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Homework Help: Parallel plates & Point charge ?

  1. Feb 28, 2005 #1
    1. parallel plate with capacitor of 20uF, plate is 0.5mm apart.

    Find
    1. Area of each plate
    2. potential difference if there's a change of magnitude of 30uC on each plate.
    3. stored energy
    4. electric field between the two plates
    5. charge density on the plate.


    |_| |_|
    |_____|
    .5mm

    I know [tex]C=\frac{Eo*A}{d}[/tex]

    so would [tex]A=\frac{Eo*C}{D}[/tex] or A=Q / Eo *E

    A= (8.85*10^ -12 ) * 20 / (.5 * 10^ -3) = 3.54 * 10 ^7 ?


    2. three point charges on the x axis q1= (10uc) at x= -4m, q2= (-5uC) at the origin, q3= (-20uC) at x= 4m.


    Q1(-4,0)...... Q2(0,0)...... Q3(4,0)
    (+)----------(-)-----------(-)
    10uC........ -5uC........ -20UC


    Find
    1. electric field at origin
    2. force on q2


    would electric field on orgin = Zero & force on q2 equal

    [tex]F1= Ke\frac{q1|q2|}{r^2}[/tex] , [tex]F2=Ke\frac{|q3|q2|}{r^2}[/tex]

    [tex]F1=\frac{(8.99* 10^9)* (10 * 10^ -6)*( 5*10^-6)}{(-4)^ 2}[/tex]

    [tex]F2=\frac{(8.99* 10^9)* (20 * 10^ -6)*(5*10^-6)}{(4)^ 2}[/tex]

    After this would i add components ( x i + y j )
     
  2. jcsd
  3. Feb 28, 2005 #2

    dextercioby

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    Science Advisor
    Homework Helper

    For the first problem,the area u computed is incorrect.You began with the correct formula,but somehow screwed up the arithmetics.What about the other 4 points...?

    Daniel.
     
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