First Law of Thermodynamics
The energy of an isolated system is constant; energy can be converted between forms but never created or destroyed.
Formula / equation
ΔU = q + w
Unit
J
In depth
The first law is the principle of energy conservation applied to thermodynamic systems. It rules out perpetual motion of the first kind but says nothing about direction — it permits heat to flow from cold to hot, which the second law then forbids.
Examples in the real world
A gas absorbing 500 J of heat while doing 200 J of work gains exactly 300 J of internal energy.