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Voltage Across a Resistor Problem

Discussion in 'Homework Help' started by frozone45, May 15, 2018.

  1. frozone45

    frozone45 New Member

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    Hi everyone, I was having trouble understanding a problem that I have attached. Work for the problem is in the attachment. The problem asks me to find a voltage across a resistor. While doing the problem, I found two different voltages, one positive and one negative. Why does the formula for power absorbed by a resistor give me two voltage values, when only one voltage value is correct?

    Could someone please help me understand this?
     

    Attached Files:

  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I got the same answer using a different method.
    Total voltage=10V
    Total resistance=14 ohms
    -----------------------
    10V/14 ohms = 1V/1.4 ohms.
    ------------------------
    10V/14=?V/6 ohms 10V*6 ohms/14 ohms = ?V =4.29V
     
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  3. JimB

    JimB Super Moderator Most Helpful Member

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    The absolute value of the voltage is correct at 4.29 volts.

    The way that you have calculated the voltage, you are taking a square root which you found by doing a power calculation.
    From that, mathematically, both +4.29 and -4.29 volts are solutions to the problem.

    Electrically, the current could flow in either direction through the 6 Ohm resistor and the power dissipated would be the same.
    IN THIS CASE, the conventional current will flow from the positive side of the supply to the negative side.

    JimB
     
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  4. dave miyares

    Dave New Member

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  5. frozone45

    frozone45 New Member

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    I understand now that the negative voltage is not possible due to the conventional current direction. Thank you all so much!
     
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Consider this. Label a node as being a reference node (the ground symbol), with respect to which all voltages at the other nodes are measured. In this example, I arbitrarily choose the lower left node to be ground, and labeled the other three a, b, and c.

    4.png

    Obviously, the current in the circuit = I = E/Rt = 10V/(5+3+6) Ω= 10/14A = 0.7143A
    It is easy to see that the power in resistor R3 is (I(R3)^2)*R3 = (0.7143^2)*6 = 3.061W

    There is no sign ambiguity.
     
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  7. frozone45

    frozone45 New Member

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    I see. That makes sense. Thank you so much!
     

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