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 distance
by the gas.
The pressure
pushing down on the piston exerts a force
To force the piston back a distance
against a force
an amount of work equal to
must be done by the gas.
is the increase in volume
of the gas so we can write
This 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.