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Questions on capacitors. help~

  1. Nov 16, 2005 #1
    hey guys, i did these problems out of my book for review and it does not give me the answers. So if you could please, tell me which ones i've got incorrect and if you don't mind explaining and writing down the correct answer for the wrong answer will be awesome! Thanks in advance. (BTW, I think I got #4 and #5 incorrect because I didn't know how to do it.)
    1)If three capacitors of values 1.0, 1.5, and 2.0 mcirofarads each are connected in series, what is the combined capacitance?
    1) 4.5 microfarads
    2) 4.0 microfarads
    3) 2.17 microfarads
    4) 0.46 microfarads
    i got 3)
    2)Increasing the separation of the two charged parallel plates of a capacitor which are disconnected from a battery will produce what effect on the capacitor?
    1) increase charge
    2) decrease charge
    3) increase capacitance
    4) decrease capacitance
    i got 4)
    3)If three capacitors of values 1.0, 1.5, and 2.0 mcirofarads each are connected in parallel, what is the combined capacitance?
    1) 4.5 microfarads
    2) 4.0 microfarads
    3) 2.17 microfarads
    4) 0.46 microfarads
    i got 1)
    4)There are two capacitors with A bigger than B and they are connected in parallel with a battery.
    1) There is more potential difference across A.
    2) There is more potential difference across B.
    3) There is the same charge stored on each capacitor.
    4) There is the same potential difference across both capacitors.
    i got 2)
    5)There are two capcitors with A bigger than B and they are connected in series with a battery.
    1) There is more charge stored on A.
    2) There is more charge stored on B.
    3) There is the same charge stored on each capacitor.
    4) There is the same potential difference across both capacitors.
    i got 4)
    6)The plates of a parallel-plate capacitor are maintained with constant voltage by a battery as they are pulled apart. During this process, the amount of charge on the plates must ...
    1) increase
    2) decrease
    3) remain constant
    4) either increase or decrease. There is no way to tell from the information given.
    i got 2)
    7)Doubling the voltage across a given capacitor causes the energy stored in that capacitor to ...
    1) quadruple
    2) double
    3) reduce to one-half
    4) reduce to one-fourth
    i got 1)
    8)If the electric field between the plates of a given capacitor is weakened, the capacitance of that capacitor ...
    1) increases
    2) decreases
    3) does not change
    4) cannot be determined from the information given
    i got 3)
    9)A parallel-plate capacitor has a capacitance of C. If the area of the plates is doubled and the distance between the plates is halved, what is the new capacitance?
    1) C/4
    2) C/2
    3) 2C
    4) 4C
    i got 4)
    10)A dielectric material such as paper is placed between the plates of a capacitor. What happens to the capacitance?
    1) no change
    2) becomes larger
    3) becomes smaller
    4) becomes infinite
    i got 2)
     
  2. jcsd
  3. Nov 16, 2005 #2
    for series capapcitors you add them like so

    [tex] C_{eq} = \frac{C_{1} C_{2} C_{3}}{C_{1}+C_{2}+C_{3}} [/tex]

    for parallel plate capacitors the capacitance is given as
    [tex] C = \frac{\epsilon_{0} A}{d} [/tex] whee d is the separation between the plates

    also dieletrics have the certain effect on capacitance
    for a parallel plate capacitor for example :

    [tex] C = \frac{\kappa \epsilon_{0} A}{d} [/tex] where k si the dielectric constnat of whatever what placed in betweren the plates

    accordinly do your calculations
     
  4. Nov 16, 2005 #3

    ranger

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    Gold Member

    For 4 and 5

    What happens in a parallel and series circuit (speaking in terms of voltage (potential difference)? You can even forget about the capacitors, just think about the rules for series and parallel circuits for a second.
     
    Last edited: Nov 16, 2005
  5. Nov 16, 2005 #4

    ranger

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    Gold Member

    Thats correct.
     
  6. Nov 16, 2005 #5
    For question one, I'd consider: the total capacitance of a circuit comprised of a series connection of capacitors is ALWAYS less than the smallest capacitor in the circuit. From a qualitative view, you can eliminate a lot of bad choices..

    For question five, consider: since the capacitors are series connected, they have the same charge in each capacitor. Q = C x V, or V = Q/C. With smaller Capacitance and same charge Q, would the voltage across the capacitor be greater, or lesser?
     
    Last edited: Nov 16, 2005
  7. Nov 18, 2005 #6
    so for #1, its 4
    #4, its 1
    #5, its 2
    right? yes no?
     
  8. May 22, 2010 #7
    for Ques 1--- always take care that the resultant capacitor in series combination is smallest of all
    #4 voltage is same in parallel combination
    #5 charge is same in series combination
    Do follow any good book .any university level
     
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