Laws of thermodynamics


There are three laws of thermodynamics, plus a zeroth law!

Zeroth law:

two systems which are in thermal equilibrium with a third system are in thermal equilibrium with each other. We saw this in energy and temperature  in both Physics and Chemistry.

First law:

the change in internal energy, ΔU, of a closed system is equal to the heat energy into the system, Q, plus the work done, W, on the system. This is adapted from conservation of energy. This can be rewritten in terms of work done by the system by subtracting W. This can be written:


There are lots of special cases for this law.

Second law:

the entropy of an isolated system always increases over time. This means all changes in entropy must be positive. This means that heat cannot be converted completely into work and heat cannot be transferred from a cold body to a hot one as the only result of a process. There’s a derivation here.


Third law:

a perfect crystalline structure at 0 K (absolute zero) has no entropy.

Back to Contents: Physics: Thermodynamics

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