Principle

First law of thermodynamics

Δ E_{int} = Q - W

Statement

The change in a system's internal energy equals heat added minus work done by the system.

Symbols and units

$Δ E_{int}$	change in internal energy	J
$Q$	heat added to the system	J
$W$	work done by the system	J

When it applies

Any thermodynamic system and process, reversible or not.

Derivation

Energy added as heat, minus energy lost as work done by the system on its surroundings, equals the net change in internal energy — conservation of energy restated for a thermodynamic system:

Δ E_{int} = Q - W

Watch out for

Sign convention varies between sources — some define W as work done ON the system, giving ΔE = Q + W instead. Always confirm which convention a given formula uses.

Connects to

Conservation of mechanical energy