Temperature and volume are the variables for this piston.
Temperature is a measure of hotness or coldness
The volume of a gas is directly proportional to its absolute temperature. More specifically, for a fixed mass of gas at constant pressure, the volume (V) is directly proportional to the absolute temperature (T).
This is Charles' Law. V = kT, where k is a proportionality constant.
Hence, temperature and volume are the variables for this piston.
Learn more about temperature here:
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