The Bicycle Pump

A student pumping up a bike tire notices that as they pump with vigor that the bicycle valve and pump itself appears to be warm. The student points this out to some friends and they discuss the possibilities.

• A. One student says that it is simply friction of the pump handle and action that heats the air and the pump.
• B. A second students says it's a thermodynamic effect, that the work being done goes both into compressing and also heating the air.
• C. A third student says it's really the interaction of the compressed air and the valve. As the compressed air in the pump tries to rush into the small throat of the valve, many air molecule collisions take place heating the air and the valve.
• D. Where do you get these questions?

See if you picked correctly.

Answer

The Bicycle Pump - Answer = B

Answer: The correct answer is B. While friction, both by the mechanical pump action and air molecule collisions, adds some small amounts of heat, the main effect is that the pumping process is basically an adiabatic (no heat loss) process. That is, the pumping takes place over a short enough period of time that effectively no heat is lost to the outside. Therefore the work done compressing the gas goes into the change in internal energy of the gas. The internal energy of the gas is proportional to the absolute temperature. So as the internal energy increases so does the temperature of the gas which in turn heats the pump and the valve.

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