First-Law Energy Balances, State Functions, and Process Work

All flashcards

What does the first law of thermodynamics state?

The change in a system’s internal energy is the heat transferred to it minus the work done by it: $\Delta E_{\mathrm{int}}=Q-W$.

When is $Q$ positive?

$Q$ is positive when heat is added to the system.

When is $W$ positive?

$W$ is positive when work is done by the system, such as when an expanding gas pushes a piston.

What sign represents work done on a system?

Work done on the system is negative under this convention, so $W<0$.

What happens to the internal energy of an isolated system?

It remains constant when the isolated system exchanges neither heat nor work with its surroundings.

Quiz questions

1. For a quasi-static gas process, how is work done by the gas calculated when pressure may vary with volume?Answer: $W=\int_{V_1}^{V_2}p\,dV$
2. For a mass $m$ whose temperature changes by $\Delta T$, what relation connects internal-energy change to specific heat capacity $c$ under the stated assumptions?Answer: $\Delta E_{\mathrm{int}}=mc\Delta T$
3. What is the internal-energy change of an ideal gas during an isothermal process?Answer: $\Delta E_{\mathrm{int}}=0$
4. What expression gives the heat required to vaporize a mass $m$ of a substance with latent heat of vaporization $L_v$?Answer: $Q=mL_v$
5. Does the first law require every intermediate state of a process to be in equilibrium?Answer: No; it applies between equilibrium endpoints even if the intermediate process is not quasi-static.