6

Compressing air without heating or cooling

Air can be compressed quickly in a bicycle pump. If we do this repeatedly, the end of the pump becomes quite warm. We can perform the same process in a cylinder with a piston and put a small piece of tinder in the cylinder. Already after a single quick stroke the tinder is observed to ignite in a flash (Movie 1).

Interpretation

The fact that the tinder ignites tells us that the air has become very hot. This explains why a bicycle pump becomes hot after a few stokes. Heat flows from the hot air through the pump casing into the surroundings.

Why does the temperature of the air rise so dramatically as the result of a quick, strong stroke? After all, there is no fire to heat the air, heat cannot flow from the surroundings into the air (the air is hotter than the surroundings!). So we do not add heat from outside. There could be friction in the air, and indeed there is, but only very little. We can understand this as follows: The compressed air is almost ideally elastic, the volume would be restored if we just let the piston move out again. Since we do not add heat, and since heat is not produced, the quantity of heat (*) of the air stays constant. The temperature of the air rises because its heat is compressed into a much smaller space.

The example demonstrates that temperature and heat cannot be the same quantity. Here, the quantity of heat stays constant while temperature changes.

(*) Note: The technical term for quantities of heat is entropy. What we call heat, and what is officially called entropy by engineers and physicsist is closely related to what scientisits before 1850 called caloric. When presenting phenomena in terms of standard language, it makes sense to keep using the word heat for what we normally call entropy.