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When a DC circuit is carrying current, large amounts or small, is there induced-emf due to the inductance? Or is it only applied to AC circuits?

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3 Answers 3

up vote 8 down vote accepted

In the limit of long times, the currents are steady, so the magnetic fields they create are steady so there is no induced EMF. This situation is usually tagged "steady state".

That said, there will be a period of time when you have just switched a circuit on or off during which things have not settled down and then there will in general be effects not seen in the steady state (including induced EMFs). This is called the "transient" behavior.

Transient behavior analysis is a important component of electronics design.

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I have a question in relation to this. If there is a change in current flow in the circuit, the inductor would instantly induce and EMF to oppose the change and the current flow would stay the same unaffected? When I think about the formula V = L di/dt that makes me think that the current from the applied PS or whatever, would first go to zero then there is an induced EMF? Or is it the moment there is a change, the EMF is induced, and the current would stay the same I'm confused about the timing. – XCIX Oct 13 at 14:02
The inductance (usually small unless designed in) resists and slows the change, but does not prevent it. After all, if the was no change the would be no inductive back EMF. – dmckee Oct 13 at 14:06
Well, I'm thinking of really high inductance(1H) that could prevent the change? – XCIX Oct 13 at 16:53
No, you can not prevent the change. Think about it: the back EMF only forms if the current is changing. The thing you want to compute is the time constant for the current change. – dmckee Oct 13 at 16:56

Here the use of DC in your question is a bad choice. DC does not imply constant current, it means that the current have same polarity in a period that we refer to. So you would like to use steady state in your question.

That said, in steady state the the current is not changing with respect to time so the flux is not changing and hence there is no emf. But if the current is not steady then there I will be an induced magnetic field produced which changes as the flux changes due to varying current. Ultimately this changing magnetic produces a non-conservative electrical field.

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Yes, inductance exist in a DC circuit. The problem here is the similarity between the words inductance and induction. Inductance is not about change. In fact, inductance is measured in Henrys, which is a Weber per Ampere. Hence, there is no change. In contrast, induction is about change and does not exist is a DC circuit. You would be amazed how many really smart people get this one confused. I believe it goes back to the poor teaching of this subject.

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You completely side-stepped the part about emf which is really what the question is about. – Brandon Enright Sep 26 at 0:34

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