1. What Is Internal Energy?
Internal energy is the total microscopic energy stored inside a system. It comes from the motion of particles (kinetic part) and the forces between them (potential part).
Even when a system looks still from the outside, its particles are constantly moving and interacting. This motion and interaction contribute to internal energy.
2. Components of Internal Energy
Internal energy has two main contributions:
2.1. Kinetic Energy of Particles
Particles in a system are always in motion—vibrating, rotating, or moving freely. The energy of this microscopic motion forms the kinetic part of internal energy. Higher temperature means faster motion and therefore higher kinetic energy.
2.2. Potential Energy of Interactions
Particles also attract or repel each other. The energy stored in these interactions contributes to the potential part of internal energy. Changes in volume or phase can change this energy.
3. Dependence on State Variables
Internal energy depends only on the state of the system—not on how the system reached that state. It is a state variable.
3.1. Factors Affecting Internal Energy
- Temperature – higher temperature increases kinetic energy.
- Volume – changing volume can change potential energy.
- Number of particles – more particles mean more total microscopic energy.
4. Change in Internal Energy
The internal energy of a system can change in only two ways:
- By adding or removing heat
- By doing work on the system or by the system
This relation is captured in the First Law of Thermodynamics.
4.1. Heat and Internal Energy
Supplying heat increases internal energy by increasing particle motion. Removing heat decreases internal energy.
4.2. Work and Internal Energy
When work is done on the system, particles are compressed and internal energy increases. When the system does work by expanding, internal energy decreases.
5. Internal Energy in Different States of Matter
The microscopic structure of solids, liquids, and gases affects how their internal energy behaves.
5.1. Solids
Particles mainly vibrate around fixed positions. Most internal energy is potential energy due to strong bonding.
5.2. Liquids
Particles can move and slide past each other. Both kinetic and potential contributions are significant.
5.3. Gases
Particles move freely and rapidly. Most internal energy comes from kinetic energy because intermolecular forces are very weak.
6. Everyday Examples of Internal Energy
- A hot cup of tea has higher internal energy than a cold one because its particles move faster.
- Compressing air in a pump increases its internal energy, making the pump warm.
- Melting ice absorbs heat and increases internal energy even though temperature stays constant.
- Steam contains much more internal energy than boiling water because of latent heat.