- Joined Feb 4, 2008
Of course, I'ze interested. Just my paranoid server at work won't let me watch youtube. Haveta wailt till I get home.Here is a video in youtube (two parts) I found interesting to me and I would like to share it with you. Take some of your time and watch them. I would like to hear your comments on the topic they present!!
Agreed. When dealing with a new situation, or when trying to answer a mystery, start with first principles (in this case Maxwell's Equations), and carefully derive the simplifications that are allowable based on the assumptions you are convinced you can make.I does go to show what I am always banging on about - Make sure the conditions of validity apply before you use an equation. This is all too often overlooked.
Personally I don't think he was being disingenuous at all. Perhaps he is a bit melodramatic, but I like his theatrical approach since it is a very effective teaching method.I wonder if perhaps Proff Lewin was being a tad disingenuous with this demonstration.
Old fashioned versions of Kirchoff's law state
The sum of the EMF's = The sum of the voltage drops (or potential differences)
They went on to state that this sum is known as the total EMF in a circuit.
Used in this form it appears to me that the law is satisfied.
The total EMF in the circuit is 1 volt (the EMF induced by the coil) and the total voltage drop is 0.9 + 0.1 volt.
I believe it used to be phrased in this way to allow for just such a situation.
It also brings out the difference between EMF (which is distributed around the circuit in this case) and Potential Difference.
This shows there are cases where you can't just work your way around a loop, counting batteries as positive and resistors as negative and sum to zero.
Why?It's clear that Kirckoff's Law is intended for circuit equations and we can only apply them to lumped circuit elements. In the experimental setup he is showing, there are two possible voltages one could assign to the upper node (point B). That is, 0V or 1 V. You just can't solve circuit equations like this - it's a field problem. There is definitely a 1V EMF generated, but where do you put it in an equivalent circuit? Any equivalent circuit you make for one physical arrangement, will no longer work if the physical arrangement changes, even if the electrical connections are identical. (for example moving the position of the voltmeter).
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