Define current density. Write an expression which connects current density with drift speed.

A metallic wire having a resistance of $20 \Omega$ is bent in order to form a complete circle. Calculate the resistance between any two diametrically opposite points on the circle.

A uniform copper wire having a cross-sectional area of $1 {\mathrm{mm}}^2$ carries a current of $5 \mathrm{A}$. Calculate the drift speed of free electrons in it. (Free-electron number density of copper$=2\times {10}^{28}/{\mathrm{m}}^3$)

An electric bulb is rated as $250 \mathrm{V}, 750 \mathrm{W}$. Calculate the electric current flowing through it, when it is operated on a $250 \mathrm{V}$ supply. 

An electric bulb is rated as $250 \mathrm{V}, 750 \mathrm{W}$. Calculate the resistance of its filament.