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What causes a spark (Eletricity)

  1. Nov 11, 2012 #1
    Hi all,

    I am currently reading a book that tries to compare Hertz transmission and reception of radio waves with the photoelectric effect. (I am aware that Hertz discovered the photoelectric effect and what it is although not why it occurs).

    The book states that; the magnitude of sparks produced at the spark gap during transmission was greater when electromagnetic radiation (Ultraviolet) was being projected towards the spark gap.

    So now I hope you can see my need to understand what causes a spark. I have searched the web but too many suggestions left me confused. So I was wondering whether someone might be able to help.

    Thank you.
    Last edited: Nov 11, 2012
  2. jcsd
  3. Nov 11, 2012 #2


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    You actually are mixing, and probably confusing, two separate phenomena here.

    One is the photoelectric effect, the other is your "spark". They are two different things, even though when combined, the photoelectric effect can "enhance" the formation of such sparks.

    A spark, or breakdown phenomena in air, is often thought to be initiated by a "field emission". This is where, when a surface is under a high electric field, an electron (typically a conduction electron in a metal) tunnels through the surface barrier, escapes from the bulk material, and then ionizes the air molecules to cause a "cascade" effect. This sets up a transient plasma and bam! You see a spark.

    Now, this "barrier" that the electron has to tunnel through is associated with something called the "work function". This is where we connect to the photoelectric effect. If while all this tunneling is going on, and you shine a light on the surface, even if this light has energy less than the work function, what it is doing is it is "assisting" some of the electrons to be in energy states closer to the vacuum state. When that happens, these electrons will have LESS of a barrier to tunnel through, and thus, increase the probability of field emission. Consequently, you tend to get more sparks, or sparks can occur more easily.

  4. Nov 11, 2012 #3
    Thanks a lot. You've helped a lot.
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