Re: [Phys-l] Sparks

[John Denker <jsd@av8n.com>]

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.