Electromagnetic Theory

electromagnetismThe study of electromagnetic theory is a very large subject, which this overview will only attempt to summarise in the context of foundation science. Today, much of electromagnetic theory is still linked to Maxwell’s equations, which were first published in 1864, which preceded the re-emergence of the wave-particle duality debate that arose out of the following development in the early 20th century:

So, although the wave-particle duality debate had existed since the time of Newton, the publication of Maxwell’s theory took place in a time when light was general perceived to be a wave. This wave-centric position had slowly strengthen in the 100 years following Newton’s death in 1727 and although the foundations of classical electrodynamics had many contributors, Maxwell’s equations owe much to the work of Coulomb, Ampere, Gauss and Faraday, who laid down many of the basic laws of electromagnetism. These laws helped define and specify the nature of some of the basic terms, which this subject now takes for granted, i.e. charge, force, field, voltage, capacitance, inductance and flux.

  • In 1785, Coulomb published an equation relating the force [F] between two charged particles as being proportional to the magnitude of the charges [q1 & q2] and inversely proportional to the square of the distance [r] between them, i.e.

[1]      F = K(q1*q2)/r2;             where K = 1/(4πε).

  • The similarities between this equation and Newton’s gravitational equation [2] is highlighted along with the fact that both forces depend on the relationship between two particles, i.e. the force is not an attribute that can be assign to a single particle.

[2]      F=G(m1*m2)/r2

  • However, it should also be highlighted that the gravitational force is always an `attractive` force, while charge can be both an `attractive` and `repulsive` force.

  •  In 1835, Gauss presented an equation that related the total electric flux out of a closed surface to the charge enclosed divided by the permittivity, i.e. φ=Q/ε.

  • In 1820, Oersted discovered that an electric current could cause a compass needle to deflect. Equally, a moving electric charge caused a magnetic field. These observations were interpreted by Ampere and published in 1826 as Ampere’s law. This law relates the magnetic field in a closed loop to the electric current passing through the loop. It is the magnetic equivalent of Faraday's law of induction.

  • In 1831, Faraday published his discovery that a changing magnetic field causes an electric voltage. If the magnetic flux through a loop of wire changes, a voltage drop appears at a small break in the wire.

As such, we might begin to recognise the importance of this body work to Maxwell’s later insights. In fact, Maxwell’s equation might be traced backed to the correction he made to Ampère's circuital law in his 1861 paper entitled ‘On Physical Lines of Force’.  Subsequently, in 1864, he published entitled ‘A Dynamical Theory of the Electromagnetic Field’ , although the original form consisted of some 20 equations that essentially combined and corrected many previously laws:

  • A corrected version of Ampere's law.
  • Gauss' law for charge.
  • The relationship between total and displacement current densities.
  • The relationship between magnetic field and the vector potential.
  • Faraday’s relationship between electric field and the scalar and vector potentials.
  • The relationship between the electric and displacement fields
  •  Ohm's law relating current density and electric field.
  • The continuity equation relating current density and charge density.

Finally, in 1884, Oliver Heaviside reformulated Maxwell's original system of equations into a differential format based on vector calculus; although today Heaviside’s differential form have also been complemented by an integral form. This said, there are essentially only 4 equations to be discussed, which we might sub-divided into 2 distinct classes that separate the time-independent and time-dependent nature of electromagnetic waves. The term ‘time- independent‘ essentially defines the scope of 'electrostatics', while ‘time-dependent’ defines the scope of 'electrodynamics'

wave

The diagram above tries to initially illustrate the concept of a self-propagating, self-perpetuating EM wave moving through the vacuum of space, i.e. with no direct reference to any supporting media. As such, we might initially visualise an EM wave as some sort of transverse wave, analogous to the motion of simple harmonic oscillation, in which energy is somehow conserved in propagation.