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Work done enclosing sphere with spherical shell - for tommorow!

  1. Feb 2, 2006 #1
    Hi guys!
    I have been working on this question all day and am getting no where :cry: I really cant get to grips with Electromagnetism, arghh!
    The question is :http://www.zen96175.zen.co.uk/problem.GIF
    I would NOT like anyone to post a solution as its assessed work but I would IMMENSELY appreciate some pointers on how to get started on this...
    I have a few hours left to complete this, I hope somebody has a few pointers :!!) :smile:
    Thank you ,

    ~Gaz
     
  2. jcsd
  3. Feb 2, 2006 #2

    siddharth

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    So what have you done with this problem? If you show where you are stuck, it will be easier for us to help.
     
  4. Feb 2, 2006 #3
    Hiya,
    Well, thats the point really, I need a little push to get me started!
    I am thinking it involves seeing how much work it takes to bring a hoop of charge from the spherical shell to the sphere, and then integrating over the whole shell. And then, integrate from r to a to make up the final large sphere is mentions? Pretty vague I know :cry:
    Just looking for some pointers if anyone can!

    Thank you!
    ~Gareth
     
  5. Feb 2, 2006 #4

    siddharth

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    I don't think you need to bring a hoop of charge, but you are on the right track. First find the work done in bringing the spherical shell from [itex] \infty [/itex] to [itex] r [/itex] and then integrate r from 0 to a.

    To find the work done in bringing the shell, you will have to know the electric field due to sphere at the center, at a distance x and also the charge dq on the spherical shell (Hint: when the shell has finally enveloped the sphere, the charge dq is still going to be the same, so you can write dq in terms of r and dr). Then you can find the force on the spherical shell and hence the work done in bringing it to the sphere

    I have not evaluated the double integral myself, but I think it should give you the answer.
     
    Last edited: Feb 2, 2006
  6. Feb 2, 2006 #5
    Phew, after a whole day on it, I think I have cracked it! This feeling is what makes physics and maths the greatest!!!!

    Thanks for your help,
    ~Gareth
     
  7. Feb 2, 2006 #6

    siddharth

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    Way to go, Gareth!
     
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