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May I get a physical interpretation on this question? What is happening in the inductor when the current is running through it and what is physically happening when the current starts changing?

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What is happening in the inductor when the current is running through it and what is physically happening when the current starts changing?

In order to explain what is physically happening it might be helpful to consider the mechanical analogue of kinetic energy and the inertia of mass. The analogy is not exact, but it may hopefully give you a physical "feel" for what's going on, that is not so easy to feel with electrical concepts.

As @niels nielson pointed out an inductor with a constant current produces a magnetic field. That magnetic field represents stored energy in the inductor, in this case, in the form of kinetic energy. (A capacitor has stored energy in the electric field between the plates and, in that case, the stored energy is electrical potential energy).

Now think of a mass moving at constant velocity and having kinetic energy. It will resist any attempt to slow it down (reduce its kinetic energy) or speed it up (increase its kinetic energy) analogous to an inductor resisting any attempt to change its current (and thereby changing the kinetic energy of its magnetic field). The mass has inertia. The inertia (to current change) of an inductor is analogous to the inertia (to velocity change) of the mass. The analogy can be seen when one compares faradays law of induction.

$$V_{L}(t)=L\frac{dI(t)}{dt}$$

With Newtons's second law of motion

$$F=M\frac{dv(t)}{dt}$$

Very roughly speaking, we can consider:

  1. Voltage as the analogue of force
  2. Inductance as the analogue of mass
  3. Velocity as the analogue of current.

The diagram below shows other mechanical analogues for resistance and capacitance.

I would like to stress that inductance is not mass, velocity is not current, and voltage is not force. The analogy is simply intended to help you get some feel as to what is going on.

Hope it helps

enter image description here

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Here is one way of looking at this.

We start with an inductor that has a steady current flowing through it from a power source. Because of this, there is a magnetic field extending into space surrounding the inductor.

Now we attempt to cut off the flow of current through the inductor, by switching off the source. At the instant the current goes off, the magnetic field begins to collapse around the inductor, which induces a current flow in the inductor in the same direction as our original current. The quicker the field collapses, the greater the induced current flow- and we observe a big fat spark jumping across the switch terminals as they move apart.

We conclude that the current flow in the inductor really wants to keep flowing, and the inductor "fights" any change in the magnitude of the current flow we try to assert.

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  • $\begingroup$ We conclude that the current flow in the inductor really wants to keep flowing, and the inductor "fights" any change in the magnitude of the current flow we try to assert. But doesn't the OP want to know why this itself happens, not how we know it does happen? $\endgroup$ – Aaron Stevens Aug 1 at 20:43
  • $\begingroup$ I left out the d(i)/dt terminology because I did not think he was at that level. What I was shooting for was the idea that the field collapse drives the current and tries to maintain its flow. My usual way of explaining this is to move into the mechanical analogue and represent inductance as a mass but I thought that might not be suitable. $\endgroup$ – niels nielsen Aug 1 at 22:07

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