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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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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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  • $\begingroup$ 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. $\endgroup$ – Pupil Oct 13 '15 at 14:02
  • $\begingroup$ 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. $\endgroup$ – dmckee Oct 13 '15 at 14:06
  • $\begingroup$ Well, I'm thinking of really high inductance(1H) that could prevent the change? $\endgroup$ – Pupil Oct 13 '15 at 16:53
  • $\begingroup$ 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. $\endgroup$ – dmckee Oct 13 '15 at 16:56
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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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  • $\begingroup$ You completely side-stepped the part about emf which is really what the question is about. $\endgroup$ – Brandon Enright Sep 26 '15 at 0:34
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Yes, there is Inductance in a DC circuit and if the Inductance changes which is the magnetic flux to current ratio then Inductance can and will change the original current flow. if you have a continuous moving positive contact that changes the amount of inductance per the amount of current then one can control the current flow with induction as current and induction are interchangeable. either one can control the other as long as the contact and or the loop count is changing. $$V_L=N\frac{\mathrm{d}\phi}{\mathrm{d}t}$$ where $V_L$ is the induced voltage in volts, $N$ is the number of turns in the coil, and $\frac{\mathrm{d}\phi}{\mathrm{d}t}$is the rate of change of magnetic flux in webers per second.

The equation simply states that the amount of induced voltage ($V_L$) is proportional to the number of turns in the coil and the rate of change of the magnetic flux ($\mathrm{d}\phi/\mathrm{d}t$). In other words, when the frequency of the flux is increased or the number of turns in the coil is increased, the amount of opposing induced voltage will also increase thus with a moving contact you have an on going change in current flow from self induction.

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If the change is withing the time constant of the circuit, DC will in fact produce on going back emf that opposes the original current flow thus control current. inductive reactance then comes into play initiated by the inductance change thus when the current flow changes a frequency is applied to the DC thus inductive reactance initiated by inductance. inductance or current change can cause inductive reactance. Thank you user191954.

Marathonman

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  • $\begingroup$ This appears to be a comment to this post. $\endgroup$ – Kyle Kanos May 17 at 2:35

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