Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can a spark form through vacuum?

  1. Aug 23, 2012 #1
    Hey guys,

    I was looking online at sparks and how they form, and the amount of energy needed to form them when this question came up to my mind.
    can a spark form through vacuum? or it actually needs particles to go through them?
    and if it can form...what is the value of the breakthrough for vacuum??

    Thanks
    Error404
     
  2. jcsd
  3. Aug 23, 2012 #2

    berkeman

    User Avatar

    Staff: Mentor

    The traditional tool for estimating the breakdown at very low pressures is the Paschen Curve:

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

    I'm not sure what happens at very hard vacuum levels, though...
     
  4. Aug 23, 2012 #3
    It actually needs a medium. The breakdown voltage is relative to medium. No medium (vacuum) hence no spark
     
  5. Aug 24, 2012 #4
    Berkeman, thanks for your reply. I can understand from this figure that either at very low pressure or at very high pressure, we need a HUGE voltage to make a spark, is that correct?

    Haxo489, thanks for your clear answer. however, i am wondering how much vacuum we could actually achieve in real life, is 100% vaccum possible?

    Error404
     
  6. Aug 24, 2012 #5
    An arc through a vacuum gap is possible, but it will be invisible, since there is no medium to light up.
    Since there is no medium, there is nothing to break down. But there is another mechanism: the electric field just has te be large enough to pull the electrons out of the metal. You could probably calculate this using the work function of the metal crystal.
     
  7. Aug 24, 2012 #6
    An arc through a vacuum is called an electron beam. It was used in CRTs (cathod ray tubes).
     
  8. Aug 24, 2012 #7

    NascentOxygen

    User Avatar

    Staff: Mentor

    Do I correctly recall reading that there is lightning between one of the large planets and one of its moons?
     
  9. Aug 24, 2012 #8
    Vacuum is a good insulator but hardly reproducible. Brutal discharges do happen, but their mechanis isn't very clear yet; it may involve X-rays in the anode-to-cathode feedback, and is not purely a tunnelling emission.

    At usual distances, the breakdown field is similar to the best solid insulators, like 50 to 100MV/m. It depends a lot on the smoothness and external layer of the electrodes; values of 200MV/m have been reported.

    Even more so than in solids and liquids, the breakdown "field" depends much on the distance between the electrodes and on the insulating volume; the breakdown "field" varies still more slowly with the distance than the breakdown voltage.

    Vacuum insulation is heavily used industrially at medium and high voltage to make switches, protections and so on, because vacuum insulates better than a gas does. Hence it is the subject of much ongoing research.
     
  10. Aug 24, 2012 #9

    nsaspook

    User Avatar
    Science Advisor

    HV breakers, switches and electrical equipment are increasingly using SF6 (an ELECTRONEGATIVE GAS) as it has a higher dielectric strength under pressure than high vacuum (it suppresses the effects of field emission in a vacuum)and makes a better switching medium for some types of loads.

    http://www.gigavac.com/apps/relays/physics/index.htm [Broken]
     
    Last edited by a moderator: May 6, 2017
  11. Aug 24, 2012 #10
    We have had some very expensive failures in space craft caused by arcing and sparking in a vacuum or near vacuum. That is why NASA tests all equipment in a vacuum chamber before sending it up into space.

    Every once in a while some bean counter will save money by not doing these tests, and we get to see another spectacular failure.

    I once toured a lab at JSC where they do the design of tools used on EVAs by the astronauts. I now understand why a power drill you buy at Home Depot is used inside the ISS, but the one they use outside exceeds a million dollars. Much of that related directly to arcing in a vacuum or near vacuum.
     
    Last edited: Aug 24, 2012
  12. Aug 24, 2012 #11
    I don't know about perfect vacuum, but there is no perfect vacuum. Any molecule or anything in vacuum will eventually arc if you put high enough voltage. We do 10EE-9 in our spectrometer, we still have to pump it down, let it arc to smooth out the rough machined edges!!!
     
  13. Aug 24, 2012 #12

    phinds

    User Avatar
    Gold Member
    2016 Award

    If you could not spark through a vacuum, you could make an infinite-voltage capacitor just by putting the two plates in a vacuum since you would never get dielectric breakdown. See how silly that sounds?
     
  14. Aug 24, 2012 #13

    davenn

    User Avatar
    Science Advisor
    Gold Member

    yeah Jupiter and its moon Io

    I dont know if its in the form of actual lightning type zaps tho
    but there are huge discharges between the two. Io looses a lot of surface material to space, this trails behind the moon in its orbit and forms a huge torus ring around Jupiter

    Dave
     
  15. Aug 24, 2012 #14

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Well in RF electronics we do use vacuum capacitors that wont break down under EHT voltages


    Dave
     
  16. Aug 24, 2012 #15
    This is really empty talk. In real instrument or circuit, creepage is a much bigger problem than jumping through space. You might have a perfect vacuum, but in order to have two electrodes facing each other, you have to have support, feed through, so there is a continuous surface connecting between the two. The surface it what it's going to break down. You look at CE or even UL spec, insulation distance is less than 1/10 the creepage distance, it's the surface creepage that is the limiting factor.

    You really worry about two planet arcing across? seriously. I am not expert in vacuum, but even if you can somehow conquer the creepage and have a perfect vacuum, if you put enough potential to create some serious E field, don't the surface of the metal start ionized and form some sort of conductive plasma or something. Again, I don't know the physics behind it, just a guess. But like vacuum tubes, if you put enough E field across, something is going to happen!!!
     
    Last edited: Aug 24, 2012
  17. Aug 24, 2012 #16

    davenn

    User Avatar
    Science Advisor
    Gold Member

    maybe you are missing the point ?

    yes there will be a voltage that the vacuum cap will eventually fail at, but its many many 1000's of volts higher than a standard capacitor with a ceramic or other dielectric

    Dave
     
  18. Aug 24, 2012 #17
    I am responding to the original post and in general. The post go into arcing between planets and vacuum cap. As I said, the creepage break down is more likely to be the limiting factor in real life case. The important thing is there is no perfect vacuum, as long as there is ions floating around, it will break down. From my experience, people count on about 400V/mil either in good vacuum or dielectric as the limiting factor in practical situation. And our problems are mainly on creepage, not on insulation that you are talking about.
     
  19. Aug 26, 2012 #18
    Thanks for replies everyone.
    As i can understand from the replies above... A 100% vacuum has a dielectric constant almost the same as air, but prevents the formation of a spark like....a 100 times more?
    Note: my question might sound very silly but i am not an expert in the field of electrical engineering...and i had some questions for you....experts!

    Error404
     
  20. Aug 26, 2012 #19
    This is what Gigavac would like customers to believe.

    Vacuum has nearly replaced SF6 in high-voltage circuit breaker. As a gas, even at pressure, SF6 is nowhere near the dielectric strength of vacuum, which exceeds solids and liquids.

    SF6 is being phased out, alas, because it's both an ozone killer and a greenhouse gas. Same for CF4.
     
    Last edited by a moderator: May 6, 2017
  21. Aug 26, 2012 #20
    This is true and so much known that designers have even found solutions.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Can a spark form through vacuum?
  1. Spark Gap (Replies: 6)

Loading...