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Homework Help: Need help with electric field questions thx

  1. Aug 8, 2006 #1
    need help with electric field questions thx:)

    Hey everyone! I'm currently knew to the site, i found it searching for help on electric field problems n such. If anyone could help me that would be great! thx again

    1) A proton orbits a long chared wire, making 1.0x10^6 revolutions per second. The radius of the orbit is 1.0cm. What is the wire's linear charge density?

    2) Show that the on axis electic field of a ring of charge has the expected behaviour when z<<R and when z>>R.

    The answer to the first questions is -2.29nC/m, but im clueless about how to go about it, i've also tried applying circular kinematics but have not been successful. The second question i am absolutly dumbfounded by. Thx again anyone who helps!:smile:
  2. jcsd
  3. Aug 8, 2006 #2


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    What has one tried with respect to circular kinematics?

    When a charge moves in a circular trajectory, the centripetal force must balance the electric force, and the electric force must be 'attractive' if the moving charge moves around the source of the electric field. In the case of a line charge, the charge must be negative in order to be attractive to a positively charge proton.

    F = q E = centripetal force. I leave this to the student to complete.

    The second questions makes no sense. There has to be more to it, as in a description of the expected behavior.

    This might help -
  4. Aug 9, 2006 #3
    k thx, just checking im on the right track, i found the force by F=(mv^2)r.
    then by using E = F/q (q = 1.6x10^-19), but this is where im stuck any more pointers please? thx again
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