The SI unit of temperature is Kelvin (K), named after the British physicist Lord Kelvin.

Temperature is a measure of the average kinetic energy of particles in a substance.

Heat can be transferred from a hotter object to a colder object through conduction, convection, and radiation.

Fourier's Law states that the rate of heat flow through a material is directly proportional to the temperature difference between the two sides of the material.

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or transformed.

The second law of thermodynamics states that entropy always increases in an isolated system.

The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero is zero.

Expansion of a gas in a vacuum is an example of a reversible process because it can be reversed by compressing the gas.

Melting of ice is an example of an irreversible process because it cannot be reversed by freezing the water.

The efficiency of a heat engine is defined as the ratio of the work output to the heat input.

The Carnot efficiency is given by $1 - \frac{T_C}{T_H}$, where $T_C$ is the temperature of the cold reservoir and $T_H$ is the temperature of the hot reservoir.

The refrigerator's coefficient of performance (COP) is defined as the ratio of the heat output to the work input.

The heat pump's coefficient of performance (COP) is defined as the ratio of the heat output to the work input.

An air conditioner is an example of a heat pump because it transfers heat from the inside of a building to the outside.

A refrigerator is an example of a refrigerator because it transfers heat from the inside of a refrigerator to the outside.