# NCERT Physics Class 12 Exemplar Ch 8 Electromagnetic Waves Part 4

Q 28 An infinitely long thin wire carrying a uniform linear static charge density λ is placed along the z-axis (Figure). The wire is set into motion along its length with a uniform velocity. Calculate the Poynting vector Infinitely Long Thin Wire Carrying a Uniform Linear infinitely long thin wire carrying a uniform linear

Ans:

Q 29 Sea water at frequency Hz has permittivity, permeability μ ≈ μo and resistivity. Imagine a parallel plate capacitor immersed in sea water and driven by an alternating voltage source what fraction of the conduction current density is the displacement current density?

Ans:

Let the distance between the plates be d. Then the electric field

The conduction current density is given by the Ohm’s.

Where.

The displacement current density is given as

Q 30 A long straight cable of length is placed symmetrically along z-axis and has radius the cable consists of a thin wire and a co-axial conducting tube. An alternating current I flows down the central thin wire and returns along the co-axial conducting tube. The induced electric field at a distance s from the wire inside the cable is

(i) Calculate the displacement current density inside the cable.

(ii) Integrate the displacement current density across the cross-section of the cable to find the total displacement current Id.

(iii) Compare the conduction current with the displacementcurrent .

Ans:

(i) Displacement curing density can be found from the relation be

(ii)

iii) The displacement current

Developed by: