Here, `r = 6.0 mm = 6 xx 10^(-3)m`,

`l = 3.0 xx 10^(-2)m`,

`rho = 3.5 xx 10^(-5)Omega m , P = 2.0W`.

(a) As `P = I^(2)R = (JA)^(2)R = J^(2)A^(2)R`

Current density ,

`J = 1/A sqrt(P/R) = 1/A sqrt(P/(rho l//A)) = sqrt(P/(rho l//A)`

` = sqrt(2.0/((3.5xx10^(-5)) xx (3.0 xx 10^(-2)) xx pi xx (6 xx 10^(-3))^(2))`

` = 1.3 xx 10^(5)Am^(-2)`

(b) As `P = IV =JAV`

or `V = P/(AJ) = P/(pi r^(2)J)`

`20/(3.14 xx (6 xx 10^(-3))^(2)xx(1.3 xx 10^(5))`

`=0.136 V`