AP Physics 2 / Unit 9: Thermodynamics
9.4 The First Law of Thermodynamics
Notes
$$\Delta U=Q+W$$
($W$ = work done ON the gas.)
- Isothermal: $\Delta U=0$. Adiabatic: $Q=0$. Isochoric: $W=0$. Isobaric: constant $P$, $W=-P\Delta V$.
- Work equals the area under the curve on a PV diagram.
Practice
A gas absorbs 50 J of heat and does 20 J of work on its surroundings. $\Delta U=$
$W_{on}=-20$, so $50-20$.
In an adiabatic process:
$Q=0$.
A gas expands at constant pressure 100 kPa by 0.02 m³. Work done BY the gas:
$P\Delta V$.
Ready for more? Take the Unit 9 test.