# Laws of thermodynamics

There are three laws of thermodynamics:

The total amount of energy in the universe is constant: $\displaystyle \Delta{} E_{\text{universe}} = 0$. Energy can be neither created nor destroyed; it can only be transferred from system to surroundings or vice versa, or transform from one form to another. (This is also called the law of conservation of energy.)

All changes either directly or indirectly increase the entropy of the universe: $\displaystyle \Delta{} S_{\text{universe}} > 0$. This means the entropy of the universe is always increasing, never decreasing. If entropy decreases in a system, it is only possible because there is an even greater increase in the surroundings, so the overall change in the universe is positive.

The entropy of a perfectly ordered pure crystalline substance is zero when its temperature is absolute zero: $\displaystyle T = 0 \Rightarrow S = 0$.