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Difficult Capacitor problem

  1. Dec 7, 2015 #1
    1. The problem statement, all variables and given/known data
    https://isaacphysics.org/api/images/content/questions/physics/circuits/capacitors/level4/figures/Circuits_potentiometer_capacitor_otp_2.svg [Broken]
    PQ is a slide-wire of uniform resistance, and J is a moveable contact at some point along it, such that the length from P to J is a known distance l. The total length of PQ is also known to be 1.00m. V=24.0V, C=10.0mF.
    Find the energy dissipated. Given that initially S1 is closed and S2 is open, and l=326mm. The circuit is left in this arrangement for long enough that the circuit reaches a steady state. Then S2 is opened and S1 is closed simultaneously.
    2. Relevant equations
    E=0.5cv^2

    3. The attempt at a solution
    So using the above eqn i assumed that the pd across the capacitor will be 24v(because the same voltage flows through the variable resistor branch and the capacitor branch. Now as the current stablies there should be no current flowing through the capacitor. Hence the pd across the capacitor is 24v. Now the total energy of the capacitor = total energy dissipated by the resistor ) then i calculated the energy E=0.5*10*(10^-3)*24^2=2.88
    Is this right baecause the answer is different
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Dec 7, 2015 #2

    TSny

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    Please include the subscripts on the switch labels; i.e., the S's.

    Think a bit more about whether or not the pd across the capacitor is 24V initially.
     
  4. Dec 7, 2015 #3

    NascentOxygen

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    Staff: Mentor

    What do you think is achieved by those switches? Nothing?
     
  5. Dec 7, 2015 #4
    S1 basically charges the capacitor but S2 helps discharge the capacitor and time constant =RC=R*10*10^-3
    I have a doubt that the pd across the capacitor is not 24v.
    Otherwise the energy dissipated in the resistor is not equal to the energy the capacitor posseses
     
  6. Dec 7, 2015 #5

    NascentOxygen

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    Staff: Mentor

    PD across capacitor is never even close to 24v here. What is the function of the slider on the resistance wire?

    Does the question ask for the energy dissipated in specifically resistor R?
     
  7. Dec 8, 2015 #6
    But shouldnt the voltage across a parallel circuit be the same regardless of the resistance?
     
  8. Dec 8, 2015 #7

    TSny

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    Line PJQ is a slide resistor. So section PJ has significant resistance. Likewise for section JQ. Might be helpful if you included resistor symbols in these two sections.
     
  9. Dec 8, 2015 #8
    Thanks alot got it
     
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