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iantresman
2006-Feb-18, 07:14 PM
Lightning is a movement of charged particles. The electromagnetic force between charged particesl is quite large (1039 times that of gravity. So why doesn't a lightning bolt blow itself apart?

Regards,
Ian Tresman

Dragon Star
2006-Feb-18, 07:44 PM
Bumping this to try and get some answers.

iantresman
2006-Feb-18, 09:48 PM
Lightning is a movement of charged particles. The electromagnetic force between charged particesl is quite large (1039 times that of gravity. So why doesn't a lightning bolt blow itself apart?

I've done a bit of research myself, and think it has something to do with the "Bennett Relation", the "Carlqvist Relation", and electromagnetic pinches (z-pinches). More when I find it.

Regards,
Ian Tresman

papageno
2006-Feb-18, 10:06 PM
Lightning in atmosphere (http://thunder.msfc.nasa.gov/primer/primer2.html) is surrounded by air.

antoniseb
2006-Feb-18, 10:18 PM
There was some discussion off-line about moving this thread to Q&A, and I have decided it belongs here in ATM. Thanks to Ian for making a good choice about where to post it.

Concerning the answer, my take is that the thunderclap is evidence that nothing hold the lightning bolt together. The air gets hot very fast and expands. Note well, my answer is not based on research or even OOM estimates.

TheBlackCat
2006-Feb-18, 10:30 PM
I know in charged particle beams, which are basically controlled lightning bolts, the charged particles interacting with the air creates a magnetic field that holds the beam together. I do not know whether this principle applies to lightning bolts or not, though.

iantresman
2006-Feb-19, 03:46 PM
Lightning in atmosphere (http://thunder.msfc.nasa.gov/primer/primer2.html) is surrounded by air.

I don't think that air pressure would be sufficient. As has been pointed out on numerous occassions, the force between charged particles is rather high (astronomical as compared to gravitational forces).

Just as the gas from an escaping valve diverges, the electrons in a lightning bolt should diverge even more drastically?

So something must be holding the lightning bolt "together" which does not hold escaping gas jets "together"?

Regards,
Ian Tresman

iantresman
2006-Feb-19, 03:49 PM
There was some discussion off-line about moving this thread to Q&A, and I have decided it belongs here in ATM. Thanks to Ian for making a good choice about where to post it.

Well, I don't plan any "Against the Mainstream" discussion, it's a general question, to which I think I know the answer... and it should be mainstream.


Concerning the answer, my take is that the thunderclap is evidence that nothing hold the lightning bolt together. The air gets hot very fast and expands. Note well, my answer is not based on research or even OOM estimates.

Absolutely, the air expands, but not the charged current-carrying channel. Consider a spark in the "vacuum" of space. Why don't the electrons (or ions) in the spark diverge, as does an escaping jet of gas?

Regards,
Ian Tresman

iantresman
2006-Feb-19, 03:50 PM
I know in charged particle beams, which are basically controlled lightning bolts, the charged particles interacting with the air creates a magnetic field that holds the beam together. I do not know whether this principle applies to lightning bolts or not, though.

This would be my inclination.

Regards,
Ian Tresman

Jerry
2006-Feb-19, 04:18 PM
A lightning bolt does blow itself apart - the air molecules are superheated and collide with nearby molecules creating a shock wave just like the leading edge of a jet. It is during the compression phase of this phenomenon that the ionized air discharges. But the expansion is rapid and the bolt rapidly dissipates...most of the time. Rarely, a torridal ball will form and the ionized plasma remains trapped, literally rolling across the sky.

iantresman
2006-Feb-19, 05:04 PM
A lightning bolt does blow itself apart - the air molecules are superheated and collide with nearby molecules creating a shock wave just like the leading edge of a jet. It is during the compression phase of this phenomenon that the ionized air discharges. But the expansion is rapid and the bolt rapidly dissipates...most of the time. Rarely, a torridal ball will form and the ionized plasma remains trapped, literally rolling across the sky.

I agree that the air through which the lightning bolt travels, may blow itself apart, but the actual discharge channel itself, containing unequal numbers of oppositely charged particles (?), does not.

Regards,
Ian Tresman

papageno
2006-Feb-19, 05:11 PM
I don't think that air pressure would be sufficient. As has been pointed out on numerous occassions, the force between charged particles is rather high (astronomical as compared to gravitational forces). See Jerry Jensen's post.




Just as the gas from an escaping valve diverges, the electrons in a lightning bolt should diverge even more drastically? There are not only electrons in the lightning bolts.
The electrons come from the ionization of atoms, so there are positive ions as well.

The gas escaping from a valve diverges if the initial velocities of the particles diverge and if the particles scatter in the medium.



