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while studying self-inductiance, I got into a tricky point...

Consider this video: https://www.youtube.com/watch?v=0IVJaXO_0XA (minute 13.00).

At this moment, opening of the R-L circuit is described. Let's say that at t= 0 we open the switch: we would see that a small current continues to flow in an open circuit, even when t > 0. From a microscopic point of view, I can't figure out how electrons continue to flow, on a given direction: I would imagine electrons as stopping at one side of the open switch... ...or not?

Thanks for helping!

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  • $\begingroup$ The information given in the video (starting at the 13:00 mark) is simply wrong. The solution for the current would be correct if the voltage source were zeroed but not for opening the switch. $\endgroup$
    – Alfred Centauri
    Commented Apr 13, 2015 at 22:23
  • $\begingroup$ Okay, so I'm a little bit worried, because I've found that same discussion on various textbooks, when they come to self-inductance theory $\endgroup$
    – graziano governatori
    Commented Apr 14, 2015 at 16:05
  • $\begingroup$ graziano, I think that, if you'll carefully check these discussions, you'll find that there is always a path to 'discharge' the inductor. If you find an example otherwise in a textbook, I would be very interested in seeing it so that I may contact the author. $\endgroup$
    – Alfred Centauri
    Commented Apr 14, 2015 at 22:27
  • $\begingroup$ graziano, I left a comment at the video for the author. $\endgroup$
    – Alfred Centauri
    Commented Apr 16, 2015 at 2:26
  • $\begingroup$ Thanks, Alfred! Actually, you can find this discussion on self-inductance on several ppt on the net, and on some italian high-school textbooks (Zanichelli's, for example). Really thanks again for helping me understanding! $\endgroup$
    – graziano governatori
    Commented Apr 17, 2015 at 8:50

1 Answer 1

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For a series RL circuit with DC source and switch, there is a problem with opening the switch after it has been closed for some time.

In the context of ideal circuit theory, the current through an inductor must be continuous since the voltage across is proportional the time derivative of the current through.

Put less rigorously, if the inductor current is discontinuous, the voltage across is 'infinite' (in magnitude) at the discontinuities; when the ideal switch is opened, the inductor voltage 'goes to infinity' for an instant and then becomes zero.

For a physical circuit, there will likely be an arc across the switch contacts that allows a brief but not instantaneous decay of current. There will also be some decaying oscillation due to stray capacitance in the circuit.

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  • $\begingroup$ Thanks for answering! Being an electronic practitioner, I know that arcs are there in case of open/close of switches (actually, you can also see them in common domestic light switches). In this sense, can it be correct to say that electrons "flows through" the open switches? $\endgroup$
    – graziano governatori
    Commented Apr 14, 2015 at 16:03

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