4.1 Thermal physics - Heat and Energy

All matter (solids, liquids, and gases) is made of particles (atoms and molecules).

Internal Energy is the total energy stored inside a system. It is the sum of:

• Kinetic Energy of the particles (from their movement).
• Potential Energy of the particles (from their position relative to each other).

Heating an object increases its internal energy. This can either increase its temperature (particles move faster) or change its state (particles break their bonds).

During a change of state (like melting or boiling), energy is put in, but the temperature does not change. This energy is used to break the bonds holding the particles together, increasing their potential energy, not their kinetic energy.

The Specific Heat Capacity (c) of a substance is the amount of energy needed to raise the temperature of 1 kg of that substance by 1 °C.

A substance with a high specific heat capacity (like water) takes a lot of energy to heat up. A substance with a low one (like metal) heats up very quickly.

Change in Thermal Energy = Mass × Specific Heat Capacity × Change in Temperature

ΔE = m × c × ΔT

Where:

• ΔE is the energy transferred in joules (J)
• m is the mass in kilograms (kg)
• c is the specific heat capacity in joules per kilogram per degree Celsius (J/kg°C)
• ΔT is the change in temperature in degrees Celsius (°C)

Specific Latent Heat (L) is the amount of energy needed to change the state of 1 kg of a substance without any change in temperature.

• Specific Latent Heat of Fusion: The energy needed to melt (solid to liquid) or freeze (liquid to solid).
• Specific Latent Heat of Vaporisation: The energy needed to boil (liquid to gas) or condense (gas to liquid).

Energy for a Change of State = Mass × Specific Latent Heat

E = m × L

Where:

• E is the energy transferred in joules (J)
• m is the mass in kilograms (kg)
• L is the specific latent heat in joules per kilogram (J/kg)