# Chapter 21: Thermal Physics

Internal Energy
The internal energy of a substance is the random distribution of kinetic and potential energy of its atom or molecules. It is due to the vibrations/movement energy of the particles and the energy due to bonds holding them together.

Solids: In a solid the particles are arranged in a fixed structure, they cannot move from their position in the structure but can vibrate. The internal energy of a solid id due to the vibrating particle and the potential energy is from the bond between them.

Liquid: In liquid, the particles vibrate and are free to move but are still in contact with each other. The forces between them are less. The internal energy of liquid is due to the kinetic energy and potential energy of the particle.

Gases: In gas particles are free to move and there is almost no forces of attraction between them. The internal energy of gas is almost entirely due to kinetic energy of particles.

Energy Change
A solid has fixed volume and shape. During the transition between solid and liquid, the energy supplied does not increase the K.E., hence the temperature of the solid, instead it is used to overcome the intermolecular forces between atoms which increases the potential energy and this increases the latent heat of fusion.

A liquid has a fixed volume and no shape. During the transition between liquid and gas, the intermolecular forces in the liquid overcome the latent heat of vaporization.

A gas has no shape and volume. It widely separates and is free to move around within their container.

First Law of Thermodynamics
The increase in the internal energy of the body is equal to the thermal energy transferred to it by heating and the mechanical work done to it.

ΔU = q + w

Temperature
Temperature is the measurement of the kinetic energy of the particles in the substance. Higher temperature means that the particles are vibrating with higher speed and low temperature means that the particle are vibrating with less speed.

Thermometer is the device which is used to measure the temperature of the substance. Temperatures on the thermodynamic (Kelvin) and Celsius scale of temperature can be related as:

K = 0 C + 273.15

0 C = K – 273.15

Specific Heat Capacity
The energy required per unit mass of substance to raise its temperature by 1 K or (10 C). Its unit is Jkg-1K-1.

specific heat capacity= (energy supplied)/(mass ×temperature)

Specific Latent Heat of Fusion
The energy required per unit mass of substance to change it from solid to liquid without changing temperature. Its unit is Jkg-1.

Specific Latent Heat of Vaporization
The energy required per unit mass of substance to change it from liquid to gas without a change in temperature. Its unit is Jkg-1.