As an explanation to Poynting vector(P), I read the following example:

A DC source is connected to a bulb (receiver) by means of 2 conductors. The direction of the electric field between the conductors is given to be from the conductor at higher potential to the one at the lower(return line) and the $\vec{H}$ field in the dielectric (air) around the conductors along with this $\vec{E}$ field is shown to result in a $\vec{P}$ which has a small component perpendicular and towards the conductors and a large component parallel to the conductors directed towards the receiver.

The text goes on to say that this proves that the energy in electric circuits is supplied to receivers via the dielectric surrounding the wires and not the wires themselves.

If that is the case why don't we feel the energy transfer when we are around or touch an insulated conductor and the shock is felt only when we touch an un-insulated conductor.


1 Answer 1


Your text is correct. So then you ask about a shock. You feel a shock when the field gets strong enough to form a dielectric breakdown which can happen of you get really close to something at a different potential.

If a conductor is uninsulated and has a potential then you can get very close to it. If you were a bird then your entire body can get closer and closer to that potential and you can touch that reasonably high voltage wire. If you are a person and you are standing on the earth which is at one potential and you try to touch that same wire then you can't really be at those two different potentials and you are already in contact with the earth. So there are many many layers of equipotential surfaces between your hand and the wire and as your hand gets closer they eventually become so dense the electric field is at the breakdown level and the dielectric of the air starts to pass current. That's the shock.

Now for the wire with an insulator let's look at insulators. One picture is to imagine air another insulator as being like resistors with a huge resistance. Wired up in parallel to the conducting wires very little current flows through the resistors so you can almost ignore them in that direction. The main idea of the insulator is that you touch the insulator while still a distance away from the wire. Plus all those equipotential surfaces are stacked inside the insulator which has a different breakdown voltage than the air.

So you get the twofold effect that its like it makes you keep your distance and it allows stronger fields in it. If it broke down at a smaller field strength it would actually be pointless.

So if you wonder about shocks you have to look at what causes them. The real concern should be that if you block the area around the wires enough you could block the flow of energy, either it lushes around yo find another path or else the power supply might actually discharge less quickly.


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