I’ve read that the Poynting vector points into an incandescent light bulb at all instants of time, whether there is direct/constant or alternating current through the wire. This means the bulb is obtaining energy from the electromagnetic field.
But, the bulb is getting hot, and it is emitting light, so shouldn’t there be Poynting vectors pointing outside the bulb? To indicate the bulb is releasing energy, in form of heat and light.
Furthermore, shouldn’t the net Poynting vector be zero at the bulb? To indicate that all of the energy the bulb receives is again released (as heat and as light). I mean, if the Poynting vector wasn’t zero, it’d mean energy is being stored in the bulb, but that can’t happen in an incandescent bulb.
More generally, since (ideal) resistors can’t store energy, I think the net Poynting vector at the resistor should also be zero.
What are your thoughts? Is my analysis wrong? How?
Best Answer
The Poynting vector only accounts for electromagnetic energy transfer. It doesn't account for kinetic energy, which is what the heat produced by a resistor is.
In the case of an incandescent bulb, the light emitted is only a small fraction of the power input. Maybe a few per cent. We could calculate the contribution of the light to the Poynting vector if we want to, but often we're actually interested in the power delivered to the bulb at low frequencies, so it would be counterproductive to do so.