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Homework Help: Electric field (easy at least I thought so)

  1. Jan 14, 2007 #1
    The electric field strength 5.0 cm from a very long charged wire is 2000 N/C.
    What is the electric field strength 10.0 cm from the wire?

    Okay I thought that since the radius is double what it was before using the equation:

    E= K*q*(r head)/(r^2)

    I said that it decreases 1/4 and becomes 500.

    Thats wrong. But obviously because I ignored r head...the vector thingy. But how do I apply it to the equation and solve for its new value?

  2. jcsd
  3. Jan 14, 2007 #2


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    Homework Helper

    think about the symmetry.... for a point charge the field is distributed in all direction (a spherical gaussian surface), for a line of charges (the lateral surface of a cylinder)... so in other words, you have used the wrong formula. Use Gauss's law to derive a new a formula (E as a fn of r)
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