Paper 2 Thermal Physics B Answers

Q1 Ideal gas equation

1. $V=nRT/p=0.240(8.31)(310)/(2.10\times 10^5)=2.94\times 10^{-3}{\mathrm{m}}^3$. [2]
2. $N=n{N}_A=0.240(6.02\times 10^{23})=1.44\times 10^{23}$. [2]
3. $⟨E_k⟩=\frac{3}{2}kT=1.5(1.38\times 10^{-23})(310)=6.42\times 10^{-21}\mathrm{J}$. [2]

Q2 First law

1. $\Delta U=850-320=530\mathrm{J}$. [2]
2. Temperature increases. [1]
3. Some supplied energy leaves as work done by the gas; only the remainder increases internal energy. [2]

Q3 p-V work

1. $W_{by}=p\Delta V=1.50\times 10^5(3.0\times 10^{-3})=450\mathrm{J}$. [2]
2. Work done on the gas is $-450\mathrm{J}$. [1]
3. Work by gas is the area under the pressure-volume graph for expansion. At constant pressure this area is a rectangle $p\Delta V$. [2]