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

Which way does a high voltage spark jump?

  1. Sep 12, 2008 #1
    Hello All!

    Quick question...

    When a high voltage capacitor is charged to say 3000 volts, a spark

    will jump across the terminals.

    Does the spark jump from the positive to negative terminal, or from negative to positive?

    Thanks Very Much!

  2. jcsd
  3. Sep 12, 2008 #2


    User Avatar
    Homework Helper

    Well since current is carried by electrons, then one should expect it to jump from negative to positive.
  4. Sep 13, 2008 #3


    User Avatar

    i've always thunk that the spark was from the ionization of the air molecules in between when the E-field is intense enough to yank electrons offa some molecules and attach them to others. while the air molecules close the the negative terminal will get extra electrons, the molecules next to the positive terminal will have missing electrons. the negatively-charged ions will fly toward the positive terminal and the positively-charged ions will, just as quickly, fly to the negative terminal. i think the spark starts at both terminals and sorta meets in the middle.
  5. Sep 13, 2008 #4


    User Avatar
    Homework Helper

    Oh yeah, I forgot about ionisation.
  6. Sep 23, 2008 #5
    Thank you very much!
  7. Sep 23, 2008 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    rbj is correct -- plasma (ionized air) forms around around both terminals. When the plasma meets in the middle of the gap, a conducting path is formed, and the current rushes across. The whole path lights up simultaneously, as the current is continuous throughout the gap. Even if you use a super-slow-motion camera, you wouldn't see a "spark" jump from one terminal to the other; you'd just see the entire gap light up at once.

    - Warren
  8. Sep 24, 2008 #7

    The video link below shows what has already been said, but in quite a dramatic way.


    Notice how the lightening seems to form all at once along its entire length? From stuff I have read on websites etc, I think the differences in intensity along its length are to do with the temperature and humidity of the air its travelling through. It looks pretty sweet though, whatever is causing it..unless by chance you were one of the passengers on that plane..

  9. Oct 2, 2008 #8
    The quick simple answer to your question about spark jump is that electrons flow from neg to pos. This has been illustrated on science shows using high speed cameras.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook