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Author Topic: Can a spark travel across a vacuum?  (Read 9439 times)

paul.fr

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Can a spark travel across a vacuum?
« on: 31/07/2007 16:53:11 »
Can a spark travel across a vacuum? What happens to the air between two charged objects to allow a   spark to jump between them?


 

Offline chris

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Can a spark travel across a vacuum?
« Reply #1 on: 31/07/2007 17:38:20 »
This is a very interesting question. A "spark" is the consequence of electricity flowing through a medium, like air, and exciting atoms in that medium so that they emit visible light. You can pass electricity through a vacuum, as a stream of charged particles, but there won't be a spark. Older style televisions (like mine) with a "cathode ray tube" (CRT) do precisely this. A beam of electrons is boiled off from a coil and then accelerated through a vacuum at high speeds towards the screen. When they hit the screen they excite a "phosphor", making it glow so you see a spot of light.

But what "is" a spark? Sparks generally occur when the electric field between two bodies at different electrical potentials (e.g. the positive and negative electrodes) is sufficient to accelerate electrons to the point where they have enough energy to knock other electrons out of their orbits around other atoms. This is known as ionisation and it creates the electron-equivalent of an avalanche. As a result, the increasing numbers of charged particles moving through the field carry the current between the electrodes. So for a spark to occur you need some atoms or molecules to become ionised in the first place.

A vacuum is therefore an even better insulator than air.

Chris
 

Offline G-1 Theory

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Can a spark travel across a vacuum?
« Reply #2 on: 31/07/2007 17:41:16 »
YES ;

Have you ever seen the old surchlight that they used in WWII, well the are what is called Ark lights, they were two carben post in a vacuum and used an electeical ark to make the light.


Chris you came in as I was typing this so will help you to understand that it is the EM-fields of the two carben post along with the hight energy of the power added here makes the ark in this type of light.

ED
« Last Edit: 31/07/2007 17:49:05 by G-1 Theory »
 

Offline chris

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Can a spark travel across a vacuum?
« Reply #3 on: 31/07/2007 19:13:24 »
Hi Ed. Thanks for raising the issue of carbon arc lights - it gave me an excuse to remind myself about their history.

However, I don't agree with what you have written. Carbon arc lamps did not use evacuated chambers - the carbon electrodes (hence the name) were enclosed with a gas, sometimes air, occasionally a noble gas.

The point is that the arc was initially "struck" in these lamps by bringing the electrodes together and then separating them slowly as they warmed up. As they did so they would be releasing particles, including ionised particles, which could convey the current between them.

Hence this is not a spark jumping across a vacuum - it is an arc created by initially touching two charged bodies and then separating them once a flow of ionised particles between them is already established.

http://en.wikipedia.org/wiki/Arc_lamp
 

Offline syhprum

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Can a spark travel across a vacuum?
« Reply #4 on: 31/07/2007 19:29:47 »
Congratulations Chris I can add nothing!
 

Offline Bored chemist

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Can a spark travel across a vacuum?
« Reply #5 on: 31/07/2007 19:52:45 »
The emission spectrum of various sorts of arc lights depends on (among other things) the gas between the electrodes. As such it can't come from a vacuum.
In thermionic tubes the electrons can jump the gap in an evacuated enclosure but there's no light emited from them as they do so.
 

Offline G-1 Theory

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Can a spark travel across a vacuum?
« Reply #6 on: 31/07/2007 20:11:40 »
Hi Chris,

Sorry, I never was that close to one of the ark lights and just assumed that the light was in a vacuum!

But, back in the early 60ís as radio / radar / & sonar operator on SSBN 598 GW
Everything at that was half-bread of vacuum tube and transistors, but the finials of the radar units was still very high voltage vacuum tubes and there was always a very bright blue ark in those when you opened the housings.

Ed
 

Offline syhprum

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Can a spark travel across a vacuum?
« Reply #7 on: 31/07/2007 20:31:27 »
Thermonic vacuum tubes (valves) although nominaly evacuated always contained traces of gasses.
 

Offline syhprum

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Can a spark travel across a vacuum?
« Reply #8 on: 31/07/2007 21:27:51 »
Early radar sets used magnetron tubes to generate the high power transmitted SHF pulse and a superhetrodyne receiver using a klystron local oscillator and a point contact semiconductor crystal as a mixer on a common antenna.
To avoid the transmitted power reaching the mixer crystal a system of waveguides were employed with a node that could be shortcircut-ed by a gas filled tube that was fired by the same power source that pulsed the magnetron.
It was probably this tube that  G-1 Theory could see glowing.
 

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Can a spark travel across a vacuum?
« Reply #8 on: 31/07/2007 21:27:51 »

 

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