Re: [Phys-l] Sparks

[Bernard Cleyet <bernardcleyet@redshift.com>]

Adding:
The reason for the spring loading in conventional switches Is to make  
the connection quickly.  I suggest dismantling one to observe the  
engineering, i.e. how the "springing" is designed to both break and  
make the connection.  In all the switches (light) I've dismantled, it  
is a knife switch wherein the handle or toggle is a lever driving the  
spring.  The only high current and PD switch I've seen "up closely" *  
was a large conventional knife in a tank of transformer oil to  
suppress the arc on break.  A list member has posted the very long  
arc resulting from opening a transmission line switch.  Some Tesla  
spark units employ rotating electrodes to extinguish the arc.

If one assumes linearity*  an electrostatically initiated discharge  
at 12 V. requires a separation of approx. four micrometer.  (air  
break down ~ 3 E+6 V/m)

* W/ a telescope check out the distribution line switches on the  
utility poles.
**Not a good one, I think, as very short points become more important  
when the electrodes are in very close proximity.  One may contest  
this by using JD's Laplacian spread sheet.

http://www.av8n.com/physics/laplace.htm


Finally, I tried initiating an arc using both steel (opened large  
paper clip) and Cu wires, and a 'mobile  emergency power pack.  I  
failed, which may explain the ~ minimum arc welding PD of ~ 24 V.    
The Cu did weld (poorly) to the clamp and the steel fused into a  
large drop at the contact end of the wire.


bc wonders if it was a matter of insufficient current instead of PD.




On 2008, May 05, , at 22:08, 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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