# The Electromagnetic tensor

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Maxwell's equations for the electromagnetic field [ in units with ] are:

Defining the anti- symmetric tensor with components:

the electric and magnetic fields are given by

If we also define a current four- vector :

Maxwell's equations can be written as [ Assignment 4 ]

where . We have now expressed Maxwell's equations in tensor form as required by Special Relativity.

The first of these equations implies charge conservaton

By performing a Lorentz transformation to a frame moving with speed *v* in the *x* direction, one can calculate how the electric and magnetic fields change:

We find [ Assignment 4 ] that is unchanged, while

where and is the electric field parallel and perpendicular to . Thus and get mixed.

The four- force on a particle of charge *q* and velocity in an electromagnetic field is [ Assignment 4 ]:

The spatial part of is the Lorentz force and the time part is the rate of work by this force.

By writing , Maxwell's equations give [ Assignment 4 ]:

where

This is the energy momentum tensor of the electromagnetic field. Note that is symmetric as required and the energy density is [ Assignment 4 ]