(a) Power is defined as the rate of doing work, it is the work done in unit time. The SI unit of power is Watt (W) which is joules per second $(\mathrm{J}/\mathrm{s})$. (b) Given : Potential difference $=V=5\ \mathrm{V}$$\;\text{Current}\;=I=500\ \mathrm{mA}=\;\frac{500}{1000\ \mathrm{A}}=0.5\ \mathrm{A}$(i) Power $=V \times I=5 \times 0.5=5 \times \;\frac{5}{10}=2.5\ \mathrm{W}$(ii) Resistance $=R=$ ?By Ohm's law: $V=I R$$R\;=\;\frac{V}{I}$$R\;=\;\frac{5}{0.5}$$\;=\;\frac{50}{5}=10\ \mathrm{ohms}$(iii) Energy consumed when it is lighted for $2\;\frac{1}{2}$ hoursEnergy $=$ Power $\times$ Time$t=2.5 \times 60 \times 60\ \mathrm{sec}.$So,$\;\text{Energy}\;\;=2.5 \times 2.5 \times 60 \times 60$$\;=22500\ \mathrm{Joules}/\mathrm{sec}.$