# Q & A: energy flow in a dc circuit

Q:
Poynting vector in circuits: I've been considering the energy transfer in a simple circuit consisting of a battery connected to a resistor. If you look at the poynting vectors you see that the energy is passing not through the wires, but through the space (waveguide I suppose) around the wires, and even across the centre of the circuit loop. If this was an AC circuit I might think of this as electromagnetic radiation, but at DC, what is it that is moving in that space? (photons? at what frequency/colour?) (do the fields move with a momentum of their own?) (If that energy is quantised, can it be caught somehow?) Apologies for the diffuse nature of the questions, but I am trying to get a handle on what is going on conceptually.
- Peter (age 36)
Southampton, UK
A:
This is a very nice question. Looking just at some stretch of wire with current flowing through it, the Poynting vectors around it point into the wire. That makes some sense, since energy must be flowing into the wire to account for the Joule heating. If you look at the whole circuit the energy flow starts in the battery, goes out through the space, and then into the wires.

I guess you could say that photons are indeed flowing through space. After all, there's a momentum density that goes along with the energy flow density. (The Maxwell equations implied E=pc, where E is energy and p is momentum, well before anyone thought of relativity or photons.)

I'm not quite prepared to answer the question about the frequencies, but thought the partial answer might help for starters.

Mike W.

(published on 11/08/2012)