Temperature describes the state of thermal equilibrium between systems. Two common scales are used in physics. The Celsius scale fixes the freezing point of water at 0 °C and the boiling point at 100 °C, while the Kelvin scale is the absolute thermodynamic scale with its zero at absolute zero, the lowest possible temperature. Conversion between the two is straightforward, with \(T(\text{K}) = T(°\text{C}) + 273.15\). Heat is the energy transferred between objects at different temperatures, and the amount of heat required to change the temperature of a substance is given by \(Q = mc\Delta T\), where \(c\) is the specific heat capacity, a property that varies from material to material.
The laws of thermodynamics formalize the behavior of energy and heat. The zeroth law establishes the concept of temperature itself: if two systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other. This seemingly obvious statement allows temperature to be used as a consistent, comparable quantity. The first law is a statement of energy conservation applied to thermodynamic systems: the change in internal energy \(\Delta U\) of a system equals the heat \(Q\) added to the system minus the work \(W\) done by the system, written \(\Delta U = Q - W\). The second law introduces the concept of entropy, a measure of disorder or of energy that is no longer available to do useful work. In any spontaneous process, the total entropy of the universe either increases or stays the same, with \(\Delta S_{\text{universe}} \geq 0\).
For a gas that is dilute and at moderate temperature and pressure, the ideal gas law gives a simple relationship among pressure, volume, temperature, and amount of substance, expressed as \(PV = nRT\), where \(R\) is the universal gas constant. This law is a good approximation for many real gases and is a powerful tool for analyzing thermodynamic processes.