Hey there! Sign in to join this conversationNew here? Join for free

Capacitance Question [Edexcel A2 Physics - Unit 4] Watch

    • Thread Starter
    Offline

    0
    ReputationRep:
    Hey guys, I've come across this questions and although a shady method gives me the correct answer, I would appreciate it if anyone can provide a detailed working.

    "Mains-operated power supplies have large-value capacitors to help keep the output voltage constant. In a full-wave rectified supply, without a capacitor the output falls to zero every 10 micro seconds. If the output must be maintained at a value at least 90% of its maximum value when a load of 1.0 Kilo Ohms is connected to the output terminals, show that the minimum capacitance needed in the power supply circuit is about 100 micro Farads."

    Quick answers, even if brief will be wholly appreciated.
    • Offline

      9
      (Original post by Aahmsil)
      Hey guys, I've come across this questions and although a shady method gives me the correct answer, I would appreciate it if anyone can provide a detailed working.

      "Mains-operated power supplies have large-value capacitors to help keep the output voltage constant. In a full-wave rectified supply, without a capacitor the output falls to zero every 10 micro seconds. If the output must be maintained at a value at least 90% of its maximum value when a load of 1.0 Kilo Ohms is connected to the output terminals, show that the minimum capacitance needed in the power supply circuit is about 100 micro Farads."

      Quick answers, even if brief will be wholly appreciated.
      Let's see your "shady method". It may be ok.
      If not we can suggest a better one.
      • Thread Starter
      Offline

      0
      ReputationRep:
      (Original post by Stonebridge)
      Let's see your "shady method". It may be ok.
      If not we can suggest a better one.
      Okay, I just simply equated this:

      e^(-t/RC) = 0.9

      And solved for capacitance, C. Is that alright? Is there any other way?
      • Offline

        9
        (Original post by Aahmsil)
        Okay, I just simply equated this:

        e^(-t/RC) = 0.9

        And solved for capacitance, C. Is that alright? Is there any other way?
        Yes. That's how to do it.
        V=Voe(-t/RC)
        And you need the value of C such that in a time t the voltage V can be no less than 90% of Vo
        So V/Vo = 0.9 = e(-t/RC)
        • Thread Starter
        Offline

        0
        ReputationRep:
        (Original post by Stonebridge)
        Yes. That's how to do it.
        V=Voe(-t/RC)
        And you need the value of C such that in a time t the voltage V can be no less than 90% of Vo
        So V/Vo = 0.9 = e(-t/RC)
        Thank you very much for your time!
        Much appreciated.
       
       
       
    • See more of what you like on The Student Room

      You can personalise what you see on TSR. Tell us a little about yourself to get started.

    • Poll
      What newspaper do you read/prefer?
    • See more of what you like on The Student Room

      You can personalise what you see on TSR. Tell us a little about yourself to get started.

    • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

      Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

      Quick reply
      Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.