I am not an engineer, I am a journeyman electrician. I've been trying to wrap my head around the difference between inductive coupling and capacitive coupling. Both of them appear to do the same thing. Can anyone explain the differences in how they work?
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I will give it a try.
Capacitance is the phenomenon whereby an electric field causes charges in conductors to move toward the gap between the conductors. A simple capacitor is two plates separated by an insulating gap. When you apply a voltage to the capacitor terminals electrons move from one plate to the other, leaving one positively charged and one negatively charged. The current into a capacitor is proportional to the rate of change of the voltage and so leads the voltage in phase when you apply AC.
Capacitive coupling occurs because any two conductors form a capacitor, just not a very good one.
You can shield against capacitive coupling by putting a grounded shield around one or both wires.
Inductance is the phenomenon whereby a current in a coil causes a magnetic field which opposes the flow of current in the short term. The voltage across an inductor is proportional to the rate of change of the applied voltage but is lagging rather than leading.
Inductive coupling is actually what is properly called mutual inductance. It happens when two coils share a common location for the magnetic field (as in a transformer). The changes in current in one wire then induce a voltage from end to end in the other wire.
The magnetic field passes almost unobstructed through shielding that would effectively block capacitive coupling.
The capacitance between two wires lying next to each other in the same raceway can cause phantom voltages on the second wire if it is not connected to a source or a load.
The inductance between two wires in the same raceway is also significant to analog signals but does little to affect power wiring in a house.
If the second conductor is disconnected at both ends inductive coupling will result in a voltage difference between the two ends of that wire, but no voltage to ground at any point along the wire that would show op on a typical meter. But even with one end connected to a current source and the other end to a load, there will be a (very small) voltage difference from inductive coupling in addition to the resistive voltage drop.
Finally, to give a real world situation, if you have a third wire in a conduit which contains the hot and neutral for a circuit or the two sides of a switch leg for a circuit you can have capacitive coupling from those two wires to the third wire, but very little inductive coupling.
On the other hand if you have only a neutral and a second wire in a conduit you can get inductive coupling but little or no capacitive coupling since the current in the neutral is non-zero but the voltage is effectively zero.
