I have non-contact voltage detector that I use when I am working on electrical wiring. I have read that these devices work by measuring the electromagnetic field generated by the electricity in the wire. This makes sense to me in a circuit carrying electricity (for example, wire leading to a light bulb that is illuminated).
However, I noticed that the device appears to also indicate the presence of a field on an extension cord that has nothing plugged into the end of it — so there is the potential for electricity to flow, but the circuit is open, so it’s not actually flowing. I would think that would, in turn, mean that there should be no electromagnetic field. But that intuition seems to be wrong.
Why does the electromagnetic field seem to be there regardless of if electricity is actually flowing?
In: Physics
It’s an extension cord, so it’s probably AC electric. That means that the voltage is constantly changing on the wire–even if nothing’s plugged into the other end–and so an electrical field is generated.
Your common non-contact voltage detectors sense changing *electric* fields
Since the wiring in your house is AC that means that the voltage on the hot line is changing at 50 or 60 Hz even if no current is flowing. This changing voltage creates an electric field around the wire that can be detected.
This method wouldn’t work on DC power lines as their constant voltage creates a constant electric field
Now if we built one that sensed magnetic fields then we would need current flow, but if you’re sensing electric fields then you just need voltage on the wire
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