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Re: [Phys-l] high voltage arcing



A question -- are the helicopter blades conductors? How about the induced emf from the blade's motion through the earth's magnetic field?

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From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker [jsd@av8n.com]
Sent: Tuesday, July 06, 2010 5:54 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] high voltage arcing

http://www.youtube.com/watch?v=9tzga6qAaBA

Interesting video.

On 07/06/2010 02:57 PM, Stefan Jeglinski asked:

1) If suit is faraday cage, how come it can be open at the face?
Thinking in terms of something like an infinite conducting plane with
small hole (see Jackson) - electric field lines will "intrude" but
maybe the potential and the size of the opening are safe here?

It's not just a Faraday cage.

There's also a lot of voltage dividering and current
dividering going on.

2) Can't tell if sitting platform is insulated from helicopter.

It's not.

Audio
implies helicopter is being brought to same potential but somewhat
unclear (I would think this more or less required to prevent any risk
of arc (wire to helicopter), even if possibility is remote.

The arcs you see are, for all practical purposes, arcs to the
helicopter.

3) Once charged, clamps appear to be used to maintain potential while
moving over to the line, but of interest is when copter pulls away.
He unclamps while using probe to presumably maintain potential, but
as separation occurs, significant arcing again occurs. Why exactly
would this be? Presumably the copter and line and suit are in
equilibrium at this moment just before separation. Perhaps the copter
cannot not fly off and maintain a massive charge on it (if it somehow
did, think of the landing). But why not? Why would it all arc off
right at that time and not just eventually "drain" off into the air
due to it's high electric field?

There is no such thing as "once charged".
It's an AC system. Let's do the numbers:


e0 8.85E-12 F/m

r 1.5 m radius of helicopter

c 1.67E-10 F self-capacitance of helicopter
167 pF

f 6.00E+01 Hz

z 1.59E+07 ohm effective impedance

v 5.00E+05 V

i 3.14E-02 A

iv 1.57E+04 VA




Summary:

high E-field causes initial arcing to bring to equilibrium
disconnect while at equilibrium causes equivalent arcing

No. The arcs would continue forever.

The self-capacitance of the helicopter is constantly being
charged one way and then the other.

This current *is* going to flow whenever the helicopter is
in the vicinity. The purpose of the probes and clamps is
so that when the current flows, it doesn't flow through
the person.

=============

Interesting questions.
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