Where does the magnetic field go???

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fluoronator
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Where does the magnetic field go???

Post by fluoronator » Thu Mar 10, 2011 4:59 am

I received my new Rigol DS1052E Oscilloscope yesterday and spent a night of bliss with my new nerd toy. I was observing the counter EMF produced from the magnetic field collapse when switching off a relay and began to ponder... If I charge a capacitor and disconnect it, it will hold it's charge until it leaks down or is discharged. But when I store energy in a magnetic field and disconnect it, the field collapses and the power is gone. Where did it go??? Why does it not hold it's "Charge" like the cap?

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MrAl
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Re: Where does the magnetic field go???

Post by MrAl » Thu Mar 10, 2011 9:15 am

Hi there,


To start with, the inductor isnt as efficient as the capacitor simply because most inductors can not be made as well as we would like. The other problem is that the inductor is the 'dual' of the capacitor, and one meaning of dual is that the inductor works on current like a capacitor works on voltage, and current is also the dual of voltage. What this means is that since the capacitor requires an open circuit to hold its charge, the inductor requires a short circuit to hold its charge. This means that in real life you would see the inductor hold its charge longer if the leads were shorted out instead of left open. This isnt that hard to do but it has to be done right. Once the current builds to maximum you have to short the inductor out rather than remove it from the circuit, then you can disconnect the power supply. Of course this means the power supply must be able to handle a short circuit without blowing out.
As i was saying above, the inductor isnt as efficient as the cap so the inductor, even if shorted out, will still dissipate its energy after some time. This is because there is a series resistance and that series resistance allows a voltage to develop across the coil and that voltage is what causes the energy loss, similar to how shorting out a capacitor would causes its energy to dissipate.
When the leads of an inductor are opened up, the inductor wants to maintain the original charging current assuming no core saturation has occurred. This means a high voltage is be developed that could hurt the wire insulation too. You also have to be careful you dont blow the input to your scope when doing this test, because the voltage can go very high if the inductor is good enough.
The discharging time constant for the capacitor is R*C, but for the inductor it is L/R. From this alone we can see that when R is increased for the capacitor, it takes longer to discharge it, but when R is increased for the inductor it actually discharges faster. With very large R (which is similar to an open circuit) the inductor would discharge very quickly, while the cap would stay charged for a long time.
Also note that not all caps stay charged for a very long time as they have leakage which discharges them too.
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sofaspud
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Re: Where does the magnetic field go???

Post by sofaspud » Thu Mar 10, 2011 10:52 am

fluoronator wrote:But when I store energy in a magnetic field and...
I don't believe a magnetic field can store energy. Remember, either the field or conductor has to move for work to be (or
potential to be) performed. The capacitor is different because you've actually moved some electrons.

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MrAl
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Re: Where does the magnetic field go???

Post by MrAl » Thu Mar 10, 2011 11:26 am

Hi Sofa,


It is "said" that the energy is stored in the field (air core inductor) because the field is built up with the passage of free electrons. If we remove the power source and maintain that field nothing happens and electrons stop moving. If we allow that field to collapse though, it again forces electrons to flow in the wire (and to the rest of the circuit assuming a closed loop of course).
Thus the field is created when the electrons are forced to move by some external source and when the field collapses the field then pushes the electrons out of the wire at one end and back in the other.
This can be compared to another system where we use a water pump to pump water though a pipe and there is a water wheel (or turbine) inside the pipe set up so that it turns when the water rushes past, and the axle of the water wheel is connected to a large rotational mass (like a big cylinder made out of the element Lead). At first the mass rotates slowly, but as time goes on the mass rotates faster and faster and that rotation itself is what stores the energy. When we turn off the pump assuming the water is still free to flow, the rotational inertia starts to drive the water wheel rather than the water driving the wheel as before when the pump was running. The energy gets transferred back into the water and the water is pushed through the pipe. Now with a lossless pipe and wheel and no friction, the water would circulate indefinitely (as a shorted inductor would conduct that current indefinitely) but if there are any losses they would show up as a drop in pressure and that would dissipate energy so the mass would run out of rotational momentum and would eventually come to a stop.

