Re: A question about current....
 

All sorts of different stuff happens in lightning- I found this and it seems to do a good job explaining it.

From the article:

Four primary types of lightning- each of which has different properties.

Re: A question about current....
 

I dont understand what they mean by positive lightning?

what is it that travels? air molicules with a positive charge? the amount of mass in motion would be huge compaired to electrons.

Last time I looked into how lightning happens scientists were at a loss to explain it all - there is not enough voltage present to strike an arc several miles long, so something else is going on.

Lightning is scary stuff - esp Blue Lightning :^)

Re: A question about current....
 

Somethings about "Positive Strikes:" (from http://www.ucar.edu/communications/i...tning/faq.html)


What is a positive flash?

The renegade of the lightning family is the positive flash--one that lowers positive charge to earth. Comprising 10-20% of all cloud-to-ground flashes, these powerful bolts carry as much as ten times the current of negative CGs and often last longer. They frequently emerge from the cirrus anvils that sweep downwind of thunderstorms, rather than from a storm's core. Some storms feature many more positive flashes than usual. The presence of smoke, dust, or pollution (such as downwind from urban areas) seems to encourage the development of positive flashes. This is probably because of the particles' effect on the number and sizes of ice crystals within storms. A study in the journal Science (10/2/98) examined thunderstorms in the Southern Plains during the spring of 1998, when smoke from Mexican forest fires was flowing northward over the region. Up to three times the usual number of positive flashes were observed in these smoke-altered storms.

Re: A question about current....
 

Well we've already seen that electrons travel pretty slowly so why not have some o3+ traveling even slower?

This article also has some more explanation, but it it doesn't really explain how it happens. The fact that the very first part of the discharge is stepped is very interesting. Its just my speculation, but maybe this indicates that there is some sort of brief moment were just enough current can travel to ionize a piece of air and that the steps occur because the bolt sort of needs to recharge after each high current ionization period.

Re: A question about current....
 

I think that in a lightning strike, or an arc in general, electrons actually are travelling the distance from one end to the other very quickly

lightning does not travel at the speed of light like an EMF wave through a conductor, but it travels very fast - I think lighting IS like water flowing through an empty hose - the electrons from the clouds are physically moved to the surface of the earth in a fraction of a second.

Re: A question about current....
 

That defenitley must be true because lightning is caused by a charge difference and the charge difference won't go away until charge is transferred.

Re: A question about current....
 

From what I'm reading, it seems like these "streamers" are doing most of the work, setting the stage for the discharge. Perhaps the streamers are caused by subtle shifts in electromagnetic fields. My best guess is as a cloud gains more and more negative particles (electrons), the sheer volume creates a large enough electromagnetic field to influence the electrons on the surface. Like charges repel, so the field of influence of the cloud's electromagnetic field would drive away the better portion of the negative particles on the ground. That assumed, with mostly positive particles left on the surface, and mostly negative particles in the cloud, opposite charges attract. Just like a magnet would start to pick up iron shavings from a distance, either the cloud starts to draw up positive particles from the ground (perhaps oxygen ions who lost their electrons to the ground?), or the ground starts to yank at the electrons in the cloud. Either or both ways at once, you have these Streamers of charge reaching for each other. If they get close enough to initiate a discharge, bam- lightning (rather, flash, then bam... thunder results from superheated/rapidly expanding gas due to lightning).

Therefore, I'd imagine the bulk of the buildup (creating the path to the ground) happens without notice, until they're close enough to strike an arc. Once an arc is in place, I'd guess the charges will want to flow whichever way is necessary to balance out the charges, until there's no longer enough of a differential for the magnetic fields to support the streamer. In any case, I don't think the particles would have to run the entire length of the arc (potentially a mile, but most likely less), but only bump into the next atom in the streamer, which in turn bumps the next atom, and so forth, until enough charge is transfered where it has to go. Think domino effect. So in essence, the water/hose analogy still holds true, but instead of a solid wire as a hose, the streamers/charged particles pulled by electromagnetic fields serve as the medium...

IANAM, (I am not a meteorologist), but from reading the information in the posted links, that's the most logical explanation I can infer.

Re: A question about current....
 

