# Inductor questions (generate a voltage opposing the source voltage?)

• I
• annamal
In summary, the self-inductance of a circuit loop can create a back EMF, but it is usually very small and only comes into play when the change in current is significant, such as when opening or closing a switch. The inductance of a circuit loop is typically much smaller than that of an inductor due to the smaller number of turns and the larger surface area. Inductance can also create a forward EMF, similar to the inertia effect in a mechanical system, when the switch opens and the current begins to fall.
DaveE said:
Switches are nearly always assumed to be ideal (instantaneous) in thought experiments like this. IRL, you would add a bit more engineering, like snubbers, clamp diodes, transient suppressors, make-before-break types, etc.

That really wasn't his point. It's a diversion from the basic physics here. You could use a step function for the voltage source without any switches, which is also a magical creation.
Not trying to trivialise it but I was trying to convince myself that we can usefully extract all the energy from the inductor. I cannot yet find a suitable circuit which does not waste energy.

tech99 said:
Not trying to trivialise it but I was trying to convince myself that we can usefully extract all the energy from the inductor. I cannot yet find a suitable circuit which does not waste energy.
I guess I don't understand your question. Short of perpetual motion and such, SMPS (that almost all use inductive energy storage) can be extremely efficient, well above 90%. Even then most of the losses are in other components, like the semiconductors, capacitors, etc.

You could start by looking at a simple boost converter.

PS: if you want to extract ALL of the inductive energy, then look into "discontinuous conduction mode" in boost or flyback SMPS.

DaveE said:
I guess I don't understand your question. Short of perpetual motion and such, SMPS (that almost all use inductive energy storage) can be extremely efficient, well above 90%. Even then most of the losses are in other components, like the semiconductors, capacitors, etc.

You could start by looking at a simple boost converter.

PS: if you want to extract ALL of the inductive energy, then look into "discontinuous conduction mode" in boost or flyback SMPS.
My question started because I was conscious of the related phenomenon whereby if you connect a capacitor to an already charged capacitor, the system loses (useful) energy.

tech99 said:
Michael Faraday knew little mathematics yet he noticed the electromagnetic nature of light and provided the basis of Maxwell's Equations.
Which goes to show he was probably much smarter than a lot of us on PF.

artis said:
Thanks, not that often when I hear positive feedback from you,
Sorry for being a miserable old git. I can be very positive sometimes, though.

tech99 said:
My question started because I was conscious of the related phenomenon whereby if you connect a capacitor to an already charged capacitor, the system loses (useful) energy.
That depends on how you connect them. If you use an inductor and switches, you can recover most all of the energy. You can look into resonant or quasi-resonant power converters to learn more.

However, you are correct in seeing a relationship. Circuits have a duality principle that basically says you can swap (appropriately) inductors and capacitors, loop circuits and nodes, voltage sources and current sources voltages and currents, etc. Then you will end up with the same analysis, the same solution. So, parallel capacitors are the dual case for series inductors. For ideal elements, the infinite currents you get from connecting capacitors in parallel is the same thing as the infinite voltage you get from connecting inductors in series.

malawi_glenn
artis said:
A copper wire conducts as little as 0.001 volts and as much as millions of volts. Semiconductors on the other hand do have bandgaps that need a certain eV of potential to get across.
I think most of the energy used by an electron to cross a bandgap comes from thermal energy. Apply any amount of voltage across a semiconductor and you should get at least some current flowing through it.

sophiecentaur said:
Which goes to show he was probably much smarter than a lot of us on PF.
I think Faraday was specially gifted from God (or from the universe circumstances if you don't believe in God), he had advanced intuition and qualitative understanding. Pretty much like you @sophiecentaur!

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sophiecentaur and vanhees71
Faraday was a very gifted experimentalist with an amazingly mathematical intuition although he indeed had no formal education in math. For me it's a miracle that he could discover the very fundamental field concept in contradistinction to the then established concept of actions at a distance, about which already Newton was in doubt. It's just ingenious. His ideas provided the empirical foundation for Maxwell's theory of electromagnetism.

artis, sophiecentaur and Delta2
Delta2 said:
I think Faraday was specially gifted from God (or from the universe circumstances if you don't believe in God), he had advanced intuition and qualitative understanding. Pretty much like you @sophiecentaur!
vanhees71 said:
Faraday was a very gifted experimentalist with an amazingly mathematical intuition although he indeed had no formal education in math. For me it's a miracle that he could discover the very fundamental field concept in contradistinction to the then established concept of actions at a distance, about which already Newton was in doubt. It's just ingenious. His ideas provided the empirical foundation for Maxwell's theory of electromagnetism.
Well there are always some people who have a very strong talent. Faraday saw the fields behind his experiments.
I recall how when I was reading the "Sacred and Profane" by David Weiss, how I still remember the descriptions of Mozart's musical abilities when he was young.
Sure there are many good even excellent musical professionals in the world, but how many of them could compose and play a classical musical piece at the age of 5?

