There is a current of 40 A in a wire of `10^(-6)` square metre area of cross section , If the number of free electrons per cubic metre is `10^(29)` , then the drift velocity will be

Options

A. `(mg)/(2r) sqrt(mg)/(pi rho)`
B. `(mg)/(r ) sqrt(mg)/(pi rho)`
C. `(2 mg)/(r ) sqrt(mg)/(pi rho)`
D. `(mg)/(r ) sqrt(mg)/(2pi rho)`

Answer

A

Explanation

Thrust `= F = rhoAv^(2) " " [A = pir^(2)]`
at equilibrium `rhoAv^(2) = mg`
Power `P = Fv = rhoAv^(3)`
`= (mg)/(r ) sqrt((mg)/(p pi))`