Re: Useless questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

In sparks (particularly white ones), it's black body radiation.  The actual
spark isn't very big (less than 1 mm in diameter, probably), so the
radiated power isn't all that high.  And, the temperature is more like
5000-7000K.  

A very different matter than 100,000K (since radiation goes as fourth power
of temperature)  1E5 degrees vs 5E3 degrees is a change of 160,000 in
radiation flux.

If it wasn't heating, then there wouldn't be enough ions to conduct the
current.

I seem to recall a number like 166 watts radiate from 1 square meter at
300K, or something like that.  Scaling for temperature, to 6000K ( a factor
of 20), that would be about 26 Megawatts/square meter.  Now, figure a spark
a meter long and 1 mm across (i.e. 0.001 square meter in area), so the
spark is going to radiate, give or take, about 26 kW, during the time that
it is hot.  It's actually hot only for a few microseconds during each
bang... let's assume 2 periods of a 100kHz coil (spark loaded Q around
2-3), or 20 microseconds, and a rep rate of 120 Hz (8.3 mSec).  So, the
duty factor is .0024.  Multiplying that by the radiated power of 26 kW, you
get an average radiated power of about 63 watts.

This is in the general magnitude area of what you'd expect.  After all,
you're putting in a few hundred watts to the TC, and most of that power
gets dissipated as heat in the sparks.  The air gets heated by the current
flow, and cools by a combination of radiation and conduction (or more
likely, diffusion, given the time scales).


For what it's worth, these sorts of energy balance calculations are what a
lot of the spark development theory is based on. Then, folks have made
measurements of the temperatures in the sparks using other techniques (i.e.
spectroscopic) to experimentally confirm it.  Similar techniques have been
used to measure the temperatures in lightning strokes (which run a bit
hotter.. 11000K, due to the high currents).

Tesla list wrote:
> 
> Original poster: "Alexander Rice by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <ajjrice-at-yahoo-dot-com>
> 
> Jim,
>         Are you sure the colour of ths streamers is not more to do with the
> nitrogen in the air being excited into photoemission rather than because
> thy are at several
> tens of thousands of kelvin - a body this hot would be radiating a
> stupendous amount of heat and surely you would feel it, i seem to remember
> once calculating in our
> phsyics class that a kilo of iron heated to 100,000k would produce a
> similar radiation intensity to the sun 70miles away!
> 
> Alex
>