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PX384 Electrodynamics

Lecturer: Robin Ball
Weighting: 7.5 CATS

Einstein's 1905 paper was called "On the electrodynamics of moving bodies" and derived the transformation of electric and magnetic fields when moving between inertial frames of reference. This module works through this transformation and looks at its implications. The module starts by covering the magnetic vector potential, A, which is defined so that the magnetic field B=curl A and which is a natural quantity to consider when looking at relativistic invariance.

The radiation (EM-waves) emitted by accelerating charges are described using retarded potentials, which are the time-dependent analogs of the usual electrostatic potential and the magnetic vector potential, and have the wave-like nature of light built in. The scattering of light by free electrons (Thomson scattering) and by bound electrons (Rayleigh scattering) will also be described. Understanding the bound electron problem led Rayleigh to his celebrated explanation of why the sky is blue and why sunlight appears redder at sunrise and sunset.


To introduce the magnetic vector potential and to show that electromagnetism is Lorentz invariant.

At the end of the module, you should:

  • Be familiar with the vector potential and Lorentz invariant form of Maxwell's equations
  • Be able to manipulate Maxwell’s equations and solve representative problems using 4-vectors
  • Understand the physics of EM radiation and scattering and be able to describe the propagation of EM waves through free space
  • Know how Maxwell's equations can be solved to calculate the EM field from known source distributions.


  1. Revision of special relativity. Revision of Maxwell's Equations in vacuum and in a macroscopic medium. Simple models of polarization. Displacement current; Potentials ϕ and A. Coulomb and Lorenz gauge. Laplace's and Poisson's equations and the solution of Maxwell's equations. Retarded potentials.
  2. Lorentz invariance of Maxwell’s equations. Four vectors. Covariant and contravariant representation (examples, exercises on-line). Minkovsky’s metric tensor (exercises on-line). Four vector formulation of Maxwell’s equation (examples, exercises on-line).
  3. Generation of EM waves and retarded potentials. The power radiated by accelerating charges.
  4. The scattering of EM waves. Rayleigh scattering and Thompson scattering.

Commitment: 15 Lectures

Assessment: 1.5 hour examination (85%), coursework (15%)

Recommended Text: IS Grant and WR Phillips, Electromagnetism, Wiley

This module has a home page.

Leads from: PX263 Electromagnetic Theory and Optics

Leads to: Further modules on Classical Physics