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Charged straight wire Electrostatics

  1. Mar 31, 2014 #1
    1. The problem statement, all variables and given/known data
    We have a charged straight wire with radius ra = 0.2 cm and with charge ta = 3nanoC/m. Around the wire there are two hollow concentric cylinders, so that the wire is in their axis. Let the charge on the first cylinder be of radius rb = 3cm and its charge tb = -1.5nC/m and on the second cylinder with rc = 5cm with charge tc = 2 NC / m What is the force on the charge Q =-5NC in the distances
    R = 0.8 cm,
    R = 2.5 cm,
    R = 100 cm?

    I do not have the slightest idea what to do. I do not want you to compute it for me but give samo advice, please.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 31, 2014 #2

    BvU

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    What you want to do is collect some relevant equations and list them under 2.)
    Hint: They might have to do with electric field calculations and with forces on a charge in an electric field.

    Then you attempt a solution, e.g. at calculating the field in different areas. If that's too difficult, start with leaving out the outer cylinder.
     
  4. Mar 31, 2014 #3

    Simon Bridge

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    What he said:
    Do you know the relationship between the force on a point charge and the electric field?
    How would you normally calculate the electric fields around charged objects?
     
  5. Apr 1, 2014 #4
    The attempt at a solution
    In the wire there should be electrical intensity E = 0. From the Gauss Law for cylindric symmetry the electric intensity E1 between the wire and the first cylinder should be E1=ta/(2pi*r), where 0.2< r< 3 thus E1(R)=ta/(2pi*R). Analogically for E2 = (ta+tb)/(2pi*r), whre 3<r<5 and E3=(ta+tb+tc)/(2pi*r), where r>5.
    Does it make sense? Thank you in advance!
     
  6. Apr 1, 2014 #5

    BvU

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    Yes. There might be a factor ##1/\epsilon_0## somewhere as well.
    And then there still is the relationship between Force, test charge and E field.
     
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