So something must be holding the lightning bolt "together" which does not hold escaping gas jets "together"? It depends on the timescales.
We can produce beams of electrons in vacuum (for example, in a Scanning Electron Microscope), which keep together over a macroscopic distance, with nothing "holding" the electrons together.
We can do the same with neutral atoms (for example, Vacuum Deposition).

korjik
2006-Feb-19, 06:38 PM
There is also the fact that the lightning is a discharge. The (generally) positive ground end is attracting the negative end. Basically tho, I think that once the conducting path to ground is formed, it just discharges so quickly that there isnt time for the bolt to disperse.

how much charge is there in a 'bolt? If there really isnt much charge, there may not be much dispersive force.

iantresman
2006-Feb-19, 07:53 PM
There is also the fact that the lightning is a discharge. The (generally) positive ground end is attracting the negative end. Basically tho, I think that once the conducting path to ground is formed, it just discharges so quickly that there isnt time for the bolt to disperse.

how much charge is there in a 'bolt? If there really isnt much charge, there may not be much dispersive force.

It seems that (from Wikipedia (http://en.wikipedia.org/wiki/Lightning)):

An average bolt of negative lightning carries a current of 30 kiloamperes, transfers a charge of 5 coulombs, has a potential difference of about 100 megavolts and dissipates 500 megajoules
.
An average bolt of positive lightning carries a current of 300 kiloamperes, transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt


And from a site on Charge and Charge Interactions (http://www.glenbrook.k12.il.us/gbssci/Phys/Class/estatics/u8l1b.html):

If the amount of charge carried by a lightning bolt is estimated at 10 Coulombs, then the quantity of excess electrons is carried by the lightning bolt is as follows:
.
Multiply the charge in Coulombs (10 C) by the conversion factor: (1 electron) / (1.6 x 10-19 C). The units of C cancel; the answer is in electrons, ie. 6.25 x 1019 electrons


Regards,
Ian Tresman

papageno
2006-Feb-19, 11:02 PM
If the amount of charge carried by a lightning bolt is estimated at 10 Coulombs, then the quantity of excess electrons is carried by the lightning bolt...
Why excess electrons?
Why not 5 C of electrons and 5 C of positive ions, which would give the same net transfer of 10 C of charge?

Gsquare
2006-Feb-20, 01:51 AM
Lightning is a movement of charged particles. The electromagnetic force between charged particesl is quite large (1039 times that of gravity. So why doesn't a lightning bolt blow itself apart?

Regards,
Ian Tresman

Ian; just prior to a lightning 'discharge' there is a ground to cloud "leader" that forms which is basically a ionized path, a path of lower resistance, in which the discharged electrons follow.
Thus, the discharge 'stays together' because the charges are following the path of least resistance.

This is well known, and in fact, discharges can be stimulated from a thunder cloud simply by creating a ionized path by sending up a small rocket into the cloud, the exhaust of which ionizes the air. The lightning discharges down the exact path the rocket took. It is done regularly these days.

Gsquare :D

iantresman
2006-Feb-20, 09:53 AM
Ian; just prior to a lightning 'discharge' there is a ground to cloud "leader" that forms which is basically a ionized path, a path of lower resistance, in which the discharged electrons follow.
Thus, the discharge 'stays together' because the charges are following the path of least resistance.

I agree with your description. So what is this "resistive force", that causes electrons to overcome the repulsive forces between them, and apparently maintain a constricted channel of charged particles?

And the repulsive force between the electrons means that they lightning channel should start to diverge immediately.

Regards,
Ian Tresman

papageno
2006-Feb-20, 08:42 PM
I agree with your description. So what is this "resistive force", that causes electrons to overcome the repulsive forces between them, and apparently maintain a constricted channel of charged particles?

And the repulsive force between the electrons means that they lightning channel should start to diverge immediately.

*cough*post #12 (http://www.bautforum.com/showpost.php?p=685364&postcount=12)*cough*

iantresman
2006-Feb-20, 09:59 PM
*cough*post #12 (http://www.bautforum.com/showpost.php?p=685364&postcount=12)*cough*

But isn't there an excess of electrons, 6.25 x 1019 excess electrons, according to *cough* post #14 *cough*, for a lightning bolt estimated at 10 Coulombs?

Regards,
Ian Tresman

papageno
2006-Feb-20, 11:51 PM
But isn't there an excess of electrons, 6.25 x 1019 excess electrons, according to *cough* post #14 *cough*, for a lightning bolt estimated at 10 Coulombs?
See post #15 (http://www.bautforum.com/showpost.php?p=685534&postcount=15):


Why excess electrons?
Why not 5 C of electrons and 5 C of positive ions, which would give the same net transfer of 10 C of charge?
After all, the air molecules in the channel are ionized, giving an equal number of positive ions and electrons.

iantresman
2006-Feb-21, 12:51 AM
After all, the air molecules in the channel are ionized, giving an equal number of positive ions and electrons.

