In thermodynamics, what is the system?

For this section's sign convention, is \(Q\) positive or negative when heat enters the system?

A gas pushes a piston outward. Is work done by the gas positive or negative in this section's convention?

A gas in a cylinder is compressed while heat leaves the gas. Taking the gas as system, state the signs of \(Q\) and \(W\).

For \(0.75\,\mathrm{mol}\) of ideal gas at \(300\,\mathrm{K}\) in volume \(0.020\,\mathrm{m^3}\), find the pressure.

A gas has \(p=1.2\times10^5\,\mathrm{Pa}\), \(V=0.030\,\mathrm{m^3}\), and \(T=290\,\mathrm{K}\). Estimate \(n\).

A piston-cylinder gas absorbs \(400\,\mathrm{J}\) as heat while the surroundings do \(150\,\mathrm{J}\) of work on the gas. State \(Q\), \(W\), and \(\Delta U\) using this section's convention.

A sealed rigid tank is heated. Identify the system boundary, whether volume work occurs, and the sign of \(Q\).

The same air in a pump can be treated either as the system or as part of the surroundings. Explain how the signs of work change when the system choice is reversed, using compression as the example.

A gas, piston, and surrounding weights are considered as one combined system during a slow expansion. Explain which energy transfers across the system boundary remain, and why the piston work between gas and weights is no longer boundary work for the combined system.