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LAN_403

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Sun Aug 27 2006, 04:12PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

With your 2 caps back to back, lets say you ground the wire in the middle. Now the middle plates are at the same potential. Put a voltmeter on the outer plates, one will be at + voltage relative to ground, one will be at - voltage to ground. The charge didn't move at all.

AndrewM

Sun Aug 27 2006, 05:32PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Charge HAD to move. One inner plate has an excess of charge, the other has a deficit. If they are to reach the same potential when connected, charge HAS to move from the negative plate to the positive one.

...

Sun Aug 27 2006, 06:31PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You have to remember that charge is relative... The electrons didn't move, just your reference point did. The caps don't know if they are sitting at ground or floating at a gazilion volts, they only know the voltage across themselves. The caps both had the same charge across the plates, but one of the caps is floating at a diferent potential.

Perhaps this very crappy m$ paint drawing can clarify; on the left are 2 charged caps sitting there. On the right they were connected in series, and a ground added to give a point of reference. The left side of the mess has a negative potential to ground (or the other cap) because the side connected to ground is at a higher potential; so this side must be lower than ground. The opposite is true for the other cap, the grounded side is at a lower potential so the floating side must be at a higher potential than ground. If you were to connect the other 2 sides of caps (the - and + terminals on the right) then charge would flow from the + charged plate on the right to the - plate on the left and from the + charged plate in the middle to the - plate in the middle --and everything would be equal.

That is the best I can do

AndrewM

Sun Aug 27 2006, 07:13PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

I think your pictures just made things worse. Now we have a plate maintaining a net positive charge while connected to another plate AND ground.......... oops.

I realize that the potential issue is relative, which is why I'm avoiding that concept all together.

Consider that you have two caps, with a quantity of electrons on each plate as follows:

Cap1: 3, 2
Cap2: 1, 0

That is, both caps have a charge difference (potential) of 1. Now when you connect the 'negative' plate of one to the positive plate of the other, the charges redistribute causing current. The only case there this would not happen is if the total charge on the connect plates was exactly the same, a statistically impossible case. the only thing that the 'relative' nature of potential ensures is that its difficult to say which direction the current flows... but there MUST be current.

Dr. Drone

Sun Aug 27 2006, 09:17PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Steve Conner

Sun Aug 27 2006, 09:31PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

OMG, OMG, someone put the show online so we can see it!

EVR: The arc between the helicopter and the line most certainly is due to the line voltage and the helicopter's capacitance. Think of the chopper as a humongous Tesla coil topload. I'm guessing this because

a) there's a helipad near my house and I don't see hot white arcs jumping to the ground any time a chopper comes in to land.

b) I once saw a video of some guy testing one of those Faraday suits at the Leatherhead 1MV AC test facility. They took him up to the output terminal of the transformer on a plastic platform hoisted by a crane with a nylon rope. He held his hand out and when it got within about 18" of the terminal, a loud cracking arc started to jump.

Nik

Sun Aug 27 2006, 10:18PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I saw the helicopter tv show as well. At one point they leave the workers ON the lines and the helicopter goes some where else while the man/men on the lines do their work.

Simon

Mon Aug 28 2006, 05:49AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

It looks to me like this guy is safe from the obvious "danger" of touching the HV but still extremely stupid considering all the other dangers.

Nik

Mon Aug 28 2006, 06:28AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

Now had the power been on at the full 25kv would the gradient from his hands to his feet have prevented him from letting go or breatheing? Say the wire is 15 feet up thats still ~1.6kv per foot (between the wire and ground). That would probably not have been the best feeling in the world.

*edit* asuming its an AC line

Steve Conner

Mon Aug 28 2006, 12:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

He's a conducting object, so he would modify the E-field around himself and wouldn't see 1.6kV per foot across his body.

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