When explosives are detonated large amounts of gas at high pressure are produced. As the gas expands it does work by expanding against the force exerted by the atmosphere. We can find an expression for the work done by a gas when it expands against a constant pressure.

Suppose the piston is pushed back or upwards a distanceby the gas.

The pressurepushing down on the piston exerts a force

To force the piston back a distanceagainst a forcean amount of work equal to must be done by the gas.is the increase in volumeof the gas so we can writeThis equation is general and applies not only to pistons. It must be noted however in general an expanding gas cools down. The work supplied to expand the gas generally comes from the internal energy of the gas – the heat energy mostly. In the example of the explosion above, the gas is created and heated by the explosion, then expands. In expanding it cools down.