# What is the first law of thermodynamics in simple terms?

Sep 17, 2016

Formally, we define it as the change in internal energy, $\Delta U$, is equal to the sum of the heat flow $q$ and the pressure-volume work $w$. We write that as:

$\Delta U = q + w$

• The internal energy is just the energy in the system.
• The heat flow is the component of the energy that goes into heating whatever is in the system, or cooling it. It is said to be negative for cooling and positive for heating.
• The pressure-volume work is the component of the energy that goes into expanding or compressing whatever is in the system. Often, it is defined to be negative for expansion and positive for compression because expansion is work done by the system and compression is work done on the system.

In simpler terms, we could say that energy is conserved for a closed system, and is put into heating, cooling, expansion, compression, or some combination of those.

(An interesting note is that the universe can be considered a "closed" system, since it is so large compared to us.)