A source of constant potential difference is connected across a conductor having irregular cross section as shown.

Options

A. electric field intensity at P is greater than that at Q
B. rate of electrons crossing per unit area of cross-section at P is less than that at Q
C. the rate of generation of heat per unit length at P is greater than at Q
D. mean kinetic energy of free electrons at P is greater than that at Q

Answer

B

Explanation

When a source is connected, a current starts to flow through the conductor. Let it be i. Then, current density at a section is equal to 1/A, where A = cross-sectional area.
Since cross-sectional area at P is maximum, therefore current density at P is minimum. Hence (B) is correct. Since electric field is`(J.)/(sigma)(=(I)/(Asigma)=(Irho)/(A))`, therefore at P electric field is minimum while that atQ is maximum
Rate of generation of heat per unit length at a section will be equal to `(I^(2)rho)/(A)` It is minimum at P and maximum atQ
. The mean kinetic energy of free electrons `=(1)/(2) mv^(2)/(d)` which is minimum at P and maximum at Q.therefore Hence (B) is correct