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Electrical Sparks

  1. Nov 14, 2011 #1
    say 12v @ 1 amp.

    When you touch the + & - together there is a spark.

    What is that spark? Why is it so bright and hot from that initial contact, but after the contact point isn't as bright and hot.


    I was told by someone that it is reisistance.

    All I can think of is as the charge heads out to the + it meets resistance there and only part of that charge makes it into the air as heat / light. So is there some sorta high initial resistance?
     
    Last edited: Nov 14, 2011
  2. jcsd
  3. Nov 14, 2011 #2
    resistance IS the culprit.

    First of all, 12 volts is insufficient to ionize air...so there will be no spark thru air with a 12 volt battery, for example, as there is, say, with natural lightning or with a spark plug. You'd usually need several thousand volts to jump even a small air gap...in other words, air is a pretty good insulator.

    In a typical contact spark, there is inadequate cross sectional area to carry the current....so the tiny contact area is overloaded and rapidly heats...sparks. Similar heating can occur in house wiring for example, say if a #22 wire (too small) were used in a 20 amp kitchen circuit.

    An example of where this unwanted phenomena occurs is aboard boats between fuses and fuse holders. Because of the humid marine environment, it is common for brass or copper fuse holders to develop surface oxidation...corrosion.... and these "growths" cause poor contact across the full surface area of the fuse holder and contact heating occurs.

    This frequently becomes so hot that a normal current within the conduction range of a fuse now gets really hot because of the few points of limited contact...the fuse melts and it can appear there is an overload of some electrical appliance... The cure is the rotate fuses in their holders at least annually the scape off oxidation and restablish proper surface contact and conductivity.

    Another similar problem with poor contact surface area is shorepower plugs aboard boats...these typically carry 30 amps or 50 amps and are known problem spots where actual fires originate. No protection device "trips" because current is normal...but contact area is minimal.
     
    Last edited: Nov 14, 2011
  4. Nov 14, 2011 #3
    I should have mentioned in my prior post:

    Mathematically, this can be seen via "resistivity: R = pl/A. As the surface conduction area, A, becomes smaller,resistivity R gets bigger and so too the power (heat) generated:
    P =i2r
     
  5. Nov 16, 2011 #4
    Not so.

    I wouldn't suggest it, but if you take 2 wires and plug them into the slots of a 110 volt receptacle, I can guarantee that there will be a spark if you touch the wires together. I see it every day with relays from 24 volt DC to 480 volt AC. The spark is what causes pitting on the contacts, and the spark itself is caused by the current "jumping" the gap, both on contact closure (make) and contact separation (break).

    Although, it does take several thousand volts to sustain a spark across "even a small gap."
     
  6. Nov 16, 2011 #5
    zgoz...your post seems self contrdictory:
    thats a CONTACT (touching wires) spark....not the ionization of air.
     
  7. Nov 16, 2011 #6
    here is an example of ionization of air voltage:

    http://en.wikipedia.org/wiki/Electrical_breakdown#Mechanism
     
  8. Nov 16, 2011 #7
    Sorry, I wasn't detailed enough. As you start to make contact, sparks occur just before contact (and just after separation).

    As I stated, the spark is caused by the voltage jumping the gap. After you make (and hold contact) there is no sparking because there is no gap.
     
  9. Nov 16, 2011 #8
    No spark will occur BEFORE contact.....
     
  10. Nov 16, 2011 #9
    Hmmm, thinking of it, seems like voltage would have little to do with the sparks (upon contact kind, not the gap jumping kind) were talking about. I can't think of an example of voltage heating wires. So seems like a big factor in "contact sparking :smile:" is amperage. Would 12v and 600 amp cause a big spark lol
     
  11. Nov 16, 2011 #10

    Drakkith

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    Staff: Mentor

    The dielectric strength of air is about 3x10^6 V/M, so wouldn't it require less voltage the closer you get the two wires? Especially if there are impurities in the air?
     
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