Temperature, Thermal Equilibrium, and Zeroth Law

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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.