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RC circuit voltage

  1. Nov 4, 2007 #1
    Why the voltage across the capacitor can be measured by connecting a CRO across both the resistor and capacitor ?

    actually,i have think of the phase difference and the overall resistance.Still,i can't find the answer.May I get some hints from you?
    Last edited: Nov 4, 2007
  2. jcsd
  3. Nov 4, 2007 #2


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    Staff: Mentor

    How else would one propose to measure the voltage across a RC cirucit as a function of time, especially for short time constants?

    Using a CRO (oscilloscope) is pretty standard. Of course, the impedance of the CRO needs to be high enough so as not to effect the measurement.
  4. Nov 4, 2007 #3
    For ac voltage,voltage of the power supply equals voltage across both capacitor and resistor.
    If i connect a CRO across both capacitor and resistor,i in fact measure the voltage of the power supply,right?
    According to my lab manual,it states that i can mesure the voltage across the capacitor by connecting a CRO across the capacitor and resistor.
    i really dont understand...

    Last edited: Nov 4, 2007
  5. Nov 4, 2007 #4


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    Gold Member

    The reason has to do with the grounds on your instruments, and I suspect you haven't read the entire instructions that accompany your lab. The voltage across the cap is found as the difference between that across both R-C, which you mention, and that across R, which is found on the other channel of the CRO. You can't measure Vc directly because your CRO and source both have grounds. Once you connect the source as shown, the CRO ground must go to the same place, that is, you cannot put gnd and probe across the cap anymore.
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