Internal Energy

Definition of Internal Energy

Internal energy is the total energy stored within a system due to the microscopic motion and arrangement of its particles. It includes:

• Kinetic energy of particles moving randomly
• Potential energy from the forces between particles depending on their positions

Internal energy is measured in joules (J), the standard unit of energy in the UK and SI system.

Components of Internal Energy

The internal energy of a substance depends on two main components:

• Kinetic energy of particles: All particles (atoms or molecules) are constantly moving. The faster they move, the greater their kinetic energy.
• Potential energy of particles: Particles attract or repel each other depending on their positions. The energy stored in these forces is the potential energy.

When a substance is heated, energy is transferred to the particles, increasing their internal energy by raising either their kinetic or potential energy.

For example, when heating a solid, particles vibrate faster (kinetic energy increases). When melting, particles move further apart, increasing potential energy as bonds weaken.

Changes in Internal Energy

Internal energy changes when energy is transferred into or out of a system:

• If energy is added (e.g., by heating), the internal energy increases.
• If energy is removed (e.g., by cooling), the internal energy decreases.

These changes can cause:

• A change in temperature (affecting kinetic energy)
• A change of state (affecting potential energy)

It is important to note that internal energy is the sum of both kinetic and potential energies, so a temperature change does not always mean the internal energy changes in the same way.

Internal Energy and Temperature

Temperature is a measure of the average kinetic energy of the particles in a substance. However, internal energy includes both kinetic and potential energy.

This means:

• A change in temperature usually means a change in the average kinetic energy of particles, so internal energy changes.
• But during a change of state (e.g., melting or boiling), temperature stays constant while internal energy changes because potential energy changes as particles rearrange.

For example, when ice melts at 0°C, its temperature remains constant, but energy is absorbed to increase the potential energy as particles move apart.

Understanding this difference is key to grasping how energy is stored and transferred in materials.

For instance, if 500 J of energy is added to a system, causing the average kinetic energy of particles to increase, the internal energy rises by 500 J.