Re: [TCML] Spark dynamics on Jacobs Ladder

[Bert Hickman <bert.hickman@xxxxxxxxxx>]

Hi Matt,

Possibly, except that Peter is correct - the EM force on the electrodes 
is actually repulsive and not attractive since the current is flowing in 
opposite directions. The magnitude of the repulsion force scales with 
the square of the current and is inversely proportional to the wire 
separation:
http://theory.uwinnipeg.ca/physics/mag/node10.html

BTW, for a graphic example of what these forces (at tens - hundreds of 
kA) can do to substation bus bars and standoff insulators during 
accidental short circuits, see:

http://www.youtube.com/watch?v=2j8D_N1v0tU

It would be interesting to compare the magnitude of the magnetic versus 
electrostatic forces for a typical 15/30 NST to see if they are of the 
same magnitude... I'll look into this a bit more.

Bert

Mddeming@xxxxxxx wrote:
>  
> Hi Gary,
>  
> A simple explanation, understandable by elementary school  kids, is this:  A 
> wire carrying a current has an EM field  around it. The two wires carrying 
> currents in opposite directions will thus  attract each other. When the spark 
> first forms at the bottom, the two electrodes  form effectively very short wires. 
> As the spark rises, their lengths becomes  effectively longer and the 
> attraction stronger. The maximum attraction  occurs when the spark is at the top. 
> When the spark breaks at this point,  the restorative force of the spring tension 
> of the electrodes pulls them apart  again and they try to oscillate at their 
> natural frequency. If you could get the  voltage, current, spark travel time, 
> and distance just right, you would see  significant resonant gain in the 
> movement (The pushing-a-swing analogy). The  college-level explanation is here:  
> <http://en.wikipedia.org/wiki/Lorentz_force>.
>  
> Hope this helps,
>  
> Matt D.
>  
>  
> In a message dated 1/18/08 11:06:32 A.M. Eastern Standard Time,  
> Gary.Lau@xxxxxx writes:
>
> I hope  this isn't viewed as too off-topic - I'll argue that the same physics 
> apply to  TC sparks ;-)
>
> I was giving a demonstration of various HV toys to a 4th  grade class 
> yesterday.  Among the devices was a Jacobs ladder, powered by  a 15/30 NST.  The two 
> 1/8" x 3 ft steel electrodes appeared to have been  excited into a mechanical 
> oscillation, bouncing towards and away from each  other, at very roughly ~ 
> 1Hz.  One of the students asked my why they were  moving, and I had to admit that 
> I didn't know the source of the force that was  moving them.
>
> The period of the oscillation was much faster than the arc  travel time up 
> the electrodes.  It's clear that the period was that of  the free-standing rods, 
> and that the exciting force between them varies as a  function of their 
> separation, but I don't see the source of the attraction or  repulsion between 
> them.  Any theories?
>
> Thanks,
> Gary Lau
> MA,  USA
>
>
>  
>
>
>
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