# Temperature, Thermal Equilibrium, and Zeroth Law

## Temperature

The definition of temperature is very problematic. The problem is that we cannot define temperature from a macroscopic viewpoint.

A microscopic definition can be developed from statistical thermodynamics, but that is an advanced topic.

## Units of Temperature

Temperature in thermodynamics is always given as absolute temperature.

Absolute temperature unit is Kelvin

Kelvin = °C + 273

Note that the unit is a Kelvin not a "degree Kelvin" and no degree symbol is used.

## Thermal Equilibrium

Even though we cannot define temperature, We can define the concept of thermal equilibrium

Thermal Equilibrium
When two systems (or subsystems) have the same temperature

## Zeroth Law of Thermodynamics

If system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then system A is in thermal equilibrium with system C.

Figure 1: Zeroth law of thermodynamics: If subsystems A and B have the same temperature, and if subsystems A and C have the same temperature, then subsystems B and C must also has the same temperature.