Internal Energy
Internal Energy
- Thermodynamics studies energy inside a chosen system. The system may be a gas, a liquid, a solid or another object named in the question.
- First decide what the system is. Then decide what happens to the particles inside it.
Energy of the whole object and its particles
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- A box moving across a room has kinetic energy as a whole object. That motion is not part of its internal energy.
- Inside the box, its particles move in random directions and pull or push on nearby particles. Those particle energies are internal.
What internal energy means
- The random kinetic energy comes from random particle motion.
- The potential energy depends on particle separation and the forces between particles.
- Add the two parts for every particle in the system. This gives the .
Definition
2 marksWhat is meant by the internal energy of a system?
Model answer: The sum of the kinetic and potential energies associated with the random motion of the particles in the system.
A recent mark scheme gives one mark for the kinetic-plus-potential energy sum and one mark for random particle motion (9702/41/M/J/25 Q4(a)(i)).
State, temperature and changes of state
- Internal energy is fixed by the state of the system. If the system returns to the same state, it returns to the same internal energy. The path taken does not matter.
- When temperature rises, the mean random kinetic energy of the particles rises. The internal energy therefore rises.
- Temperature can stay constant while internal energy rises. During vaporisation, the mean kinetic energy stays constant, but the particles move farther apart and their potential energy rises.
Temperature rises; state does not change
Random kinetic energy rises. Molecular potential energy may stay nearly unchanged. Internal energy rises.
Liquid vaporises at constant temperature
Random kinetic energy is unchanged. Molecular potential energy rises as particles separate. Internal energy rises.
This separate kinetic-energy and potential-energy check is used directly in recent papers (9702/42/M/J/24 Q3(b)).
The ideal-gas case
- In the ideal-gas model, there are no forces between molecules except during collisions. The molecular potential energy is therefore zero.
- The internal energy is then the total random kinetic energy of all the molecules.
- For a fixed amount of ideal gas, molecular kinetic energy is proportional to thermodynamic temperature. Its internal energy is therefore also proportional to thermodynamic temperature (9702/41/M/J/25 Q4(a)(ii)).