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

[Delta2]

I think there is a problem with the proposed circuit, because when the switch is operated, the current falls almost instantly to zero, creating very large voltage, and all the energy of the magnetic field is dissipated in a spark across the switch contacts..

It depends. @vanhees71 models it as an RL circuit where the switch is not open but it is closed and bypasses the originally present EMF source.

You have another circuit in mind where the switch is open (after again we have removed the original EMF source) and this circuit should be modeled as an RLC circuit where the capacitor C is between the switch contacts and has very small capacitance so that is charged very fast to a big voltage because all the energy of the magnetic field of the coil E, goes into energy in the capacitor $E=\frac{1}{2}CV^2$, C is very small, V must be very big for some finite E (the energy originally stored at the magnetic field of the coil, before we open the switch).

 

[artis]

It seems to me that you have achieved something very important. You have realized that many of the 'simple' EE questions you are likely to be asked are actually very complicated and seem to imply paradoxes. You have also appreciated that these apparent paradoxes can be rationalised by acknowledging the need to add extra 'components' or factors into the model or sometimes to wait for transitional effects to have died down. This is good!

Thanks, not that often when I hear positive feedback from you, I guess I've passed the test.

Science is about objectively observable and quantifiable facts. The only language we have for that is math, and it's a very concise language. The underlying theory (in this case Maxwell's electromagnetism) provides a clear, causal picture of what's going on here. There's no need for religion to understand the results of the natural sciences.

Sure. But some visual mind models and intuition does come in handy to better understand the maths. In fact for me it;'s always better to first get a "picture" of how something works in my mind and then it is far easier to learn the math side. I think that is because not all people are equally capable of high levels of abstraction. Some need to see the "connection" in the abstract math to the actual process within the circuit for example.

I just visualize the B field as a real (even though invisible) field around the coil and then I intuitively think, now energy must go somewhere it can't just stop existing, so it goes to current as that is the only way it can go, but current has no where to go, so it needs to develop a circuit through an arc, so the energy from the B field goes to the E field across the open switch contact capacitance. Boom problem solved.

Once I develop this mental model then the dry math makes sense. The problem with math alone is as I said - it's "dry", it;s basically just a language of abstraction that aims to represent some real physical phenomena.
Sure some folks have a natural "feel" for such language and for them they can learn it even without ever understanding what the symbols represent in real life physics. We call such folks theoretical mathematicians.

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.

 

[tech99]

It depends. @vanhees71 models it as an RL circuit where the switch is not open but it is closed and bypasses the originally present EMF source.

You have another circuit in mind where the switch is open (after again we have removed the original EMF source) and this circuit should be modeled as an RLC circuit where the capacitor C is between the switch contacts and has very small capacitance so that is charged very fast to a big voltage because all the energy of the magnetic field of the coil E, goes into energy in the capacitor $E=\frac{1}{2}CV^2$, C is very small, V must be very big for some finite E (the energy originally stored at the magnetic field of the coil, before we open the switch).

I think the devil is in the detail here. vanhees seems to describe a change over switch, going from battery to resistor. This will not work, as the inductor will instantly give up its energy during the changeover. Maybe someone can provide a circuit which will work, or maybe not.

 

[DaveE]

I think the devil is in the detail here. vanhees seems to describe a change over switch, going from battery to resistor. This will not work, as the inductor will instantly give up its energy during the changeover. Maybe someone can provide a circuit which will work, or maybe not.

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.

 

[Delta2]

This will not work, as the inductor will instantly give up its energy during the changeover

This is not sure, I think if the changeover is very very fast the capacitor formed by the switch contacts will not have time to charge, so not much energy transferred from the inductor to the capacitor.

 

[tech99]

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.

 

[DaveE]

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.

 

[tech99]

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.

 

[sophiecentaur]

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.

 

[sophiecentaur]

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.

 

[DaveE]

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.

 

[Drakkith]

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.

 

[Delta2]

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!

 

[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]

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!

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]

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.

 

[Dale]

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

 

[vanhees71]

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]

Me too

 

[Delta2]

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.

 

[Dale]

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

 

[fresh_42]

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!

 

[Delta2]

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.

 

[artis]

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!

 

[vanhees71]

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".

 

[rude man]

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.

 

[vanhees71]

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.

 

[sophiecentaur]

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.

 

[vanhees71]

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

 

[berkeman]

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