1.2 Zero’s Axiom of Thermodynamics

• Thermal equilibrium exists

• It is characterized by a universal temperature

• Within a system thermal equilibrium is formed

• Two systems in thermal contact will exchange heat energy $\Rightarrow$ a universal temperature in both systems result

• This exchange process allows for temperature measurement with a thermometer

• This exchange process is transitive; i.e. if a system is in thermal equilibrium with two other systems, both systems are in thermal equilibrium with each other ($\Rightarrow$ This allows to compare systems of the same temperature)

• Temperature measurements can e.g. be performed with a gas thermometer: For an ideal gas in thermal contact with another system one can e.g. measure the volume; applying the formula

  $$pV=nRT$$

  (1.1)

  this defines the temperature of the gas.

• But: the heat exchange can disturb the measured system