First this: Much of what we've been discussing lately doesn't directly relate to the Lewin experiment, but it most certainly is related and helpful to investigate. I have a pretty good grip on the dynamics involved in this experiment, but I am learning some new things here as well.
The induced voltage is always correct and fixed. Ohm's law voltage is always correct but the path must be considered. If the path includes more than one possibility your meter position will convince you the current is going either way and Ohm's law is broken.
Fellows: The PATH is important only when our measurement leads are in-plane with the loop. Ohm's law is NEVER broken when the measurement is taken decoupled.
The loop is a translator; it performs the integral of the E-field at radius r. That integrated E-field results in a loop emf. No emf can be induced if there is no conductive loop around the solenoid.
Now, once the loop is present, it is a real-time translator/integrator, and we must then treat the loop as any "normal" circuit, with an emf and loads. The only difference now between this circuit and one with a DC battery, is the emf source is localized with the battery. With an induced emf, the emf will be spread over any length (or lengths) of wire in the loop, and it will vary in amplitude with time.
The only way to measure all the points on a loop with an induced emf, is to decoupled the leads, otherwise the leads become influenced by the induced E-field. Once decoupled, all measurements and calculations we normally use on DC circuits apply to this one as well. Ohm's Law, and KVL are completely valid.
Where KVL breaks down, is when we are observing or measuring the E-field, or its integral. We inadvertently do this when our measurement leads are in-plane with the induction loop. When the measurement leads are directly in-plane with the induced E-field, they are at its mercy. What ever the E-field is doing, the meter will reflect. The meter can not measure the induced emf in the wire segments this way.
As emf is induced in the wire segments, there is an excess and shortage of electrons created at each end, due to restrictions caused by the resistors. This bunching of charges at each end of the wire segments and resistors "distorts" the induced E-field so that it too is concentrated at the junctions of the resistor elements and the wire segments. This is why moving the in-plane measurement lead probes anywhere in-between the wire segments in the Lewin experiment, makes no discernible difference in what is indicated on the scope.
Now, decouple the measurement leads (raise them normal to the loop) and the induced E-field no longer has any influence on them. You are now able to measure the true emf and potential drops anywhere on the loop, without interfering with the induced emf.
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Topic: Professor Walter Lewin's Non-conservative Fields Experiment (Read 312151 times)