Guys,
What you need to remember is that air acts like an insulator. But like any insulator, (glass, or PVC around a wire) high voltage can break the material down and turn it into an conductor. Air also contains a lot of conductive particles and in a storm the air that is in motion picks up static just like the cloud. In the case of lightning, the high voltage potential between cloud and ground builds up to a point where it is capable of jumping from conductive particle to conductive particle at which point the flash begins and the air molecules are ionized allowing current to flow in the material. At some point, enough current has passed through the conductive path that the potential difference between cloud and earth can no longer cause the jump between gap(s) in conductive particles in the air, and the current flow stops. During that brief time that current flows, there are in the order of several million amps flowing.
As with any electric circuit where current flows, a magnetic field is created around the flow path. It is this induced magnetic field which couples into nearby electronics and disrupts them. Also remember that ground is not a perfect conductor. As the lightning bolt enters the earth, current flow through the ground resistance causes a potential difference on the surface. Using Ohm's law, what voltage will be generated through a one ohm resistor when one million amps flows through it? That is why many people are hurt in near strikes that are never hit by a direct strike. They are bridging enough ground resistance to have several hundred to several thousand volts present across their feet. Any conductive devices in the general vicinity of a lightning strike will be subject to this potential and that is another reason why electronics and power is disrupted during storms.
The same voltage potenetial difference on the ground injures people near a high voltage wire that has fallen. If electric power lines have dropped in a storm or other damage, stay away! If you are close, keep your feet close together and shuffle away.
Lastly, there is only one thing you can predict about lightning and that is it is unpredictable. I can tell you many stories of ham radio operators and broadcasters who have gone to great lengths to lightning protect their shacks only to have a storm come along and burn it to the ground. I have a picture of what was left or a bird that was sitting on a wire that was struck I will have to find. The potential difference between it's feet caused it to vaporize.

Re: A question about current....
 

electricity will arc when you have 1000 volts within one centimeter

that means to strike an arc 1 meter long you need 100 thousand volts

and to strike an arc 1 kilometer long you would need 100 million volts

lightning often travels 3 or 4 miles, so you are looking at several billion volts - and the measurements they have taken during thunderstorms dont support that

I dont think rain helps the equation very much, because pure water is not conductive - as I mentioned before I think something else is going on - once you have charges up in the millions of volts I think some secondary effect is happening, something we cant measure or observe in a laboratory, and the 1000V per centimeter rule gives way to something else.

We had a big T-storm here last night. I never get tired of watching lighting - sure beats any pyrotechnic display that we are capible of.

Re: A question about current....
 

Ken et al,
One reference on the Intenet lists the conduction of air in the 20kv/cm range but as you suspect there are other factors involved. The polution (particles) present in the air and the ionized gas molecules that make up the atmosphere all cause that value to change to much lower values. As the static builds up (a potenetial difference between cloud and ground or cloud to cloud) energy is imparted to the air molecules causing electrons to begin to move between atoms. This "ionization" imparts conductive paths from molecule to molecule. Remember that a good conductor is one in which the applied voltage is capable of adding energy to an electron to cause it to move to another atom or raise it to a higher energy level within the atom. So the static discharge is not one that jumps several miles but millions that jump micro inches until a conductive path is formed, creating a pipe of ionized (conductive) gas which is sufficient to cause current to flow. A high voltage source causes great deals of ionized gas to be formed near the source. High voltage power lines produce this ionizing voltage sufficient on very high voltage power lines of jumping fourteen feet. Remember when looking at the insulators on power lines that the total path length is the surface path around each section of the insulator not the tip to tip length. Power lines do produce some current flow between the wires and ground and anyone standing underneath can hear the discharge as a constant crackle which becomes worse on humid or rainy days.

Re: A question about current....
 

That stepped ionization is what i gathered from the page referenced in one of my earlier posts. You had the idea correct, but the page claims that the steps happen in 50 ft sections. This is reasonable when lightning strikes are in the million volt range so 50*2.54*12*1000 = 1.524 million. At the Boston Museum of Science (or whatever its called), they have a show about electricity (the one with the enormous Van der Graaf generator that makes 12 foot arcs.) and they mention that lightning usually requires 1 million volts per mile. The longest recorded bolt was something like 6 or 7 miles.

Re: A question about current....
 

