Here’s a explanation:
Internal Energy (U)
- Internal energy is the total energy contained within a system due to:
- Kinetic energy of molecules (motion, vibration, rotation)
- Potential energy of molecules (chemical bonds, interactions)
- It does not include energy from the system moving as a whole or external fields.
How Internal Energy Changes
Internal energy of a system changes when:
- Heat (Q) is added or removed
- Adding heat → internal energy increases
- Removing heat → internal energy decreases
- Work (W) is done by or on the system
- System does work on surroundings → internal energy decreases
- Work is done on system → internal energy increases
Key idea:
- The first law of thermodynamics links these changes:
- Any change in internal energy = Heat added – Work done by the system
In short: Internal energy is the energy inside a system, and it changes whenever heat is exchanged or work is done.