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Inductive Circuit and Frequency

  1. Feb 13, 2015 #1
    1. The problem statement, all variables and given/known data

    A resistor and a capacitor are connected in series to a variable frequency supply. A voltmerter is connected across the inductor and another across the resistor. The supply voltage is kept constant as frequency of supply is increased.
    State and explain the changes in the readings on voltmeters across the inductor and across the resistor

    2. Relevant equations

    current is inversely proportional to frequency in an inductive circuit


    3. The attempt at a solution

    I thought across the inductor V decreases since I will decrease as a result of frequency increasing

    therefore to keep total V constant the V across the resistor will increase

    4. Actual ANSWER

    the answer however is V across resistor increases because Inductive reactance increases & Current decreases

    so V across inductor decreases

    I've never been taught what inductive reactance means???

    can anyone explain why this is the answer
    thank you very much in advance :)
    my prelim is on MOnday :( and I'm so failing
  2. jcsd
  3. Feb 13, 2015 #2


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    Homework Helper

    First phrase says R series C ... the rest talks about L ... are there all 3, or just L?
    reactance "X" is a "generalized resistance" ... how much (volt-wise) does the device "react" to conducting AC current?
    V(AC) = I X
    for a Resistor, XR = R , no matter what the frequency is.
    for a capacitor, XC = 1/ωC ... since small amount of charge collects in short time, at high frequency.
    for an inductor, XL = ωL ... since the current changes more rapidly at high frequency.

    You have R series L, so they have the same current;
    what happens to the Resistor's voltage, if the current is limited by the inductor (at high frequency)?
  4. Feb 13, 2015 #3
    oh it would increase since the electrons require more energy to move through it??

    thanks for replying
  5. Feb 15, 2015 #4


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    Staff: Mentor

    Unfortunately, this cannot be the correct answer. It is contradictory in itself. Please check whether you have transcribed this incorrectly.

    You haven't answered lightgrav's question: does this problem involve a series R + C or a series R + L. You have mentioned both, probably a careless mistake due to haste, or you may be mixing up two separate questions.
  6. Feb 15, 2015 #5

    rude man

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    Homework Helper
    Gold Member

    You did not describe the position of the inductor in the circuit.
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