do not conserve energy, although the local law is always satisfied. Exact formulation of energy–momentum conservation on an arbitrary geometry requires use of a non-unique stress–energy–momentum pseudotensor. Electromagnetism General relativity modifies the description of electromagnetic phenomena by employing a new version of Maxwell's equations. These differ from the special relativity form in that the Christoffel symbols make their presence in the equations via the covariant derivative. The source equations of electrodynamics in curved spacetime are (in cgs units) where Fab is the electromagnetic field tensor representing the electromagnetic field and Ja is a four-current representing the sources of the electromagnetic field. The source-free equations are the same as their special relativity counterparts. The effect of an electromagnetic field on a charged object is then modified to , where q is the charge on the object, m is the rest mass of the object and P a is the four-momentum of the charged object. Maxwell's equations in flat spacetime are recovered in rectangular coordinates by reverting the covariant derivatives to partial derivatives. For Maxwell's equations in flat spacetime in curvilinear coordinates see or Physics-related lists General relativity