How long can an electric lamp of 100W be kept glowing by fusion of 2.0 kg of deuterium? Take the fusion reaction as 
${ }_1^2\mathrm{H}{+}_1^2\mathrm{H}{⟶}_2^3\mathrm{He}+\mathrm{n}+3.27\mathrm{MeV}$

$2_1{\mathrm{H}}^2{\to }_2{\mathrm{He}}^3+3.27\mathrm{MeV};\mathrm{P}=100\mathrm{W}$
for 2g→N_A atoms
$\lg \to \frac{{\mathrm{N}}_{\mathrm{A}}}{2}; \mathrm{for} 2\mathrm{kg}\to \frac{{\mathrm{N}}_{\mathrm{A}}}{2}\times 2\times {10}^3$
$=6.023\times {10}^{23}\times {10}^3;\mathrm{N}=6.023\times {10}^{26} \mathrm{atoms}$
E for each ₁H² atom $=\frac{3.27}{2}=1.635\mathrm{MeV}$
for N atoms $\Rightarrow 6.023\times {10}^{26}\times 1.635\times 1.6\times {10}^{-13}\mathrm{J}$
$\begin{array}{l}\mathrm{t}=\frac{\mathrm{E}}{\mathrm{P}}=\frac{6.023\times 1.635\times 1.6\times {10}^{13}}{100}\mathrm{s}\\ =\frac{6.023\times 1.635\times 1.6\times {10}^{11}}{365\times 86400}\mathrm{yrs}\therefore \mathrm{t}=4.99\times {10}^4\mathrm{yrs}\end{array}$