# Understanding Poynting's Theorem intuitively

1. Apr 21, 2014

### sigma_

understand that, broadly speaking, Poynting's theorem is a statement of conservation of energy.

There are many forms of the theorem, so I will simply give my current intuitive understanding of it in words:

Energy density of a volume of current and charge decreases proportionally to work done on charges within the volume and by energy flux through the surface bounding the volume.

The energy flux term is the area integral of the Poynting vector.

My question is this: is this energy flux that is described by the Poynting vector a form of electromagnetic radiation? If not, how is energy radiating out of the charge and current distribution if not by work done on charges? By what agency is this energy density being exhausted?

2. Apr 22, 2014

### PhilDSP

Hi sigma_,

I'd hesitate to call the energy flux described by the Poynting vector "radiation", since the Poynting vector is often used to describe non-radiative situations. Radiation occurs when the EM fluctuations come from a traveling wave, which is also called a self-propagating wave.

Merely moving a charge without essentially accelerating it also produces EM fluctuations, but those fluctuations are effectively evanescent waves. They dissipate very quickly. Both of types of waves of course involve the same EM fields, and by the Poynting theorem the same type of energy is fluctuating in either case.