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Homework Help: Variable Resistor connected to parallel resistors

  1. Jul 10, 2012 #1
    1. The problem statement, all variables and given/known data
    In the electric circuit shown in the figure, when the value of the variable resistor R3 is 25Ω, the current which flows into the power supply is 2A. Moreover, when the value of the variable resistor R3 is 5Ω a current of 1A flows into the resistance R2.

    Q1.Find the value of parallel combined resistance of resistance R1 and R2,
    Q2.Find the value of R2,
    Q3.When the resistance R3 is 25Ω how much electric power is consumed by resistance R1

    2. Relevant equations
    R parallel combined =( R1 x R2) / (R1 + R2)

    3. The attempt at a solution
    from the first given information i know that the volt at R3 is 50 then the volt at combined resistance is 30 then I tried various equations but then I stuck ,please help me this is the first time I met a physics problem with variable resistor, what is it really?
    Last edited: Jul 10, 2012
  2. jcsd
  3. Jul 10, 2012 #2


    User Avatar

    Staff: Mentor

    Hi ordem. Welcome to Physics Forums.

    In addition to carrying 30 volts, you also know that the current through the parallel combination is 2 amps. Can you now determine the resistance of that parallel combination? :wink:

    A variable resistor is one whose resistance can be set to a different value.
  4. Jul 10, 2012 #3
    OH MY GOD, thank you so much for the answer, as soon as I read what you wrote, I feel so stupid, now I know all the answers , i don't know what hit my head before, haha, thanks again
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