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The first law

The first law is conservation of energy applied to thermodynamic systems: heat added either increases the system's internal energy or does work on the surroundings. The signs must be kept consistent, with $W$ as the work done by the system itself.

\Delta U = Q - W

first law, closed system

Symbols

$\Delta U$	change in internal energy	J
$Q$	heat added	J
$W$	work done by the system	J

Example

A system receives $Q = 500$ J of heat and does $W = 200$ J of work on the surroundings. The change in internal energy is $\Delta U = 500 - 200 = 300$ J.

W

is the work the system does on the surroundings, not the work done on it – flip the sign if a problem gives the opposite.

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Part of Thermodynamics: Basic concepts.