Why don't electrons jump between the turns of a coil?

[Guidestone]

Hey guys. I just want to know why electrons in a coil always follow the turns instead of jumping from one turn to another. I mean, in many cases the turns are very close to one another. Also, if I'm not mistaken, electrons travel the shortest distances. Thank you!

 

[jtbell]

The wire has insulation on it. If it didn't, you would indeed have a "short circuit" between the turns of a coil, if they're in contact.

 

[Averagesupernova]

Sometimes they do. Usually this is not considered a good thing. Lightning is usually thought of as 'not liking corners'. It contains very high frequencies and sometimes the inductance is high enough to cause arcing between turns when a coil is involved in lightning strikes.

 

[dlgoff]

I just want to know why electrons in a coil always follow the turns instead of jumping from one turn to another.

It's all in their speed. Not fast enough to jump that far. Here's a neat little video about their speed.

 

[davenn]

It's all in their speed. Not fast enough to jump that far.

not completely
it's more their electron volt potential. if the voltage potential is enough to break down the insulation / air-gap
they will indeed jump between turnsDave

 

[meBigGuy]

I don't understand any of the answers so far.

Also, if I'm not mistaken, electrons travel the shortest distances

That is very mistaken.

The electrons take the path of least resistance, not the shortest path. The path traveled is that which requires the least expended energy.

Wire has high conductivity, insulation does not. You can read about the causes of conductivity here:
https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

It requires more force to travel through materials with lower conductivity (the wire insulation) so the electrons travel the long way around to "save energy".

 

[dlgoff]

not completely
it's more their electron volt potential. if the voltage potential is enough to break down the insulation / air-gap
they will indeed jump between turnsDave

Well yes. The point I was trying to make is, there's not enough energy (hence velocity) to cause breakdown; as @meBigGuy explains via conductivity. The phenomenon of arcs needs high electron volt potentials to occur.

High-field emission is essentially quantum-mechanical tunneling through the potential barrier at the surface of the cathode. The current density is given by the Fowler-Nordheim equation J = CE2exp(-D/E), where C = [6.2 x 10-6/(φ + EF)](EF/φ)1/2 A/V2, and D = 6.8 x 109φ3/2 V/m. EF is the Fermi energy and φ is the work function, both in volts. E is the field in V/m. For tungsten, φ = 4.52V and EF = 8.95V. The fields required are very high. A field of 2 x 107 V/cm produces emission of only 1.7 μA/cm2 in tungsten. However, the current increases quite rapidly with electric field. With a field of 3 x 107 V/cm, the current is already 0.2 A/cm2. At atmospheric pressure, the mean free path is about 10-5 cm in air, and if the cathode drop is 10V, the resulting electric field if the cathode drop occurs over one mean free path is 106 V/cm. This is about a factor of 20 less than is required, so some investigators have questioned the importance of high-field emission in arcs. However, it is at least close to the required value, and some local strengthening of the field by the arrangment of adsorbed positive ions may make up the difference.

The Richardson-Dushman equation for thermionic emission, J = AT2exp(-b/T) is very similar in form to the Fowler-Nordheim equation, with the absolute temperature T replacing the field strength E. A = 60.2 A/cm2, and b = 11600φ. At the boiling point of tungsten, 5993K, J = 3.45 x 105 A/cm2, a quite ample result. Even at the melting point, the current density is 541 A/cm2. Mercury has about the same work function as tungsten, 4.5V, but the thermionic emission at its boiling point, 630K, is only about 10-29 A/cm2, which is wholly inadequate. Therefore, high-field emission seems to be the only alternative.

Quote taken from http://www.physics.csbsju.edu/370/jcalvert/dischg.htm.html#Arc

I did an image search trying to find a coil showing any kind of arcing across turns and couldn't find any. I wonder why?

 

[rbelli1]

The electrons take the path of least resistance,

No, no, no, no. The electrons take all paths inversely proportional to the resistances of those paths.

If the quote were true than only one electrical device would ever be able to be connected to any given generating station and only one component in that device would be operational.

In reality some current is flowing through the insulation but the resistivity of the copper is so much lower than the plastic insulation that that current may be ignored.
Bulk resistivity of copper 0.000000017
Bulk resistivity of Mylar 10000000000000000000, which is similar to the insulation materials used on magnet wire.

BoB

 

[davenn]

Well yes. The point I was trying to make is, there's not enough energy (hence velocity) to cause breakdown; as @meBigGuy explains via conductivity. The phenomenon of arcs needs high electron volt potentials to occur.

yes that's correct voltage levels were not initially mentioned, and velocity doesn't enter into it ;)
the energy of an electron can be very high but its velocity can be zero ... consider a charge on a capacitor that breaks down the dielectric

Hey guys. I just want to know why electrons in a coil always follow the turns instead of jumping from one turn to another. I mean, in many cases the turns are very close to one another.

