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Parallel plate capacitor help

  1. Aug 3, 2009 #1
    1. The problem statement, all variables and given/known data

    An isolated parallel-plate capacitor (not connected to a battery) has a charge of Q=1.4x10^-5 C. The separation between the plates initially is d=1.2 mm, and for this separation the capacitance is 3.1x10^-11 F. Calculate the work that must be don to pull the plates apart until their separation becomes 4.5 mm if the charge on the plates remains constant. The capacitor plates are in a vacuum.

    2. Relevant equations


    3. The attempt at a solution

    Last edited by a moderator: Aug 4, 2009
  2. jcsd
  3. Aug 3, 2009 #2


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    You will probably also need a formula for the relation between the capacitance between the two plates and the distance between them, right? Then use the potential energy equation. And leave the CAPS LOCK off, ok?
  4. Aug 3, 2009 #3
    ok i have been tryin to figure this out for the past hour and got a work of 4.7x10^-10 J would that be correct?
    i used


    Would that be right??
  5. Aug 3, 2009 #4
    or would i have to use

    C=Q/Ed for capacitance
  6. Aug 3, 2009 #5
    nevermind i made a huge mistake in my math
  7. Aug 3, 2009 #6
    ok after i fixed my math i get an answer of -539.07 J
  8. Aug 3, 2009 #7
    is that correct? Or is my method correct?
  9. Aug 3, 2009 #8


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    In the first case
    C1 = εοA/d1
    In the second case

    C2 = εοA/d2
    Substitute the values of d1 and d2.
    C1 is given. Find C2.
    Find the energies in C1 and C2 and find the difference.
  10. Aug 3, 2009 #9


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    Your capacitance is the only thing that changes. C_after=C_before*(1.2/4.5). Both of your energy numbers seem somewhat off to me. Isn't (1/2)*(1.4E-5*coulomb)^2/(3.1E-11*farad)=3.16J? Or is my list of constants off?
  11. Aug 3, 2009 #10
    i thought you had to convert the mm to m
  12. Aug 3, 2009 #11
    idk maybe i missed something in lecture but can you explain why C after=C before*(1.2/4.5)
  13. Aug 4, 2009 #12


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    You don't have to convert the distance to any particular units. The capacitance is proportional to 1/D. You can't directly use the e0*A/D formula because you don't know A. But you do know A is the same at both distances. Take a ratio.
  14. Aug 4, 2009 #13
    But i thought the question was asking for work not the capacitance? and I did not use that formula of e0*A/D cuz of the same reason we have no area which is why i used

    C=(C*E)/(d)... To find capacitance at 4.5 mm..... Because they gave us capacitance at 1.5 mm in the problem

    U=(1/2)(Q^2)/C)... to find potential energy at 1.5 mm and at 4.5 mm

    W=U2-U1... to find the work done

    Is what I did incorrect??, because we do not get the same answers.... Idk i may be wrong but what your doing seems like your just solving for capacitance after and not for the work
  15. Aug 4, 2009 #14


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    Yes, you need to find the capacitance at 4.5 mm. Then you can just take the difference of the potential energies. But how are you doing that? C=C*E/d doesn't even make sense. And what do you get for the potential energy at 1.5 mm?
  16. Aug 4, 2009 #15
    the problem gives you the capacitance at 1.5 so using U=(1/2)(Q^2)/C.... I get U=3.16
    and U at 4.5 mm gives me U=195.93
    then taking the difference gives me Work=192.76 J

    and C=C*E/d is an equation our prof gave us, it gives the same values if you use C=Q/Ed
  17. Aug 4, 2009 #16


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    I get the C at 4.5mm is 1/3 of C at 1.5mm. I still don't get C=C*E/d. There are two C's in there.
  18. Aug 4, 2009 #17
    what formula for C are you using?
    What U at 4.5 mm are you getting?

    yea that formula uses C to get a new C i think.... our prof gave it to use in class.... it gives the same values as C=Q/Ed
  19. Aug 4, 2009 #18


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    I'm using C=epsilon*A/d. If d changes from 1.5mm to 4.5mm C is divided by 3. If that formula gives you a new C somehow, then what do you get for C at 4.5mm? Is E electric field? You don't know that either, do you?
  20. Aug 4, 2009 #19
    How can you use the C formula if you dont know area

    I found the electric field by using E=(K*Q)/d^2

    C at 4.5 is 5.00x10^-13
  21. Aug 4, 2009 #20


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    I don't need to know area, I just need to know A doesn't change. Only d changes by a factor of 3. Look back at rl.bhat's post. E=(K*Q)/d^2 isn't right. E is proportional to charge density (charge per area). d is the separation between the plates. It doesn't have anything to do with charge density.
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