In an inductor with a metal core however there is a secondary action that takes place. The moving electrons create a field but then that field, instead of building up itself, forces tiny substructures in the metal to flip their magnetic states.
If the right external conditions are put in place, the domains flip back and act on the field the same way the field acted on them. So with the metal core we create tiny little magnets that orient in the direction of the field, and then later we allow those magnets to generate the field.

It's sort of ok if you want to say that the air core inductor does not store energy in the field, but then you have to figure out where it does store it, and how. You may want to keep in mind that work requires resistance, so if we dont have any resistance we dont have any work being done. There is a small amount of work done in the metal core inductor though that is caused by the effort to flip states, but that is only a small fraction of the entire energy and that is one of the things that makes a metal core inductor less efficienct, and that energy is never recovered but is lost as heat.
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haklesup
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Re: Where does the magnetic field go???

Post by haklesup » Thu Mar 10, 2011 11:57 am

If you operated that coil in a medium capable of aligning with the field and staying that way, then you could have a magnetic capacitor.

For example consider a coil with an iron core, heat that up above the critical temperature, turn on the coil, cool the metal and you will have a stored magnetic field in the form of a permanant magnet. Even so, it's still not a perfect analogy to capacitance because its difficult to extract the energy you stored in that rod. Now you have to move the rod around to flux the magnetic field for the coil to do any additional work.

The reason inductors don't store energy is because we don't want them to and so supply them with either air cores or magnetically stable ferrite cores. An inductor with storage capacity would exhibit hysterisis WRT its polarization.

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MrAl
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Re: Where does the magnetic field go???

Post by MrAl » Thu Mar 10, 2011 2:26 pm

haklesup wrote:If you operated that coil in a medium capable of aligning with the field and staying that way, then you could have a magnetic capacitor.

For example consider a coil with an iron core, heat that up above the critical temperature, turn on the coil, cool the metal and you will have a stored magnetic field in the form of a permanant magnet. Even so, it's still not a perfect analogy to capacitance because its difficult to extract the energy you stored in that rod. Now you have to move the rod around to flux the magnetic field for the coil to do any additional work.

The reason inductors don't store energy is because we don't want them to and so supply them with either air cores or magnetically stable ferrite cores. An inductor with storage capacity would exhibit hysterisis WRT its polarization.
Hi hackle,

Im not sure i understand you when you say "inductors dont store energy". Do you mean they dont store energy for an indefinite period of time as they do only temporarily?
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haklesup
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Re: Where does the magnetic field go???

Post by haklesup » Thu Mar 10, 2011 7:33 pm

You read between the lines correctly. I was thinking of static storage as opposed to dynamic storage. When an inductor is de-energized (i=0) its stored energy is returned to the circuit or lost as heat immedietly (but not instantly).

Inductors don't store charge at least

I was even talking loosly referring to a magnetic capacitor because the analogy is not very close.

A better model might be a motor and flywheel powered by a field generated by coils (as in an AC motor) some energy would be stored as inertia which could be returned to a load for an extended period of time. Still not quite the same since there are extra energy conversions not present in a pure inductor.

I think the traditional analogy to mechanical systems is capacitors are springs and inductors are shock absorbers (dashpots). The time domain differential equasions take the same form and formulas.

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Re: Where does the magnetic field go???

Post by Robert Reed » Thu Mar 10, 2011 9:20 pm

One way of looking at storage capabilities of capacitors and inductors is in a resonant circuit. Here we have the two components handing off all their stored energy to the other component. Each component stores an equal amount of energy and at the proper time hands it off to the other. So in that respect they both have equal energy storage capabilities and use it to the max. Back and forth with each swing of energy reduced only by circuit 'Q' until it diminishes to zero. The loss of energy is due to only one parameter, and that being resistance with in the circuit. With infinite Q, we could bang the circuit once and it would ring forever (at least in theory).This would seem to indicate that each device is just as efficient as the other when storing and releasing energy,albeit for the short time constants usually involved here.

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sofaspud
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Re: Where does the magnetic field go???