Sure the ionized air has equal numbers of oppositely charged ions. We can ignore them. But through this ionized channel flows an excess of some 1020 electrons.

Not only do these electrons experience a repulsive force, but I believe that the discharge may actually "constrict" and overcome this repulsive force.

Regards,
Ian Tresman

Gsquare
2006-Feb-21, 05:12 AM
I agree with your description. So what is this "resistive force", that causes electrons to overcome the repulsive forces between them, and apparently maintain a constricted channel of charged particles?


Good question;
I will answer with a question.
What is the 'resistive' force that causes electrons to overcome their repulsive force in a copper wire? Why don't the electrons spread out of the channel provided by the wire?

Answer: Because the wire provides a channel of lower electrical resistance (conductivity) which they are required to follow; beyond the edge of the wire is higher 'resistance' that keeps the electrons 'contained'.
Likewise for the 'wire' in the sky provided by the ionized path of a 'leader'. There is higher resistance 'outside' the ionized path which keeps the electrons contained and accelerating in one direction.




... the repulsive force between the electrons means that the lightning channel should start to diverge immediately.


That's true ONLY if they don't meet an equivilent repulsive force from the edges of the channel.

Guess what provides the repulsive force at the edges of the channel?
Thats right .... heavy atoms and molecules that have their orbitals completly filled with....
ELECTRONS! :D

G^2

-Lottery: The government's method of taxing the mathematically challenged.--

Enzp
2006-Feb-21, 05:49 AM
If a cloud of free electrons were hanging in the air, they might like to push apart from each other. But a lightning bolt is an arc - it is current flowing. The electrons aren't fighting each other any more than they are in the wires in the wall of your house.

The Charges build up between cloud and ground - or cloud and cloud - excess of electrons in one place and a dearth in the other. When the charge builds to a point it can overcome the resistance between teh two points, the arc forms and discharges the big old capacitor.

Once discharged, the arc will fail and other forces will extinguish it.

papageno
2006-Feb-21, 11:31 AM
Sure the ionized air has equal numbers of oppositely charged ions. We can ignore them. But through this ionized channel flows an excess of some 1020 electrons. And where do these excess electrons come from?
You cannot ignore the ionized atoms, because those provide the particles that compose the current.

When you connect a charged condensor to a circuit you do not get excess electrons flowing in the circuit.
Electrons that are already in the wires move under the influence of the electric field produced by the charges in the condensor.
Simililarly, once the voltage between cloud and ground has produced a conducting channel in the air by ionization, charges that are already in the channel flow.
(Look up the mean free path of an electron in air; there is a reason why cathode ray tubes are under vacuum.)



Not only do these electrons experience a repulsive force, but I believe that the discharge may actually "constrict" and overcome this repulsive force. The "z-pinch" idea?
You have to show first that the charged particles move long enough without scattering, for the magnetic field produced by the current itself to have a significant effect on the trajectory of the particles.
(There is a reason that the quantum hall effect is observable only at low temperatures.
By the way, you don't need excess electrons to produce a magnetic field; otherwise electromagnets would not work.)

papageno
2006-Feb-21, 11:36 AM
Good question;
I will answer with a question.
What is the 'resistive' force that causes electrons to overcome their repulsive force in a copper wire? Why don't the electrons spread out of the channel provided by the wire?

Answer: Because the wire provides a channel of lower electrical resistance (conductivity) which they are required to follow; beyond the edge of the wire is higher 'resistance' that keeps the electrons 'contained'.
Likewise for the 'wire' in the sky provided by the ionized path of a 'leader'. There is higher resistance 'outside' the ionized path which keeps the electrons contained and accelerating in one direction.


That's true ONLY if they don't meet an equivilent repulsive force from the edges of the channel.

Guess what provides the repulsive force at the edges of the channel?
Thats right .... heavy atoms and molecules that have their orbitals completly filled with....
ELECTRONS! :D

Actually, it's because the ions of the metal provide an attractive force to keep the electrons in the wire.
That's why it takes work to extract electrons from a metal (work function).

Gsquare
2006-Feb-21, 02:58 PM
Actually, it's because the ions of the metal provide an attractive force to keep the electrons in the wire.
That's why it takes work to extract electrons from a metal (work function).

Right, i understand clearly (and figured someone would challenge me on it) but didn't want to complicate it with work function concepts so I used "resistive' force in a very general sense to keep it consistant with Ian's termonology. Notice, that's why I put the word 'resistance' in quotations. It was only to be an analogy to show that in both cases there are what Ian calls 'resistive' forces at the boundary.

