Hi guys, this thread made me join this forum so I want to ask a few questions about prof. Lewin's lecture and his lecture supplement (View attachment sup.PDF).
OK, for you who want to read this OLD thread:
Open the PDF (read it a few times and watch the video) document on page 1 and look at the picture. Imagine you close the switch and wait a while until the induction effects die out. There will be a current in the circuit I = E/R where R is the resistance of a resistor assuming all the wires are ideal conductors (superconductors cooled down to 0 K, haha). Now think what are the fields inside the circuit elements. The induction in that large loop (circle) had died out and we can apply the concept of potential. So there is a voltage drop across the resistor and because E = -grad(potential) there must be an electric field inside it in the same direction as the conventional current. But in the ideal wires there can't be an electric field because there isn't any voltage drop across them.
(I found an interesting question on the internet about this latter conclusion. How can a current flow when the electric field is zero? I explained this to myself with the concept of force and dynamics. If the electron is already moving then you don't have to apply any electric field (force) to keep him moving. Its Newtons first (second ???) law. An object will have constant speed if there are no forces.)
So to conclude, the electric field is only inside the resistor, confined only in that location. But this is also the case if we now include the induction effects (induced EMF/electric field) because of the changing current.
On page 4 prof. Lewin tries to explain this. He makes an ''experiment'' by inducing EMF inside an uniform loop (same resistance everywhere) and concludes that the induced electric field will also be uniform at every point. I can accept that because of the symmetry of the problem. Now comes the fun stuff. What if the loop isn't uniformly resistive, one side having lower resistance than the other (that's the second picture). If you read paragraph 2 of page 4 he says: (the current) must be the same on both sides by charge conservation... bla bla and than in paragraph 4: ''Nature accomplishes the reduction in E1 compared by E2 by charging up the junctions separating the wire segments.'' and he continues with his explanation to yield a conclusion that I already mentioned.
Is this legal???
P.S: I have so many questions about this particular problem and generally about confusion that arises from wrongly applied physics in engineering.
OK, for you who want to read this OLD thread:
Open the PDF (read it a few times and watch the video) document on page 1 and look at the picture. Imagine you close the switch and wait a while until the induction effects die out. There will be a current in the circuit I = E/R where R is the resistance of a resistor assuming all the wires are ideal conductors (superconductors cooled down to 0 K, haha). Now think what are the fields inside the circuit elements. The induction in that large loop (circle) had died out and we can apply the concept of potential. So there is a voltage drop across the resistor and because E = -grad(potential) there must be an electric field inside it in the same direction as the conventional current. But in the ideal wires there can't be an electric field because there isn't any voltage drop across them.
(I found an interesting question on the internet about this latter conclusion. How can a current flow when the electric field is zero? I explained this to myself with the concept of force and dynamics. If the electron is already moving then you don't have to apply any electric field (force) to keep him moving. Its Newtons first (second ???) law. An object will have constant speed if there are no forces.)
So to conclude, the electric field is only inside the resistor, confined only in that location. But this is also the case if we now include the induction effects (induced EMF/electric field) because of the changing current.
On page 4 prof. Lewin tries to explain this. He makes an ''experiment'' by inducing EMF inside an uniform loop (same resistance everywhere) and concludes that the induced electric field will also be uniform at every point. I can accept that because of the symmetry of the problem. Now comes the fun stuff. What if the loop isn't uniformly resistive, one side having lower resistance than the other (that's the second picture). If you read paragraph 2 of page 4 he says: (the current) must be the same on both sides by charge conservation... bla bla and than in paragraph 4: ''Nature accomplishes the reduction in E1 compared by E2 by charging up the junctions separating the wire segments.'' and he continues with his explanation to yield a conclusion that I already mentioned.
Is this legal???
P.S: I have so many questions about this particular problem and generally about confusion that arises from wrongly applied physics in engineering.
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