Post by sofaspud » Fri Mar 11, 2011 8:01 pm

My thinking is that from an electronics pov using the phrase "store energy" is adequate and practical. But the question was
more physics oriented, and "store" is likely not the best term to use in understanding the answer to the op's question. The
energy isn't stored in the field or the inductor; it is induced by the expanding and collapsing field. And the amount induced is
directly proportional to this change in the field or the location of the conductor within the field. The capacitor on the other
hand does store energy, and the amount stored is directly proportional to the amount of energy used to store it. I'm a far
far cry from Feynman but I'm reasonably confident this is correct.

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MrAl
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Re: Where does the magnetic field go???

Post by MrAl » Sat Mar 12, 2011 9:18 am

Hello sofa,

That's a good point to bring up i think. But it's just a little bit of a matter of wording.
The energy isnt really stored "in" the field, the energy "is" the field. It's the relativistic energy of the field itself.
We commonly refer to this as "energy is stored in the field", but the better wording is "the energy is the field".
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Re: Where does the magnetic field go???

Post by Robert Reed » Sat Mar 12, 2011 11:20 am

If the energy is the field and the inductor created that field, then it follows that the inductor stored its energy as that field.It took its energy from energized circuitry and the at the proper time returned that energy to the circuit, thus in an ideal world no energy would have been lost. The way I see it is, if an inductor had any load across it whatsoever (diode,resistance,etc.) that when the field collapses upon it self the high voltage created by this condition would create a very large current briefly. There cold be a lot of joules in this process, but I will leave it up to the better mathematicians in this group to come up with a comparison scenario as to C versus L. If there were absolutely no load across the coil. then the energy would dissipate at the SRF of the inductor and ring until its inherent resistance ate up all the power.
Bottom line would be that the field created by a transformation of energy is returned to that form of energy and thats "where the magnetic field went"

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MrAl
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Re: Where does the magnetic field go???

Post by MrAl » Sat Mar 12, 2011 12:32 pm

Hi again,


Robert:
I have to agree with most of that except for one little part. The inductor is the dual of the capacitor, and voltage is like the dual of current. Open circuiting the inductor is like short circuiting the capacitor. Open circuiting the inductor causes a very high voltage of little current, and short circuiting a capacitor causes a very high current with little voltage.
Energy dissipation is the same though as you nicely noted earlier. The capacitor that generates the high current through the small resistance dissipates as P=I^2*Rsmall, while the inductor that dissipates through large resistance dissipates as P=E^2/Rlarge. Yeah we could instead multiply the voltage times the current in each case, and in one case the current is high and the voltage low, and in the other case the voltage is high and the current is low.
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Re: Where does the magnetic field go???

Post by Robert Reed » Sat Mar 12, 2011 2:32 pm

MrAl
Agreed. Kind of like their vectors at 180 Degrees apart and their properties in sort of a mirror image.

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sofaspud
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Re: Where does the magnetic field go???

Post by sofaspud » Sat Mar 12, 2011 4:06 pm

Yes, we're on the same page here. Maybe not the answer the op was expecting, but in many ways inductor action is as
complicated as active semiconductor components. And to say: The electromagnetic field exists around all the current-carrying
conductors. An inductor is designed to store some of that energy in that portion of the circuit. I'm happy with that. :grin:

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Bob Scott
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Re: Where does the magnetic field go???

Post by Bob Scott » Sun Mar 13, 2011 11:20 am

sofaspud wrote:Yes, we're on the same page here. Maybe not the answer the op was expecting, but in many ways inductor action is as
complicated as active semiconductor components. And to say: The electromagnetic field exists around all the current-carrying
conductors. An inductor is designed to store some of that energy in that portion of the circuit. I'm happy with that. :grin:
The way I see it, every "circuit" has at least one turn, from the power supply + terminal, through the circuit, and back to the - terminal. It's a one turn inductor.

A coil could store energy indefinitely if it weren't for resistance. Get current moving through the coil and then short circuit the coil terminals. If it weren't for resistance, the current would flow round and round forever. Just like the time constant for capacitors with a parallel resistor (T=RC) there is a time constant for inductors with a series resistance. (T=L/R)

This brings up another interesting question. If there are only two parts in a circuit, how can you tell if they are in parallel or in series? Both circuits look the same.
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