91Ó°ÊÓ

1. Fermi energy of copper, ${\mathrm{E}}_{\mathrm{F}}=7\mathrm{eV}$
2. Mass of copper coin,$\mathrm{m}=3.10\mathrm{g}$
3. Molar mass of copper,$\mathrm{A}=63.54\mathrm{g}/\mathrm{mol}$
4. Power or the rate of energy used of the lamp, P = 100 W

The transfer of energy from one body to another in unit time interval is known as power. It is also described as the rate of doing work with respect to time.

Formulae:

The average energy used by the a body,${\mathrm{E}}_{\mathrm{avg}}=\frac{3}{5}{\mathrm{E}}_{\mathrm{F}}$ (i)

The total energy released by the conduction electrons,$\mathrm{E}={\mathrm{NE}}_{\mathrm{avg}}$ (ii)

where, N is the number of electrons present in the material and is the Fermi energy.

The rate of energy consumption, P = E /t (ii)

From equation (i) and equation (ii), we get -

$\mathrm{E}=\left(\frac{\mathrm{m}}{{\mathrm{AN}}_{\mathrm{A}}}\right)\left(\frac{3}{5}{\mathrm{E}}_{\mathrm{F}}\right)$

$$\begin{gathered} \left(âˆ\mu \mathrm{where},N\mathit{=}\frac{\mathit{m}\mathit{a}\mathit{s}\mathit{s}\mathit{\left(}\mathit{m}\mathit{\right)}}{\mathit{m}\mathit{o}\mathit{l}\mathit{a}\mathit{r}\mathit{m}\mathit{a}\mathit{s}\mathit{s}\mathit{\left(}\mathit{A}\mathit{\right)}\mathit{×}\mathit{a}\mathit{v}\mathit{o}\mathit{g}\mathit{a}\mathit{d}\mathit{r}\mathit{o}\mathit{n}\mathit{u}\mathit{m}\mathit{b}\mathit{e}\mathit{r}\mathit{\left(}{\mathit{N}}_{\mathit{A}}\mathit{\right)}}\right) \\ =\left(\frac{3.1\mathrm{g}}{\left(63.54g/mol\right)\left(6.022×{10}^{23}/\mathrm{mol}\right)}\right)\left(\frac{3}{5}\left(7\mathrm{eV}\right)\left(1.6×{10}^{-19}\mathrm{J}/\mathrm{eV}\right)\right) \\ =1.97×{10}^4\mathrm{J} \end{gathered}$$

Hence, the value of the energy is .

Using the above value of energy, we can get the time till which the energy is lit up as follows:

$$\begin{gathered} t=\frac{1.97×{10}^4\mathrm{J}}{100\mathrm{J}/\mathrm{s}} \\ =197\mathrm{s} \end{gathered}$$

Hence, the value of the time is 197 s .