# Re: Light emission

```Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 05:57 AM 5/11/2006, you wrote:
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```Original poster: "Langer Giv'r" <transworldsnowboarding19@xxxxxxxxxxx>

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Hello, does an electrical arc give off light not only because of photon emission, but also due to blackbody radiation?
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Yes.. In fact, I would guess that the majority of the energy dissipated in an arc goes to heating up the gas in the gap, which then radiates. A typical spark channel temperature would be about 7000K.
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Some heat lost through conduction to surrounding air, some lost through radiation.
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If we want to estimate the latter, we could use a simple Stefan-Boltzmann calculation.
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The constant is 5.67E-8 W/(m^2 K^4), so we can calculate approximate values:

Lets assume the arc is a cylinder 1 cm long, and 1mm in diameter and at 7000K.

W = 5.67E-8 * Area * (Thot^4-Tcold^4)
Area = 0.01 * 0.00314 = 0.000314 m^2 = 3.14E-4 m^2
Assume Tcold = 0 (to make numbers easy)
Thot^4 = 7E3^4 = approx 2500E12

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So we have 5.67E-8 * 3.14E-4 * .25E16 = approx 4.5E4 Watts = 45kW (per cm of length)
```(5.67 * 3.14 * .25 is about 6*3/4 = 4.5)

Check for resonableness..
20 kV peak voltage on primary, with gap of, say, 1 cm
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Fires 120 times/second (i.e. every 8333 microseconds), and pulse lasts 30 microseconds, so duty factor is about 30/8333 or, about 1/280. 45000/280 = 450/3 = 150W.. which seems reasonable, in a NST powered coil running at 450W.
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usual TC lore has it that roughly half the power dissipated in the spark gap, and this is consistent.
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```Daniel

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