# Biological Physics/The Zeroth & First Laws

The zeroth law of thermodynamics came as an historical afterthought. It is often phrased in terms of thermal equilibrium (equal temperature), however, the basic idea may be applied to all types of equilibrium.

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

Consider an ice chest full of ice and a soda can in the chest. The inside of the ice chest is system A. The can is system B. The soda is system C. If the ice has had time to bring the can to 0oC, system A is in thermal equilibrium with system B. If the can has had time to bring the soda to 0oC, system B is in thermal equilibrium with system C. We could also say this system is in mechanical equilibrium since there is no volume change occurring once everything is 0oC.

The first law of thermodynamics deals with the conservation of energy.

${\displaystyle \Delta U=W+Q}$

Its essence is that all changes in energy, ${\displaystyle \Delta U}$ of a system can be tracked by work ${\displaystyle W}$ and heat energy ${\displaystyle Q}$.