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Please help Capacitance question!

  1. Feb 15, 2010 #1


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    Please help!! Capacitance question!

    Does the capacitance increase, decrease, or stay the same if:

    1. the charge on the plate doubles?
    2. the potential difference across the plates triples?
    3. the distance between the plates is halved?
    4. area of the plates is doubled?
    5. electric field is reduced by half.

    My answers are

    1. increases
    2. decreases
    3. increases
    4. increases
    5. same

    Are these correct?

    Also, True or false: "Charging a parallel plate capacitor requires no work provided the charges placed on the two plates are equal and opposite." Provide appropriate reasoning.
    Last edited: Feb 15, 2010
  2. jcsd
  3. Feb 15, 2010 #2
    Re: Please help!! Capacitance question!

    The capacitance of a parallel plate capacitor is equal to

    [tex] \frac {\epsilon A} {d} [/tex]

    where [itex] \epsilon [/itex] is a property of the dielectric between the plates, and A and d are the area of the plates and the distance between the plates. As long as none of these change, the capacitance doesn't change either, so a and b are wrong.

    part 2.
    When you discharge a capacitor through a resistance, where comes the energy for the
    heating of the resistance come from? How can a capacitor contain energy if charging it
    takes no work?
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