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Homework Help: Parallel plate capacitor with dielectric. Find capacitance

  1. Sep 21, 2010 #1
    1. The problem statement, all variables and given/known data
    The plates of a parallel plate capacitor measure 15cm x 15cm, and they are separated by 6cm. The capacitor is charged with a potential difference of 18V, then removed from the voltage source and kept isolated. A 15cm x 15cm x 3cm slab of dielectric material (k = 2.5) is inserted between the plates. What is the capacitance in µF after the dielectric is inserted?



    2. Relevant equations
    Vi=Ei*d (1)
    E=(ke)Q/r^2 (2)
    V=(Vi)/k (3)
    C=Q/V (4)

    3. The attempt at a solution
    I used the formulas above in that order. I solved first for E(initial) using V(initial) and d=.06. I got E(i)=300
    I then plugged E(i) it into eqn(2) to get Q. Q=1.2 x 10^-10
    Next, I found V using eqn(3): 18V/2.5 to get V=7.2

    Finally, I plugged Q and V into eqn 4 and got 1.7 x 10^-11

    This doesn't match any of my multiple choice answers. Can someone help me figure out where I went wrong or give me a better plan of attack for this type of problem?
     
  2. jcsd
  3. Sep 21, 2010 #2

    rl.bhat

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    Homework Helper

    What is the capacitance in µF after the dielectric is inserted?

    Capacitance does not depend on the charge on the plate or PD across the plates. When you introduce dielectric slab, the capacitance increases.

    C = k*Co where C is the capacitance with dielectric, Co is the capacitance without dielectric and k is th dielectric constant.
     
  4. Sep 21, 2010 #3
    Does the size of the dielectric affect the answer?
    C for parallel plates= (eps)A/d
    I used that to get C(i) and then multiply by the dielectric coefficient, but I am still incorrect
     
  5. Sep 22, 2010 #4

    rl.bhat

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    Homework Helper

    If the dielectric is completely occupies the space between the plates, then the C = Co*k applies. If the a part of the space is filled, them

    C = εο*A /[(d-x) + x/k], where x is the thickness of the dielectric slab.
     
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