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Re: [Phys-l] Sparks



The same explanation works for why you don't feel a "shock" from static electricity if you hold a metal object in your hand and touch something that is grounded. The "shock" is actually local heating of your skin caused by a relatively high current density at the point where you skin is close to the grounded object. If you hold a metal object, like your car key, creates a large area of contact and hence a low current density, and no sensation of shock.

Arc initiation is essentially as John described. Sherwood and Chabey have a nice piece on what happens in the gas phase between the two charged electrodes before they make contact. How a plasma develops.

It turns out the field is quit high for any voltage because most metals that are not carefully prepared has fine fibers of metal that rise out of the surface due to the field as the electrode get close. The field around the tip of these fibers is very high since it goes as 1/r and r is on the order of nanometers.

Perhaps that is more detail than you want.

cheers,

joe

Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On May 6, 2008, at 1:08 AM, John Denker wrote:

On 05/05/2008 09:09 PM, Ken Fox wrote:

I want to say that the conditions for the sparks happen just before contact
and again just after when the field is strong enough even with 12 V
potential.

I don't want to quibble about the distinction between "just before contact"
and "right at the first contact". The distinction is unimportant, and is
probably only a matter of definition. If the wires are close enough to
conduct electricity, it is either contact or "virtual" contact.

I would call attention to /current density/ not just voltage or E- field.
As contact is being made, there will be some very small region that makes
contact (or virtual contact) before any other region does. The current
through this small region will create a huge current density. This
vaporizes the metal, which goes flying in all directions.

This fits the facts, because what's special about jumper cables is not
the voltage (which is modest) but the huge available current i.e. low
source impedance.

To say it the other way around, if you want to make a high-current connection
without catastrophic local heating, you need to make a large-area connection.
Making a large-area connection quickly is not easy.

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