQ items often test the definition, the two contributions, or the effect of a change such as melting. A strong answer earns marks for these points. Internal energy is the sum of the kinetic and potential energies associated with the molecules of a system. The kinetic energy contribution arises from the random motion of the molecules. The potential energy contribution arises from the forces between the molecules and depends on their separation. The kinetic energies of the molecules are randomly distributed, so internal energy includes a distribution of values rather than a single molecular energy. Exam tip: Quote the definition in full, naming both kinetic and potential contributions and the random distribution.
See it
Key words
Say what each word means before you turn the card.
Internal energy
Try answering aloud or in your head before turning the card.
Kinetic energy contribution
Try answering aloud or in your head before turning the card.
Potential energy contribution
Try answering aloud or in your head before turning the card.
See every definition
- Internal energy
- The sum of the random distribution of kinetic and potential energies associated with the molecules of a system.
- Kinetic energy contribution
- Arises from the random motion of the molecules, and the kinetic energies of the molecules are randomly distributed.
- Potential energy contribution
- Arises from the forces between the molecules and depends on their separation.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Solid, liquid and gas: (d) internal energy as the sum of the random distribution of kinetic and potential energies associated with the molecules of a system
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge