Thermia

Meet Thermia, a gas in a cylinder who takes in heat and pushes the piston up.

The first law of thermodynamics is energy conservation for heat and work. Heat Q added to a system can either raise its internal energy U, the energy of its moving and vibrating molecules, or leave as work W done by the system on its surroundings. So ΔU = Q − W. James Prescott Joule's experiments in the 1840s showed that heat and work are two forms of the same thing, energy.

If a flame adds 500 J of heat to the gas and the gas does 200 J of work lifting the piston, the internal energy rises by 500 − 200 = 300 J, and the gas gets warmer.

Every heat engine, from a car engine to a power-station turbine, relies on this balance, as do refrigerators and heat pumps, which use work to move heat.

Watch the sign convention. Here W is work done by the system; many chemistry texts use work done on the system, which writes the law as ΔU = Q + W. The first law also says nothing about which direction changes go; that is the second law.