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

Homework Help: Electric Field question

  1. Oct 20, 2005 #1
    My teacher in class told me that the electric field of a equally charged object is greater at sharp points, he never explained why and i dont really understand it.

    For example if a knife was a charged object the electric field would be greatest at the point of the blade.

    Im thinking that since electric field lines cannot overlap and since two lines are converging at a point that the field lines must increase to avoid each other

    i dunno

    any help?
  2. jcsd
  3. Oct 20, 2005 #2


    User Avatar
    Gold Member

    Electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, i.e. from a sharp point or edge rather than a flat surface.
  4. Oct 20, 2005 #3
    Charge is distributed on the surface of a conductor. So you can sort of think of a point of a knife as having the more surface area per volume then the rest of it.

    This is also why you shouldn't put pointy conductors in a microwave. The small surface area of the point can cause a huge build-up of charge when the electrons are sloshing back and forth due to the changing radiation. The buildup at a point can easily have voltages high enough to inonize air.
  5. Nov 14, 2006 #4
    This would seem to explain why you get sparks in a microwave oven from crinkled foil but not from a smoother metal surface like a spoon, correct? I am also scouring this forum for a satisfactory explanation of the role of plasma in the creation of sparks in a microwave - what exactly is happening that results in the creation of a "spark", that can be seen, in a microwave? Is it like a plasma arc? Where does the light come from? Presumably photon, ok that might sound stupid, but I am curious about the underlying sequence of events that makes the visible spark.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook