I notice that the 3 drawings by .99 have been viewed over 150 times so I feel impelled to focus on why KVL cannot be applied to "Professor Lewin's Non-conservative Fields Experiment" - especially in light of the various digressions and permutations that have arisen in this thread.
I think everyone will agree that when we take an instantaneous snapshot in time, that at that moment if the fields are not changing between the beginning and ending of the snapshot, then we will have reduced the period to a state whereby we CAN apply KVL comfortably and with confidence. Much of the discussion has been based on this instantaneous nature, focusing on the "Peak" or maximum value obtained - and this peak is quite readily visible in the scope shots presented.
However, and this is of the utmost importance to this thread, Dr. Lewin's experiment was not an instantaneous experiment. No, his experiment covered a period of time and is representative to explicitly show a CHANGE in magnetic flux - extremely important factor to this very particular experiment. When we take the entire time period stated to be "About 10 Milliseconds" (which is quite long in today's nanosecond economy), we CANNOT apply KVL to the problem. In fact, there is NO time snapshot during the experiment where the beginning and ending periods do not see some small change in the field and resulting induced voltage.
Why do I say that KVL cannot be applied? Because KVL requires two things: 1. A static field and 2. A closed loop of zero volts (0V)
However, by applying Faraday's law, we can derive the various induced voltages at various stages in time and quite closely approximate the "source" needed to balance the KVL equations. And this is precisely what some of the new simulators do. Alternatively, we could arbitrarily assign the source convincing ourselves that it must match the drops so as to net to zero. But even then, we are kluging together a series of stair stepped values to approximate the reality, whereas Faraday's formula gives us a means to smoothly transition from each point to the next and map out the real curves with as high a resolution we choose to adopt.
So like many of the lessons provided by the well experienced Professor, there are deep nuances embedded in his lectures that are designed to be thought provoking and illicit a deep contemplation leading to solid understanding. That is what makes him one the best teachers on this planet.
What are some of the finer points of this experiment?
1. Results are different for Static and Dynamic fields
2. Because of inductive reactance a wire is not always just a wire
3. KVL is a subset or special case of Faraday's Law
4. Measurement equipment can become part of the circuit in very non-intuitive ways
5. The majority of textbooks and schools overlook the importance of this subject.
6. Real world situations will demand that the proper method be used
There may be more points that you have learned.
One of the most profound things I took away from this experiment is a magnetic field that is changing is no longer conservative. When a magnetic field is not conservative, energy can be extracted from it. Ponder that.
After realizing that truth, it was easy to see that the same thing holds true for other "conservative" fields, like gravity. It is because we are able to break the field(s) into non-conservative parts (think tides and
energy extraction ) that we are able to extract energy from them.
Author
Topic: Professor Walter Lewin's Non-conservative Fields Experiment (Read 329991 times)
