When we heat a pressure cooker, the heat we supply to the water is used to increase the temperature of the water, to evaporate part of the liquid and to produce mechanical work on the safety valve as the steam escapes through it. The heat we started with has been distributed into different forms of energy.

The first law of thermodynamics establishes how this distribution is made.

In the following visual we will try to rediscover this first law by exploring a very simple system: a gas is contained in a receptacle whose walls absorb a negligible amount of heat. 

The lid of the receptacle can slide up and down depending on the pressure of the gas, unless we fix it purposely to maintain the volume constant.