The first law is a general statement of energy conservation.

Work $W$ and heat $Q$ depend on the process by which the system is changed.

Thermal energy ${\mathit{E}}_{\mathbf{\text{th}}}$ Microscopic energy of moving molecules and stretched molecular bonds. $\text{Δ}{E}_{\text{th}}$ depends on the initial/final states but is independent of the process.

Work $\mathit{W}$ Energy transferred to the system by forces in a mechanical interaction.

Heat $\mathit{Q}$ Energy transferred to the system via atomic-level collisions in a thermal interaction.

The work done on a gas is

When two or more systems interact thermally, they come to a common final temperature determined by

An adiabatic process has $Q=0$. Gases move along an adiabat for which $p{V}^{\gamma }=$ constant, where $\gamma =C_{\text{P}}\text{/}{C}_{\text{V}}$ is the specific heat ratio. An adiabatic process changes the temperature of the gas without heating it.

The heat of transformation $L$ is the energy needed to cause 1 kg of substance to undergo a phase change

The specific heat $c$ of a substance is the energy needed to raise the temperature of 1 kg by 1 K:

The molar specific heat $C$ is the energy needed to raise the temperature of 1 mol by 1 K:

The molar specific heat of gases depends on the process by which the temperature is changed:

• $C_{\text{V}}=$ molar specific heat at constant volume

• $C_{\text{P}}=C_{\text{V}}+R=$ molar specific heat at constant pressure

Heat is transferred by conduction, convection, radiation, and evaporation.

• Conduction: $dQ/dt=kA(\text{Δ}T/L)$

• Radiation: $dQ\text{/}dt=e\sigma A{T}^4$