Explain the first law of thermodynamics with its equation and the meaning of each sign. What are two of its limitations?

1. 1 · State the law. Heat given to a system = increase in its internal energy + work done by it. It is the conservation of energy.
2. 2 · Write the formula. ΔQ = ΔU + ΔW, or ΔU = ΔQ − ΔW.
3. 3 · Give the signs.
   • Heat given to the system is +; heat lost is −.
   • Work done by the system is +; work done on it is −.
   • A rise in internal energy is +.
4. 4 · Note about the path. ΔQ and ΔW depend on the path, but ΔU does not.
5. 5 · First limitation. It does not tell the direction in which heat flows.
6. 6 · Second limitation. It does not tell how much heat can be turned into work.
7. 7 · Real life. Rubbing your hands turns work into internal energy, which you feel as warmth.
8. 8 · Close in one line. The first law balances energy; the second law tells which way it can go.

Scoring tip: Write the sign convention as three short lines. Many answers lose a mark by giving the formula without saying what + and − mean.