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Homework Help: Electric field

  1. Mar 27, 2005 #1
    Suppose there is a positively charged wire inside a negatively charged cylinder. What would be the net external electric field? Would I first need to combine the charges, or can I just combine the seperate fields?
  2. jcsd
  3. Mar 27, 2005 #2

    Andrew Mason

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    You would draw a Gausian cylinder around the charged cylinder and determine the enclosed charge. Then you would apply Gauss' law to determine the flux through that surface:

    [tex]\oint E\cdot dA = \frac{q}{\epsilon_0}[/itex]

    where q is the enclosed charge. To find q, just add the charges (negative subtracts from positive). If they sum to 0, there is no external field.

    Since E would be the same at all points on the Gausian surface, and since the area of the cylinder is [itex]A = 2\pi RL [/itex] the external field E would be:

    [tex]E = \frac{q}{2\pi\epsilon_0RL}[/tex]

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