Question

Calculate the electric field when the conductivity is 20 units, electron density is 2.4 units and the velocity is 10m/s. Assume the conduction and convection current densities are same.

(a) 2.4

(b) 4.8

(c) 3.6

(d) 1.2

I have been asked this question by my school teacher while I was bunking the class.

This is a very interesting question from Continuity Equation in chapter Electric Fields in Material Space of Electromagnetic Theory

Answer 1

Correct option is (d) 1.2

For explanation I would say: The conduction current density is given by J = σE and the convection current density is J = ρe v. When both are equal, ρe v = σE. To get E, put σ = 20, ρe = 2.4 and v = 10, E = 2.4 x 10/20 = 1.2 units.