# Q & A: heat from compression and friction

Q:
When the Space Shuttle re-enters the atmosphere heat is generated I know that. However, is that heat caused by compressing gas or by friction. Or both? Essentially, is the heat from compressing a gas just frictional heat?
- Alex Frasca (age 17)
Setauket, NY USA
A:

Initially those are two separate effects. Think of when you slowly compress a gas with a piston in a thermally insulated cylinder. It gets hot because you've put in energy by doing work on it. The energy can come back out as work if the piston is allowed to move back out. If you heat the gas up the same amount by say rubbing two plates together in it, you can't get as much work out because the piston wasn't compressed. So heating by compression doesn't reduce free-energy (the ability to do work) but heating by friction does. Another way to say that is that compressive heating (at least if it's slow enough) doesn't increase entropy, but frictional heating does.

If you look at what happens when the space shuttle passes through the atmosphere, the atmosphere is not compressed overall, just heated. So overall the energy loss is through friction. On the way, however, a bit of energy is stored in compressed air before getting lost via friction in the air as it re-expands.

Mike W.

(published on 08/21/2013)