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Homework Help: Capacitors question no idea?

  1. Apr 13, 2015 #1
    1. The problem statement, all variables and given/known data
    A 12V DC supply is connected in series to 2 capacitors, of 12 and 4 micro farads. A switch is in series also. The diagram shows the capacitors above the supply and the larger capacitance capacitor to the left. The names of the plates of the capacitors from left to right are as follows, A,B,C and D. the + end of the supply is on the left,

    i) describe and justify the direction of electron flow when the switch is closed

    ii) work out the total capacitance of the arrangement

    iii) deduce the charges on each plate when the switch has been closed for some time. justify your answers.

    iv) if a 1 megaohm resistor was placed between B and C, how would this affect your answers to i and ii

    2. Relevant equations

    3. The attempt at a solution
    Part i) as the supply establishes a negative potential at end D and a positive at end A, the electrons are moved anticlockwise so as to establish a negative charge at D.

    ii) was fine

    iii) i think this is where my knowledge of capacitors is strained.
    i thought that all the charge moved from A to D. and since the electrons in between B and C are iscolated there isnt a change. This is really where i need help.

    iv) naturally my inability to answer the previous part pays no dividends here.

    Help would be appreciated.
  2. jcsd
  3. Apr 13, 2015 #2


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    Part (i) may need additional work together with answering part (iii).

    What effect will be the effect on plate C, from all that negative charge on plate D ?

    Similarly, What will be the effect on plate B from charge on plate A? Is there charge on plate A ?
  4. Apr 13, 2015 #3


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    And here's my two cents:

    (ii) fine is a relative qualification. Wat came out ?
    (iii) Look under 2. relevant equations :rolleyes:.

    In fact, how can you have found (ii) without a relevant equation ? And that very equation was derived (in your notes, or your textbook) on a basis that includes solving (iii)

    (iv) can be answered without having solved (i), (ii) or (iii)
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