Question

The internal energy increases by 25 J as a gas-filled piston expands 0.30 m as the piston is pushed outward by a 70 N force. What is the heat added/removed from the system?

Answer 1

By the first law of thermodynamics, we have:

$$\Delta U=Q-W,$$

here, $\Delta U$ is the change in the internal energy of the gas, $Q$ is the heat added or removed from the gas, $W$ is the work done by the gas (since the gas volume expands, the work done by the gas has negative sign).

Then, we can find $Q$:

$$Q=\Delta U+W=\Delta U+Fd,$$ $$Q=25\ J+70\ N\cdot0.3\ m=46\ J.$$

Since $Q$ is positive, heat is added to the system. Therefore, 46 J of heat added to the system.