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Author Topic: Is electricity really attracted to the earth?  (Read 4961 times)

Offline Voxx

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Is electricity really attracted to the earth?
« on: 21/06/2012 18:09:50 »
I believe hearing people say that lightning has bypassed perfectly good conductive wiring paths to leap through air. (very resistant) So my question is, do we positively  know what attracts electricity?  I've heard a lot of controversy on the grounding rule.

*Edit:  Alright, on another post of mine JP said, "Electrons repel each other."  Is this what causes the interaction with the weak gravitational pull of the earth, interacting with the electronic interaction of repelling each other?
« Last Edit: 22/06/2012 14:52:30 by Voxx »


 

Offline JP

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Re: Is electricity really attracted to the earth?
« Reply #1 on: 22/06/2012 21:15:22 »
The very rough explanation is that electrons repel each other, so they want to spread out as much as possible.  But it's easier for them to move through some materials than other.  It's very hard for them to jump through the air, so they tend to move along wires.  If you get enough of them in one place, the repelling force is so strong that they can jump through the air (lightning, for example). 

They tend to jump towards the earth because the earth is so big that they can spread out very easily.  Even if you discharge the world's biggest battery into the ground, it won't have much of an effect on the earth.

This is a very hand-waving explanation of it, but we do know with extreme precision how electrons will move in their attempt to "spread out," which is why we can make such precise electronic devices.
 

Offline Phractality

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Re: Is electricity really attracted to the earth?
« Reply #2 on: 23/06/2012 00:50:48 »
Lightning can choose a straight path thru air instead of a sharp angle thru copper wire. The reason is that the current changes very rapidly. Inductive impedence times rate of change of current is voltage, and the lightning current can go from zero to several thousand amps in a microsecond. Even a sharp bend in the wire has as much impedence to radio frequency as a big coil of wire at 60 Hz. This makes shortcuts look very attractive to lightning.
When grounding a lightning rod, avoid sharp bends. When grounding a 60 Hz curcuit below the lightning rod, you might add a couple of loops between the circuit ground and the earth. That way, the lightning will prefer to bypass the circuits. However, a straight path to ground may be needed when grounding a radio-frequency circuit.
 

Offline Voxx

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Re: Is electricity really attracted to the earth?
« Reply #3 on: 23/06/2012 01:52:39 »
Very interesting explanations JP and Phractality.  Thank you for your input.
 

Offline evan_au

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Re: Is electricity really attracted to the earth?
« Reply #4 on: 24/06/2012 11:45:29 »
For frequencies that are common in lightning current surges (up to a few MHz), the characteristics are quite well understood. High electric currents in this frequency range can easily burn out electronics (and people).

The main factors are:
  • Electrical resistance: Copper has lower resistance than dry wood, so far more current will flow through the copper than through the wood.
  • Physical distance: Resistance is higher for a longer path, so lightning will tend to take the shortest path (all other things being equal)
  • Inductance: A coil in a copper conductor will make it harder for the high frequencies to flow through it
  • Capacitance: Two conductors in close proximity can carry small amounts of current without a spark between them
  • Skin Effect: As a result of its magnetic field, most of the current will flow through the outside of the conductor. The depth of the skin depends on the material that the wire is made of.
  • Wire Thickness: Due to the skin effect, it is better to make lightning conductors as a flat braid of thin conductors, rather than a single round conductor with the same amount of metal.
  • Wire Shape: Lightning rods have a sharp point facing the air, as this promotes the very high electric field gradients that create a corona discharge and attract lightning. (Or the converse: high voltage electric power lines have rounded fittings, to discourage corona discharge)
  • Resonant effects: Wires of certain lengths & inductance/capacitance will respond more strongly to impulses at certain frequencies. This is called resonance.

For behaviors in the order of nanoseconds, such as in a lightning leader, electrical breakdown in insulators (including air) or electromagnetic pulse (EMP) from nuclear explosions, the effects are less well understood, and harder to control. Moderately strong electric fields in this frequency range can easily burn out electronics, but don't tend to affect people.
 

Offline evan_au

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Re: Is electricity really attracted to the earth?
« Reply #5 on: 24/06/2012 11:53:07 »
Sorry - I left out the most obvious factor: What is the voltage between the different points? Lightning will jump between any two points where the voltage exceeds about 1 Million volts per meter.

Storms create voltage differences between clouds and the ground, and also between different parts of the cloud.
I have seen estimates that 90% of lightning strikes are between clouds, so I guess you would have to say that the answer is "NO: Lightning is not necessarily attracted to the earth".
 

Offline Voxx

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Re: Is electricity really attracted to the earth?
« Reply #6 on: 24/06/2012 16:34:18 »
Interesting, so if lightning is attracted towards 1million or so volts in between ground and sky.  Does that mean that lightning is in fact repelling each other, but that can also go with attraction and counter attraction.  When there is a strong enough impulsion near the ground the lightning is attracted not to the ground, but by the energies that are transpiring within the ground or maybe the tectonic plates?
 

Offline Phractality

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Re: Is electricity really attracted to the earth?
« Reply #7 on: 24/06/2012 18:03:20 »
The rule of 1 megavolt/meter applies to the point of greatest electrical gradient. As soon as the air starts to ionize at one point, the ionized air becomes a conductor, and there is little voltage drop in it; so the electrical gradient then increases above and below that point, and the rest of the path ionizes in a matter of nanoseconds, close to the speed of light (.3 m/ns).
When a conducting rod nears a conducting plane with a potential between them, the ionizing potential is less for a sharper rod. (I can't remember if it is inversely proportional to the radius or the square of the radius.) You can demonstrate this with clothes removed from the drier. Bringing your fist toward the clothes will release the charge in a few sparks. Your finger tip will do it with numerous smaller sparks. A needle point will do it with a steady glow.
In the USA, most lightning rods have needle-sharp points. In Europe, the put metal balls on top. European experts say the ball discharges a larger volume of air by allowing the voltage gradient to grow higher before arcing occurs. In both cases, the discharged air spreads downwind.
About 20 years ago, I installed an electronic truck-axle scale at a lightning-prone border crossing between Arizona and Sonora. I advised the Mexicans that the flag pole next to the scale should have a sharp point on top, instead of a round ball (about 10 cm in diameter), and it should be well grounded. They ignored my advice. A month later, they had a lightning strike, and I replaced most of the electronics. (Good thing I'm not an arachnophobe. For two hours, I rubbed elbows with more black widows in that 10-foot square pit than all the other spiders I've seen in my life. Every 10 centimeters, a big black widow. I moved cautiously, and didn't get bitten once.)
 

Offline Phractality

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Re: Is electricity really attracted to the earth?
« Reply #8 on: 24/06/2012 18:13:14 »
Interesting, so if lightning is attracted towards 1million or so volts in between ground and sky.  Does that mean that lightning is in fact repelling each other, but that can also go with attraction and counter attraction.  When there is a strong enough impulsion near the ground the lightning is attracted not to the ground, but by the energies that are transpiring within the ground or maybe the tectonic plates?
Water and ice in a storm cloud carry electric charges up and down gradually. This results in a large potential differences between parts of the cloud and the ground. Since opposite charges attract, lightning puts them back together suddenly.
It's explained in more detail here.
 

Offline yor_on

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Re: Is electricity really attracted to the earth?
« Reply #9 on: 05/07/2012 16:52:54 »
Interesting reading.
 

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Re: Is electricity really attracted to the earth?
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