Question

27. The electromagnetic field due to a point charge must be described by Lienard-
Weichart potentials, when :
(A) the point charge is highly accelerated
(B) the electric and magnetic fields are not perpendicular
(C) the point charge is moving with velocity close to that of light
(D) the calculation is done for the radiation zone i. e. far away from charge​

Answer 1

Explanation:

The Liénard–Wiechert potentials describe the classical electromagnetic effect of a moving electric point charge in terms of a vector potential and a scalar potential in the Lorenz gauge. Stemming directly from Maxwell's equations, these describe the complete, relativistically correct, time-varying electromagnetic field for a point charge in arbitrary motion, but are not corrected for quantum-mechanical effects. Electromagnetic radiation in the form of waves can be obtained from these potentials. These expressions were developed in part by Alfred-Marie Liénard in 1898[1] and independently by Emil Wiechert in 1900.[2][3]

Answer 2

The correct answer to this question is option (D) The calculation is done for the radiation zone i.e. far away from charge​:

Explanation:

• The Lienard-Wiechert potential defines the electromagnetic effect of a moving charge.  

• It is assumed that the particle is moving with a velocity much smaller than the velocity of light, c.

• The Lienard-Wiechert fields have several interesting properties, like the electric and magnetic fields are always perpendicular.

• The acceleration field depends upon $\frac{1}{r}$ as compared to $\frac{1}{r^{2} }$ for the velocity field, it rules the first term at large distances, and so is generally known as the ‘far-field’ term.