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Power Calculations

  1. Mar 9, 2014 #1
    1. The problem statement, all variables and given/known data
    Concider the circuit shown below. Find the current iR flowing through the resitor. Find the power for each element in the circuit. Which elements are absorbing power?


    2. Relevant equations
    Ohm's law: U = IR;
    Kirchhoff's Current Law and Kirchhof's Voltage Law.


    3. The attempt at a solution
    My instructor told me that the resistor is supplying power and the voltage source is absorbing power. But I just didn't get it.
    By the diagram, the current iR flows in a clockwise direction. With KCL, iR = 2A, and with Ohm's law, vR = iRR = 10V. But How can I know whether the resistor is absorbing power or supplying power? Because I can't determine the polarity between the resitor with KVL.
    It's clear that the voltage source is absorbing power, and if it were a battery, it would be charging.
    But I can't understand how can a resistor SUPPLY power to other elements in the circuit as my instructor told me.
     

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    Last edited: Mar 9, 2014
  2. jcsd
  3. Mar 9, 2014 #2

    PhysicoRaj

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    Is that 2A a current source or ammeter?
     
  4. Mar 9, 2014 #3
    It's an independent current source, and 10V is an independent voltage source.
     
  5. Mar 9, 2014 #4

    NascentOxygen

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    Hi kexanie.

    http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    I believe what your instructor intended you to understand is that through the resistor is power delivered to the voltage source. It is the constant current source that supplies this power. Resistors always dissipate heat, according to the I2•R formula, but there is nothing to say that the current can't then go on to deliver power to another element.
     
    Last edited by a moderator: May 6, 2017
  6. Mar 9, 2014 #5
    so can we tell that the power of the current source, the resitor and the voltage source are respectively -20W, 0W and 20W since the resitor just give out what it absorb from the current source?
     
    Last edited by a moderator: May 6, 2017
  7. Mar 9, 2014 #6

    SammyS

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    What is the voltage drop across the resistor? (in the direction of current flow)

    What is the voltage drop across the voltage source? (in the direction of current flow)
     
  8. Mar 9, 2014 #7

    NascentOxygen

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    All electrical power a resistor 'absorbs' is turned into heat.
     
  9. Mar 10, 2014 #8
    1)By Ohm's law, the voltage drop across the resitor is 10 V? How can I know the polarity of that voltage?

    2)Is it just 10 V? I am not sure. But by the definition of an independent voltage source, it should be 10 V.
     
  10. Mar 10, 2014 #9

    SammyS

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    Going around the circuit clockwise, the same direction in which the current is forced to flow, yes the voltage drops by 10 Volts as it flows through the resistor. (Now you can find the power )

    Continuing in the same direction, how much does the voltage change as current passes through the voltage source?
     
  11. Mar 11, 2014 #10
    Sorry for grammar mistakes.

    1) I can easily tell that the magnitude of that power is 10 W, but should it be negative or positive? I can't tell the polarity of the voltage dropped across the resistor. That's why I am asking.

    2) I don't know. Do you mean electric potential by voltage?
     
  12. Mar 11, 2014 #11

    NascentOxygen

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    Current flows through the resistor from the higher potential end to the lower potential end, e.g., from positive towards negative, and this is described as a voltage drop.
     
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