In the below page, I have two questions:

  1. How can a magnetic circuit not consume energy?

  2. How does the stored magnetic energy return to the circuit?

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  • \$\begingroup\$ Both : By Lenz's law. 1) When dI/dt = 0. 2) When dI/dt <> 0. So, study Lenz's Law. \$\endgroup\$ Jul 13, 2021 at 11:29

3 Answers 3


How can a magnetic circuit not consume energy?

If you spin up a frictionless flywheel to 1,000 RPM (or whatever), you only put in the kinetic energy to get the flywheel in motion at 1,000 RPM. That kinetic energy is now stored in the flywheel and can be used some time later. Other than friction losses, it will continue to spin and hold that energy with no further energy or power input. It's a similar story for an inductor or a capacitor.

How does the stored magnetic energy return to the circuit?

To keep the energy stored you must force current into the inductor and then short it out. When you disconnect the "short" to something more resistive, the energy stored starts to heat the resistor i.e. it returns to another circuit.


How a magnetic circuit don't consume energy?

The answer to your question is "physics".. Just like a current flowing in a superconductive ring doesn't consume energy, a constant magnetic flux in a lossless medium doesn't require power.

That's simply how physics is. If that wasn't the case, the conservation of impulse would be violated.

How the stored magnetic energy returns to the circuit?

Induction. That's what you're learning there: the relationship between changing magnetic fields, and changing currents.


The circuit consumes energy to build the magnetic field. That means building up current through an inductor. If there's resistance in series, some energy is dissipated continuously. Superconductors are free of this harm.

Breaking the current of an inductor removes the magnetic field. The field energy can be returned to the current supply if there's properly constructed circuit for it. In practical circuits some dissipation occurs also in this phase because parts have resistance.

Simply opening a mechanical switch to break the current in the inductor dissipates 100% of the magnetic field energy in an arc which occurs over the switch except in case there's a circuit which directs the inductive kickback pulse back to the source or to some other useful place. I guess you should read this: https://electronics.stackexchange.com/questions/282053/how-does-the-inductor-really-induce-voltage/282073?r=SearchResults&s=5|19.9734#282073

and learn something about induction also from other physics and electricity teaching materials.

The text does not take into the account energy loss as radiated electromagnetic waves. They appear and take something away to the space if the increasing or decreasing rate of the current in field buildup or collapse are high enough in relative to the mechanical dimensions of the structure and there's no attempt to prevent the radiation with geometric forms.


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