Hwello,

Electrical arcs and lightning bolts across substances are governed by Paschen's law. Each scenario has a breakdown voltage, above which electricity can flow through a material. This voltage is related to the distance across the gap, gas density, and other properties that I have forgotten. Once this breakdown voltage has been crossed, the gap allows electricity to flow. This is the basis for Jacob's ladder demonstrations. Take a look here for more information:

http://home.earthlink.net/~jimlux/hv/paschen.htm

Also, electrons do not actually move to carry charge. The current moves at a speed called propagation speed, which is dependant upon the medium in which the current is flowing. Electrons move at a speed called drift velocity, which is equal to the magnitude of the current divided by the number of charge carriers times the magnitude of the charge on each charge carrier times the area of the conductor.

v = I / nqA,
where v = velocity of electron, I = current, n = # of charge carriers, q = magnitude of charge on carriers, and A = conductor area.

I have always found a water system analogy very useful for explaning basic electrical concepts to people. Think of water flowing in a pipe. The pressure on the water is equivalent to the voltage and the amount of water crossing a point at a given time is analogous to the current. This model is not wholly accurate, but it makes sense to people who used to ask "How many volts is an amp?"

Re: A question about current....
 

This why lightning tends to be one of those phenomena that is actually quite difficult to explain in entirety. It is an extremely complex dynamical system which is why no two lightning bolts ever look the same. Take a year of upper level E&M and then also a year of upper level Statistical Mechanics and you might be on the right track for describing how a one inch spark propagates. You cannot take a general solution of dielectric breakdown as 20kv/cm (for dry air) to be a solution for 6 miles of air that has varying pressure gradients, temperature gradients, varying air currents, changes in air composition and varying humidity, etc.... Thanks Al, for setting us straight on this.

The electric field from the power line to the ground is quite substantial and now easily measurable with digital multimeters. For those of you with power lines nearby; obtain or borrow a digital multimeter and set it to measure voltage, place one lead in (near) the ground and simply raise the other lead over your head (6-8 feet off the ground) - REMEMBER NEVER TO GET TOO CLOSE TO OR PLAY NEAR POWER LINES - YOU SHOULD ALWAYS BE AT LEAST 25 FEET AWAY (OR MORE, IF ITS HUMID OR RAINY)!! So with that disclaimer, your voltage lead should still be at least 25 - 30 feet below the power line! Hmm, what's the voltage? Lightning strikes are common when the voltage near the ground 18 KV/m or higher. Hopefully you never measure this - If you do you are in danger and I'd put down the voltmeter probes and then bring your feet close together, curl into a ball, and stay away from trees or tall objects. Lightning kills the most people each year, more than all other natural weather phenomena put together (hurricanes, tornados, floods, etc..)!

Oh, yeah....

The terms Positive and Negative originated before the electron was discovered. Positive and Negative were used to simply describe excess and deficiency of a mysterious substance known as charge. + was more than normal and - was less than normal. I like to think of it in terms of pressure differences High and Low. Thus "current" was used to describe the flow of "charge" from high "pressure" (+) to low "pressure" (-). But then the electron was discovered and everything had to fit within the well known model. (Gee, thanks Franklin!) This has caused an enormous amount of confusion especially when today we realize that what actually travels in a current carrying wire is electrons which DRIFT from (-) to (+). They drift on average of 0.1 mm/s at 1 Amp through 18 AWG solid copper wire. However in a plasma (ionized air) the mobile charge carrier can be electrons AND positively charged ions. Now which way does it go?? (Take an upper level plasma physics class!!) Just remember that the term "current" was defined by someone who didn't know the whole story and now we're stuck with it. Use the term "electron flow" or "electron current" if you feel a need to describe the direction electrons are going!

Just another thought ... What would happen if you held a 4 foot flourescent tube underneath a power line??? (make sure it is a dark, moonless night, and please don't drop / break the flourescent tube - mercury vapor is a known carcinogen) REMEMBER MY SAFETY DISCLAIMER STATED ABOVE!!!

That's enough of me rambling on and on and on....

Re: A question about current....
 

Not quite. The electrons move, but not the ions, so therefore the charge of the ions are meaningless. Electricity always moves from neg to pos.

Sparks

Re: A question about current....
 

Yes, in wires, electrons are the mobile charge carriers. But I must warn you to stay away from using ambigous terminology like "electricity.. moving...". It just perpetuates common misconceptions about electricity.

In a wire, current, by definition, is from pos. to neg. Whereas electron flow is from neg to pos. Understanding this subtle difference is vital when you try and use applications where bias is important or say when your doing circuit analysis with Kirchoff's Laws. Remember, current was defined before they knew the electron existed and now we're stuck with it.