Birds on Electrical Wire

[casdave]

All bodies have a capacity to store electrical energy, and this will depend upon the physical make up and the mass of the body.

The actual charge that moves from one point to another is dependant upon the differance in potential between them, so that if two points are at or very near to the same potential, little or no current flows.

One does have to distinguish between high potentials and high current, you can have huge voltages, but very little actual charge stored, so a body might have perhaps 500KV, but only a tiny capacity for storage, which means it takes a minute amount of current to flow before the potential differance between two bodies has equalised and current therefore no longer flows.

In addition, the rate at which the current flows is also time dependant, energy transfer is not instantaneous.

This rate of flow depends upon the impedance betweent the two points, the greater is that impedance, then the longer it takes for the charge to flow from one point to another, and hence the smaller the rate of current flow.

The material that makes up animals is not a very good medium for storge of electrical energy, this means that if there is not a continuous circuit not much energy can be supplied to or taken from an animal. Add the natural impedance which would tend to limit the rate of flow of that very small electrical charge, and what you tend to get is no damage, perhaps you might get a tingle.

Extreme circumstances can change this, unlikely circuit paths may be formed when dealing with extreme voltages, a helicopter may distribute some of its excess chagre to atmosphere simply from the friction of its rotor blades with air, but even in such a case, the current flow is small.

When you get very high voltages and very high currents, things are very differant, and fatalities and destruction are likley, such as in natural lightening.

[Q.E.D]

@casdave

All bodies have a capacity to store electrical energy, and this will depend upon the physical make up and the mass of the body.

Mass is irrelevant, since capacitance is a strictly surface phenomenon.

[David_Simmons]

@Jinx

I was on a commuter train when the engine broke down. Another train pulled along side on a parallel track no more than 1.5 ft apart (2-ft, max, I WAG). The doors were purposely not aligned so no one would attempt to stretch across directly from one train to the other. They cautioned us NOT to touch both trains at the same time. (I understand, though, the pantograph(?) of one train was lowered as a precaution.)

Anyway, as it was told to me, the trains are not grounded AND (although hard for me to accept) the metal outside of the train IS at the same potential as the overhead lines (give or take). :eek: If this is true, … - Jinx

As has been pointed out it most definitely is not true that the train car exterior is at the same potential as the overhead lines. Grabbing the handrail to get on or off would be pretty hazardous.

I suspect that the warning was purely precautionary since they weren’t yet sure of what the problem was. If, for example, something had interrupted the ground connection (highly unlikely but possible) which would cause the locomotive to stop then that train exterior could just possibly be at some hazardous voltage.

[Cardinal]

Did anyone see the bear that did manage to complete the circuit? There were pictures on the web. Quite a fireball. It somehow survived the shock and the fall.

[Crafter_Man]

@David Simmons

The spark between probe and electric line didn’t occur until about 2’.

Things that move around build up charges so the helicopter is at a greatly different potential than any part of the transmission line. The difference in potential forces charges to flow from the line to the helicoper, or vice versa, until the potential equalizes.

When you walk around you accumulate an electric charge and when you point your finger at your dog’s nose a spark jumps as the charges flow to equalize your two different potentials.

So are you saying the spark would still occur even if there was no voltage on the power line?

[Cardinal]

@Crafter_Man

So are you saying the spark would still occur even if there was no voltage on the power line?

If the potential (voltage) is different for any reason, then when the resistance of the airspace between them gets small enough, there will be some sort of current. We don’t usually notice these happening because we don’t play with high enough voltage.

Lightning happens when the voltage gets so high that IIRC, there is a smaller flow that ionizes the path from the cloud to the ground, and then the real bolt comes whomping through. All of this happens extremely fast, of course.

Summary: If the copter has built up a charge from running the rotor through all that dry air, there could be a spark to any kind of conductor that had a different potential, especially if it were grounded. The electric pole wire wouldn’t be grounded, but it’s pretty darn big, and able to absorb or give off a good number of electrons

[David_Simmons]

@Crafter_Man

So are you saying the spark would still occur even if there was no voltage on the power line?

Pretty much everything has an electrical potential of one magnitude or another. What counts is the potential difference as Cardinal pointed out. After all the dog’s nose isn’t powered by AC from anywhere and a spark still jumps.

We once were conducting some tests that involved pulling a large weather balloon around at the end of a wire rope attached to a vehicle. The rope was about 500’ long. The first day when we stopped one of the guys jumped out and was holding onto the vehicle as he touched the ground. Fortunately he was in good physical condition because he got quite a shock. Measurements showed that we were building up a potential of over 30,000 volts. So from then on we carried a “anchor” which was a metal weight attached to a wire rope that we threw out onto the ground to discharge the vehicle before anyone got out.

I’ve a much better answer to the Jinx question about the need for a “complete circuit” for any current to flow than the one I gave before.

If you have two objects at different potentials and they are brought close enough together, without touching, so that the voltage gradient in the air between them exceeds the breakdown voltage, then the air is ionized and becomes a conducting path between two different potentials. And, Voila!, a “complete circuit.”

[Q.E.D]

@David Simmons

If you have two objects at different potentials and they are brought close enough together, without touching, so that the voltage gradient in the air between them exceeds the breakdown voltage, then the air is ionized and becomes a conducting path between two different potentials. And, Voila!, a “complete circuit.”

And as you bring them closer together, the capacitance between them increases, and therefore, so must the voltage (what with the pesky Law of Conservation of Energy and all), since the total amount of charge doesn’t change.

[Desmostylus]

Wow. This thing has gotten pretty far off track.

I’ll try and post to this again later, but in case I don’t get time:

casdave, Crafter_Man, David Simmons - you all seem to be thinking in terms of conduction currents and static charges, which is inappropriate given the nature of the problem. It involves an a.c. powerline and high d.c. resistance to ground, so you have to be primarily concerned about the displacement currents.