Question

A gas is contained in a vertical piston–cylinder assembly by a piston with a face area of 260 cm2 and weight of 450 N. The atmosphere exerts a pressure of 101.3 kPa on top of the piston. A paddle wheel transfers 3 kJ of energy to the gas during a process in which the elevation of the piston increases slowly by 0.9 m. The piston and cylinder are poor thermal conductors, and friction between the piston and cylinder can be neglected.

1) A force balance on the piston shows that, for this process, the gas pressure is:
Option A: constant and negative.
Option B: decreasing and positive.
Option C: constant and positive.
Option D: increasing and positive.

Answer 1

In a piston–cylinder assembly with a poorly conducting piston and no friction, the internal gas pressure must remain constant and positive to balance the weight of the piston and atmospheric pressure as the piston moves, making the correct answer Option C: constant and positive.

Step-by-step explanation:

The student question is asking to determine the nature of the gas pressure inside a piston–cylinder assembly as a response to a given process. Looking at the system, a force balance on the piston must account for the weight of the piston and the atmospheric pressure, in addition to the internal gas pressure. The weight of the piston and the atmospheric pressure are constants, and if we assume that there is no change in temperature and no heat loss (poor thermal conductors), according to the descriptions of processes in a piston-cylinder system, the internal gas pressure must remain constant to balance the forces as the piston moves up.

Therefore, we can infer that the gas pressure must remain constant to counterbalance the weight of the piston and the atmospheric pressure. This means the answer to the question is Option C: constant and positive.