G^2

Nereid
2006-Feb-21, 05:17 PM
So, in terms of the OP ("Lightning is a movement of charged particles. The electromagnetic force between charged particesl is quite large (1039 times that of gravity. So why doesn't a lightning bolt blow itself apart?"), you have to look at all (relevant) charged particles, not just those which at first glance seem to be those which comprise the current.

To see 'lightning' in terms of one example of a much broader class of phenomena, think of electron guns, particle accelerators, electron microscopes, static electricity discharges, cosmic ray induced discharges of electroscopes, arc welders, ... is there anything special about (Earth atmosphere) lightning (other than that it involves a lot of different things, several of which are not well understood yet)?

iantresman
2006-Feb-21, 07:32 PM
To see 'lightning' in terms of one example of a much broader class of phenomena, think of electron guns, particle accelerators, electron microscopes, static electricity discharges, cosmic ray induced discharges of electroscopes, arc welders, ... is there anything special about (Earth atmosphere) lightning (other than that it involves a lot of different things, several of which are not well understood yet)?

I think you're probably right :-)

The How Stuff Works web site has what appears to be a good description of the lightning process (http://science.howstuffworks.com/lightning3.htm):


"The strength or intensity of the electric field is directly related to the amount of charge build-up in the cloud. [..] the electric field becomes more and more intense -- so intense, in fact, that the electrons at the earth's surface are repelled deeper into the earth by the strong negative charge at the lower portion of the cloud. [..] The strong electric field causes the air around the cloud to "break down," allowing current to flow in an attempt to neutralize the charge separation."

So there is an obvious repulsive force between the electrons in the Earth causing them to disperse (and I would assume that the Earth is not a very good conductor), but when the breakdown occurs and discharge occurs, the positive ions overcome their repulsion, converge, and follow the discharge channel.

I believe that a lightning bolt emits a strong electromagnetic pulse, which seems to be consistent with your suggestions that there is a relation to particle beams.

Regards,
Ian Tresman

papageno
2006-Feb-21, 11:25 PM
The How Stuff Works web site has what appears to be a good description of the lightning process (http://science.howstuffworks.com/lightning3.htm):



When there is a charge separation in a cloud, there is also an electric field that is associated with the separation. Like the cloud, this field is negative in the lower region and positive in the upper region.(Emphasis mine.)




"The strength or intensity of the electric field is directly related to the amount of charge build-up in the cloud. [..] the electric field becomes more and more intense -- so intense, in fact, that the electrons at the earth's surface are repelled deeper into the earth by the strong negative charge at the lower portion of the cloud.
A charge is induced at the Earth's surface, exactly as it happens in a capacitor.





The strong electric field causes the air around the cloud to "break down," allowing current to flow in an attempt to neutralize the charge separation."



The electric field causes the surrounding air to become separated into positive ions and electrons; the air is ionized. Keep in mind that the ionization does not mean that there is more negative charge (electrons) or more positive charge (positive atomic nuclei / positive ions) than before. This ionization only means that the electrons and positive ions are farther apart than they were in their original molecular or atomic structure. Essentially, the electrons have been stripped from the molecular structure of the non-ionized air.
(Emphasis mine.)
So, no excess electrons.



So there is an obvious repulsive force between the electrons in the Earth causing them to disperse (and I would assume that the Earth is not a very good conductor), but when the breakdown occurs and discharge occurs, the positive ions overcome their repulsion, converge, and follow the discharge channel.



The importance of this separation/stripping is that the electrons are now free to move much more easily than they could before the separation. So this ionized air (also known as plasma) is much more conductive than the previous non-ionized air.

[...]

These electrons have excellent mobility, allowing for electrical current to flow. The ionization of air or gas creates plasma with conductive properties similar to that of metals.

[...]

Once the ionization process begins and plasma forms, a path is not created instantaneously. In fact, there are usually many separate paths of ionized air stemming from the cloud. These paths are typically referred to as "step leaders."

[...]

As the step leaders approach the earth, objects on the surface begin responding to the strong electric field. The objects reach out to the cloud by "growing" positive streamers.

[...]

Next to occur is the actual meeting of a step leader and a streamer.

[...]

After the step leader and the streamer meet, the ionized air (plasma) has completed its journey to the earth, leaving a conductive path from the cloud to the earth. With this path complete, current flows between the earth and the cloud. This discharge of current is nature's way of trying to neutralize the charge separation. The flash we see when this discharge occurs is not the strike -- it is the local effects of the strike.
Since the electrons have a much higher mobility than the positive ions, it's the electrons that determine the current along the channel.




I believe that a lightning bolt emits a strong electromagnetic pulse, which seems to be consistent with your suggestions that there is a relation to particle beams.
Of course it's an electromagnetic pulse: there is a sudden change of electric fields during the discharge.
In principle, lightning is like a discharge through the dielectric material in a capacitor.