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Circuits problem Need help by 10:00 tonight (EST).

  1. Apr 29, 2008 #1
    Circuits problem :( Need help by 10:00 tonight (EST).

    1. The problem statement, all variables and given/known data
    In the circuit shown below, R1=10 ohms, R2=45 ohms, and R3=14 ohms. The power supply delivers a voltage of 25.0 V. a) What is the current through R3? b) What is the voltage across R2?

    [​IMG]

    Possible choices for part a:
    0.92 A, 1.21 A, 1.79 A, 1.84 A, 2.50 A

    Possible choices for part b:
    25.0 V, 12.1 V, 12.9 V, 15.0 V, 0 V.

    2. Relevant equations
    V=IR


    3. The attempt at a solution

    To be honest, I have no idea how to do this problem. I don't know where to start. Do I have to find the voltage drop first? I'm just lost, heh.

    Any help is appreciated.
    Bensky
     
  2. jcsd
  3. Apr 29, 2008 #2

    rock.freak667

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    Is R3 in series or parallel with R2? Is R2 in parallel or series with the supply?
     
  4. Apr 29, 2008 #3
    R3 is in parallel with R2, R2 is in series with the supply. (I would assume)
     
  5. Apr 29, 2008 #4

    rock.freak667

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    Then what should be the p.d. across R2?
     
  6. Apr 29, 2008 #5
    Umm, I have to find the current first for that, right?
     
  7. Apr 29, 2008 #6

    rock.freak667

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    Remember how the currents through resistors in series are the same? How does the p.d. vary in resistors in parallel?
     
  8. Apr 29, 2008 #7
    The P.D. is the same.
     
  9. Apr 29, 2008 #8

    rock.freak667

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    Right so if R2 is in parallel with the supply...what is it's pd?
     
  10. Apr 29, 2008 #9
    25V

    Is it really that easy? XD
     
  11. Apr 29, 2008 #10

    rock.freak667

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    It is multiple choice...but another way to do this is by kirchoff's laws which would give you a system of equations to solve with 3 unknowns.. which sounds like way too much to do for a multiple choice question.

    So...I am hoping that is the correct thing to do...
     
  12. Apr 29, 2008 #11
    Ok, thanks...we did not cover Kirchoff's Laws. Do you have any idea how to do the first part of the problem?
     
  13. Apr 29, 2008 #12

    rock.freak667

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    R3 in parallel with R2...same idea. You will get the pd across R3 and use V=IR.
     
  14. Apr 29, 2008 #13
    How to solve:

    1. The problem statement, all variables and given/known data
    In the circuit shown below, R1=10 ohms, R2=45 ohms, and R3=14 ohms. The power supply delivers a voltage of 25.0 V. a) What is the current through R3? b) What is the voltage across R2?

    First Calculate the total resistance of the R2,R3 parallel circuit:


    1/R23 = 1/R2 + 1/R3

    .
    . . R23 = (R2 x R3)/(R2 + R3)

    Then add R2 and R1 This gives the Rt which you plug into V= IR to determine total Current. With the current you can determine the voltage drop across R1. Subtract that Voltage from the supply voltage to determine the voltage Across R2 and R3. Knowing that Voltage you can Calculate the Current through R3 using I=V/R3!

    So not only do you need V=IR, but you also need Rt = R1 + (R2 X R3)/(R2 + R3) for a circuit containing two Parallel resistance in series with another resistance.

    Edmund
     
    Last edited: Apr 29, 2008
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