A gas is held at constant temperature while its pressure and volume are allowed to change together. If the gas behaves ideally, what does the source say stays constant throughout this process?
AThe product of pressure and volume, PV
BThe pressure alone, regardless of volume
CThe volume alone, regardless of pressure
DThe rate of heat transfer into the gas
Answer & Solution
Correct answer: A. The product of pressure and volume, PV
1. A process at constant temperature is called isothermal, and for an ideal gas PV = nRT.
2. Since n and R never change and T is held fixed in this process, the product PV must also stay fixed.
3. Pressure and volume can each still change individually during the process; as one falls the other must rise to keep PV constant, so options B and C describe something that does not happen.
4. Heat transfer actually varies continuously during an isothermal process to keep temperature constant, so it is not the constant quantity here, ruling out option D.
_Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.2 The First Law of Thermodynamics and Some Simple Processes_
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