Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

A piston, a refrigerator, and a space suit

A space suit

Both sides of the equation PV = nkT have units of energy. Suppose the pressure in a cylinder of gas pushes a piston out, as in the power stroke of an automobile engine. Let the cross-sectional area of the piston and cylinder be A, and let the piston travel a small distance Δx. Then the gas's force on the piston F = PA does an amount of mechanical work W = FΔx = PAΔx = PΔV, where ΔV is the change in volume. This energy has to come from somewhere; it comes from cooling the gas. In a car, what this means is that we're harvesting the energy released by burning the gasoline.

In a refrigerator, we use the same process to cool the gas, which then cools the food.

In a space suit, the quantity PΔV represents the work the astronaut has to do because bending her limbs changes the volume of the suit. The suit inflates under pressure like a balloon, and doesn't want to bend. This makes it very tiring to work for any significant period of time.

Last Update: 2010-11-11