In the context of the OP, I was just stating that under the under the right voltage potentials arcing between turns is possible

Dave

 

[Guidestone]

Ok so if the potencial is enough then electrons can actually get through air gaps or even the insulation of the fabric that covers the wire. However, most coils I've seen don't seem to have anything covering up the wire. Do they still possesses insulation?

 

[meBigGuy]

No, no, no, no. The electrons take all paths inversely proportional to the resistances of those paths.

Of course that is correct. I oversimplified to the point of being incorrect. In the case with the coil there will be a small current through the insulation (all possible paths). The bulk of the current (in this case) will flow through the metal wire path with least resistance.

 

[meBigGuy]

Ok so if the potencial is enough then electrons can actually get through air gaps or even the insulation of the fabric that covers the wire. However, most coils I've seen don't seem to have anything covering up the wire. Do they still possesses insulation?

Most coils have an insulating enamel coating (google "magnet wire"). If it is not insulated, it will be wound on a grooved forms that separates the windings.

 

[Darryl]

The insulation around the wire of the coil makes the 'path of least resistance' the wire, but a coil or inductor stores the current as a magnetic field around the coil and the energy (of the electrons) in that magnetic field influences all the other coils (or loops) in the coil. But if you have two conductors close together but separated by an insulator then that is a capacitor, so a varying voltage across the coil will induce change in the other loops of the coil. So the electrons themselves do not jump across loops of the coil and electric and magnetic fields do.

 

[Guidestone]

Sometimes they do. Usually this is not considered a good thing. Lightning is usually thought of as 'not liking corners'. It contains very high frequencies and sometimes the inductance is high enough to cause arcing between turns when a coil is involved in lightning strikes.

by frequencies you mean lightning is a form of alternate current?

 

[Averagesupernova]

by frequencies you mean lightning is a form of alternate current?

Yes. This is why we hear it in radio receivers.

 

[thuyln2]

I felt it was really good , thank you ^^~

 

[NascentOxygen]

The insulating paint ('enamel') on the wire used for electromagnets in relays, and for inductors and transformers, is often similar in colour to copper wire, if a bit darker, so that can be why it's sometimes mistakenly thought to be bare copper. If you look inside TV sets you'll see inductors wound using bare uninsulated wire; these are wound using stiff self-supporting wire or tubing and the turns are spaced apart, so there is air insulation separating each turn from its neighbours.

 

[meBigGuy]

Yes. This is why we hear it in radio receivers.

I think we hear lightning in radio receivers for the same reason we hear any capacitive DC discharges in radio receivers. Lightning is a DC discharge. It is a impulse, and as such produces some high frequency RF components. Calling it AC is the same as saying that turning on a flashlight is AC because it produces high frequency components. Yes, it produces some AC components, but the basic phenomena is a DC discharge.

 

[CWatters]

most coils I've seen don't seem to have anything covering up the wire.

Frequently the insulation is a (semi) clear varnish or, as NascentOxygen said, it's a similar colour to copper.

 

[Averagesupernova]

I think we hear lightning in radio receivers for the same reason we hear any capacitive DC discharges in radio receivers. Lightning is a DC discharge. It is a impulse, and as such produces some high frequency RF components. Calling it AC is the same as saying that turning on a flashlight is AC because it produces high frequency components. Yes, it produces some AC components, but the basic phenomena is a DC discharge.

Yes I probably should have been more specific.

 

[NascentOxygen]

The typical cloud-to-ground lightning strike involves an initial stream of + charges heading upwards off a tall object, followed by a torrent of - charges plunging downwards along the same path. So if you say it quickly, it does sound like there's some sort of alternating flow involved near ground level. [emoji52]

 

[meBigGuy]

Yeah, I think the early low current streamers create (or initiate) the plasma path for the main discharge, or something like that.
http://www.nature.com/articles/srep15180

 

[sophiecentaur]

Lightning is a DC discharge.

I would classify (haha) it more as an impulse waveform - or a series of impulses. The main spectral components of the radiation will be Rado Frequency. in fact, 'DC' will not radiate an em wave. So we sould 'see' the optical output due to the DC but the RFI would not be DC.

 

[jim hardy]

To OP:

Ahhh, terminology

Strictly speaking DC means unidirectional, ie not crossing zero
as opposed to alternately flowing thisaway and thataway.
But it doesn't mean steady(constant) .

Inductance generates voltage in response to changing current
so a current going alternately from +1 amp to +2 amps back to +1 amp and so on but always same direction
which is direct (though not constant)
if directed pushed through a transformer primary
will show up as voltage on that transformer's secondary
provided the transformer is designed so its magnetic core doesn't saturate from so much direct current.

