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Homework Help: Potential difference in circuits

  1. Mar 19, 2010 #1
    1. The problem statement, all variables and given/known data
    In series and parallel circuits, I'm confused like let's say 2 resistors are placed both in series and parallel, how can we tell when the potential difference is at a maximum? Like if one of these resistors has a higher value than the other, do we use that in our calculation since it is the higher value or are there certain exceptions...?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 19, 2010 #2

    berkeman

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    Staff: Mentor

    Use the equations for combining resistors in series and in parallel. In series, their values just add. Do you know the equation for the equivalent resistance for two resistors in parallel?
     
  4. Mar 19, 2010 #3
    yes i do but like when they ask for the maximum potential difference, do we always have to use the equivalent resistance for both series and parallel circuits? I think it's just the maximum potential difference that's sort of throwing me off.
     
  5. Mar 19, 2010 #4

    berkeman

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    Staff: Mentor

    The term "maximum potential difference" has no meaning without more context. Maximum with respect to what? What is the voltage source, and where is it in relation to these resistor circuits you are asking about.

    Could you state the whole problem word-for-word please, and include the circuit diagram if possible. That would help a lot in answering yur question.
     
  6. Mar 19, 2010 #5
    'Two resistors with values 2 ohms and 5 ohms are each rated at 10 W. What is the maximum potential difference that can be applied when they are placed
    a) in parallel
    b) in series
    and the answers are a) 4.47 V and b) 9.9 V
    but like i used the formula P=V^2/R but when i plug it in and solve, they don't come out to the right answer...maybe i'm using the wrong formula? like perhaps i need o find the current first?
     
  7. Mar 19, 2010 #6

    berkeman

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    Staff: Mentor

    In this case, it would be easier probably to use the P = I^2R eqiation. The trick here is that you need to calculate the power in *each* resistor in those two configurations, to make sure that one does not get over its rated power.

    So what is the current I(R) (current as a function of resistance) when the two resistors are placed in series? The same current flows through both resistors, but the power P = I^R is different, right?

    And when you have the resitors in parallel, what are the two currents going to be? (they won't be equal).

    When you have the equations for each resistor's power as a function of resistance for the two configurations, set the max power to 10W and solve for the power supply voltage that would be applied to make that current.
     
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