golfbumb
Joined: 29 Sep 2004
Posts: 40
Helped: 5
Location: California
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 14 Oct 2004 15:18 Re: three parallel metal lines |
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In a time varying circuit, such as a sine wave on a wire, a magnetic field is created according to the right hand rule (if your thumb is the direction of current flow your curled fingers represent the magnetic field, B). That B field will induce a current in an adjacent wire in this case, your center wire. When the current reverses, so will the magnetic field. (This is Faraday's law of magnetic induction).
If the signals are out of phase, then at any moment in time the current flow in each of the outside wires is in the opposite direction from each other. Imagine you have two right hands (put your left one behind your back for a minute) wrapped around each of the outside wires -- one thumb heading up, one down (in a hypothetical vertical wire arrangement). At the center wire, the magnetic fields are additive. The net magnetic field on the center wire is additive, so the magnetic induction in the center wire is additive. Therefore, when out of phase, the interference creates a similar wave in the center, but lagging by 90 degrees (because the amount of induction is greatest when there is the greatest change in current flow and that exists when the waves are passing through the zeros and, correspondingly, there is no instantaneous change when the outside wires are at their peaks voltages.
If the signals are in phase, using the same logic, the fields cancel each other. The net B field is zero, induction is zero. You in effect have no interference (flat line). This assumes the wires to be equidistant, otherwise there will be a net effect from the differences in the fields.
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