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Power dissipated in resistor.

  1. Sep 16, 2014 #1
    1. The problem statement, all variables and given/known data

    You have a circuit with a single resistor of resistance R connected to a battery that has zero internal resistance and has a power output of 12W. A resistor of resistance of 6R is now connected in parallel to the original resistor. How much power is now dissipated in the original resistor?

    2. Relevant equations
    Power:
    [tex]P = iV = i^{2}R = \frac{V^{2}}{R}[/tex]
    Equivalent resistance in parallel:
    [tex]\frac{1}{R_{eq}}=\sum_{j=1}^{n}\frac{1}{R_{j}}[/tex]

    3. The attempt at a solution

    I feel that there isn't enough information in the question to get the required answer.

    If the resistors are connected in parallel, then the voltage will be the same across them, but we don't know the voltage of the battery, we only know the power it produces. We also don't know what R is.

    Would the power dissipated in the original resistor be changed if we added a second resistor in parallel? Shouldn't it remain the same?

    From the given information, is it possible to find the power dissipated in the original resistor?
     
    Last edited: Sep 16, 2014
  2. jcsd
  3. Sep 16, 2014 #2

    NascentOxygen

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    Staff: Mentor

    No, it won't change.

    Yes.

    A trick question? :uhh:
     
  4. Sep 16, 2014 #3
    So if it doesn't change, the question becomes:

    "You have a circuit with a single resistor of resistance R connected to a battery that has zero internal resistance and has a power output of 12W. How much power is now dissipated in the original resistor?"

    Is it possible to know the power dissipated in the original resistor now?

    If we don't know the value of R, it shouldn't be possible, correct?

    It is a multiple choice questions and one of the options is "Not enough information provided" so I will just choose that...
     
  5. Sep 16, 2014 #4

    NascentOxygen

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    Yes, it hasn't changed. What are the options in the question?
     
  6. Sep 16, 2014 #5
    The other options are just numbers like 10W, 5W, etc. and there is no option for "Power doesn't changed" so I think the only option I have is "Not enough info" because I can't justify choosing the options with numbers.
     
  7. Sep 16, 2014 #6

    BvU

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    Not enough information is not the right answer. Too much would be better already.
    In fact there is contradicting (or at best ambiguous) information: "a battery that has zero internal resistance and has a power output of 12W" can be a good constant voltage source - in which case the power output changes when the load changes. The voltage remains the same. The zero internal resistance hints at that.

    Or it can be a source with an output power regulator, in which case the voltage does change when the load changes.

    In both cases there is enough information. Picking the right case is a gamble. My money is on the first one -- but I haven't seen the options to pick from...
     
  8. Sep 16, 2014 #7

    BvU

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    I still haven't seen the options (like... ?). But you can ask yourself: is there any possibility of a power output less than 12 W ?
     
  9. Sep 16, 2014 #8

    NascentOxygen

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    Can you list all the options? Exactly.
     
  10. Sep 16, 2014 #9
    12W
    6W
    10W
    5W
    No enough info

    I am starting to think that 12W is the correct answer since there is no internal resistance in the battery.
     
  11. Sep 16, 2014 #10

    NascentOxygen

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    Staff: Mentor

    When the single resistor was connected to the battery, how many watts was that resistor dissipating?
     
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