To your original question
Air is a good insulator. Its insulating strength is given as about 3 million volts per meter,
which is why it takes thousands of volts to jump a spark plug gap of maybe 0.030 inch.
Bare coils always have a space between turns. Enameled ones take care to not let the end turns' leads cross for that'd impress full coil voltage across the insulation where they touch . I had some relay failures where the assemblers had not put tape over the end turns where the incoming and outgoing wires crossed. It was in important equipment so we had a new batch of relays made and replaced them all.

old jim

 

[sophiecentaur]

The frequency spectrum of the energy radiated by a unidirectional lightning strike would contain no DC component, though. The DC component would presumably go towards heating the sky and the earth. With an arc of a few km length, the radiated RF could extend down to 100kz or so (which fits with the lf and mf interference we experience. (Not sure when I actually listened to am radio, actually). The burst of RFI would be much more serious than what you would get from a single, short unidirectional pulse.

 

[meBigGuy]

All we are saying is that lightning is not AC. As you said, It is an impulse (or maybe a series of impulses).
You are saying that DC only applies to components with 0 frequency.
By your definition a power supply is not DC over the time before turning it on to the time after turning it off since it is merely an impulse.
Technically that is true but not a useful perspective.

An impulse is DC over the duration of the impulse.

I refuse to call a DC impulse AC. Lightning might have true AC components if there is some sort of ringing phenomena that I am not familiar with.
A series of DC impulses might be considered an AC signal riding on DC (like an offset squarewave).

Can anyone find a reference that classifies lightning as an AC phenomena (other than stringer reversal).

 

[Salvador]

I didin't search for definitions but to me AC is something which has some periodicity over some given amount of time, and the current reverses direction during those periods.
Lightning , atleast all of which I have seen is basically large voltage across a gap which forms a short when the breakdown of air happens and the charges flow in one direction until the voltage across the gap has fallen so low that the arc cannot be sustained anymore.I would say it's like charging a capacitor with some high voltage DC and then just short circuiting the terminals allowing all the stored energy to be drained.

 

[sophiecentaur]

I refuse to call a DC impulse AC. Lightning might have true AC components if there is some sort of ringing phenomena that I am not familiar with.

It's of no consequence what name individuals choose to give a phenomenon but the following is what happens. The lightning discharge generates RF and it is caused by a burst of unidirectional current from what is, essentially, a DC source (charged clouds). RF is not DC, it's AC. Describing a waveform in terms of its frequency spectrum must be acceptable. There is more than just DC in the current waveform in a lightning strike. If there were not, we would not hear it on our mf radios. This is basic theory that relates the time domain description of signals to the frequency domain. "Unidirectional" would be a more suitable word to describe the current waveform that you get in lightning. DC is what you get out of batteries - after a suitable time for its value to settle down.

 

[jim hardy]

derivative removes any DC component

current i may not reverse but its derivative di/dt does else i would never get back to zero

 

[meBigGuy]

Mathematically there is no difference between a battery being discharged and a lightning strike. They are impulses with different timeframes. They both generate RF.

Let's get clear here--- are you saying that lightning is AC? Do you have references to support that label? I think you said it is an impulse or series of impulses, and impulses have AC components. That doesn't make it AC.

 

[Salvador]

I think what sophie said is that lightning itself is DC , we have known that for quite some time now but the very burst itself the arc through the air creates some extra components with it that may resonate after the initial strike has run short of its potential, and those extra components are more like high frequency AC.

not a brilliant but still an analogy would be that take any other DC source , a battery or more closely a capacitor charged to high potential and it matters through what medium you discharge the cap.the discharge itself could create AC for brief amount of time.
it matters if you discharge the cap with a short thick screwdriver or with a few turns of conducting path wrapped in a coil fashion , it could even be coreless still a very high potential would be shorted through it , and some resonant EM field would exist for some time around the coil.yet the source was and still is DC.

 

[sophiecentaur]

See what a waste of time classification can be?
DC from a battery and the conditions that we use in 'DC calculations' are all predicated on the steady state - or the 'near enough' steady state to allow good predictions. A lightning strike does not satisfy those conditions. It is brief, non linear and generates significant RF to tell me that there is 'AC' there. AC is not just 50 or 60 Hz; if there is an alternating component at 200kHz then I would call that AC. Would you call it DC? What would Mr Fourier have to say about it? I don't need to give references to him, do I?

 

[NascentOxygen]

A lightning bolt doesn't neatly align with the concept of either DC or AC. Nor need it.

The question of why electrons follow the tedious circuitous path in a coil has been adequately answered. Thanks to all contributors.

Thread closed.

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