Or how many could enter a chapel and hear Allegri's "Miserere mei Deus" and remember it by memory from listening to it just once while being in your teens.

These folks are just "freaks of nature" but their example gives me inspiration to advance my own knowledge and intuition.

vanhees71 and Delta2
Somewhere above in this thread, I've solved the simple equation for the case of switching of a current through a coil via a resistance. What happens there is that the change in the current causes the magnetic field change with time, which induces an EMF (not a voltage, because there's no electric potential but rather a "vortex field", i.e., ##\vec{\nabla} \times \vec{E}=-\partial_t \vec{B} \neq 0##. This EMF drives the electrons through the resitance, where there is energy dissipated into heat due to collisions of the conduction electrons with the ions making up the resistor. A qualitative classical model is the Drude model. This dissipated energy was stored before as the energy of the magnetic field of the coil.

artis and Delta2
Temporarily closed for cleanup

Edit: thread reopened. A large number of posts were cleaned up. Many published sources state that an inductor does have an EMF. Such a debate is off topic for this thread and if participants wish to have that debate in another thread then they should do so in a new thread, specifically citing the relevant professional scientific literature in support of their position

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hutchphd and Delta2
Can you explain, why one of my postings was canceled? Is Faraday's Law, one of the fundamental Maxwell equations, in doubt? Of course, a time-varying magnetic field induces an EMF. That's the content of this fundamental Maxwell equation: ##\vec{\nabla} \times \vec{E}=-\partial_t \vec{E}##! I hope, it's allowed to contradict obviously unscientific claims in the very thread, where they are made!

hutchphd and Delta2
Me too

Lets face it @vanhees71 and @hutchphd , you are not mighty professional scientists, you are puny ungifted amateurs like myself. LOL. The professional scientific literature killed us all.

vanhees71 said:
Is Faraday's Law, one of the fundamental Maxwell equations, in doubt?
Of course not. The issue is that many posts that were themselves perfectly fine were referring to one of several problematic posts which I deleted. Then the fine posts no longer made sense in context and had to be deleted.

If you did not also receive a warning in conjunction with the deletion, then your post fell into this category

hutchphd and vanhees71
Delta2 said:
Lets face it @vanhees71 and @hutchphd , you are not mighty professional scientists, you are puny ungifted amateurs like myself.
This might turn out to be completely wrong!

hutchphd and Delta2
fresh_42 said:
This might turn out to be completely wrong!
Yes ok it was meant as a joke, we all know vanhees is really excellent and hutchphd is quite good as well.

Well for what it's worth I don't believe in such "history rewriting" aka cleaning up unless the topic is complete garbage. In this case I think the posts that argued against the viewpoint of @cabraham were I believe informative for anyone else in the future that might stumble upon the same stone.

But as they say, history is written by ment...victors that is!

hutchphd, vanhees71 and Delta2
I've been only puzzled, why my posting was deleted (I don't even remember which specific one it was), because this is really an issue, where there is nothing to argue about. It's just a direct consequence of Faraday's Law, which (in differential form) is one of the fundamental laws of Nature (i.e., one of the Maxwell equations).

I'd also prefer such threads simply to be closed soon enough with the hint that the question has been clarified by arguments from well-established physics rather than "cleaning them up".

hutchphd and Delta2
vanhees71 said:
What happens there is that the change in the current causes the magnetic field change with time, which induces an EMF (not a voltage, because there's no electric potential...
After the switch is shunted from the battery to a short there is emf developed by L di/dt across the inductor. However, there is also voltage, established as an electrostatic E field equal in magnitude but opposite in polarity to the electromagnetically generated E field, both fields resident within the inductor (zero inductor resistance assumed).
Across the resistor there is no emf but voltage only, equal to that of the inductor (obvious, since they are connected in parallel!).
And of course there is potential also in the electrostatic field.

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Delta2
You are contradicting yourself since ##L \dot{i}## is nothing else than ##\dot{\Phi}## where ##\Phi## is the flux of the magnetic field and this is equal to the vortex of the electric field, according to Faraday's Law, and this implies that the electric field has no scalar potential.

Dale
artis said:
My former math teacher could solve all of Maxwell's equations and much more but she doesn't know a single thing about how current behaves in actual circuits.
Necessary but not always sufficient.

There is one and only one E-field. That's what I tried to point out all the time :-(.

hutchphd
This thread needs to be closed; some posts have been deleted as part of the cleanup. Thanks everybody for participating.

vanhees71 and DaveE