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Homework Help: Maximizing dissipation

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    3. The attempt at a solution

    First I decided to find the equivalent resistance.

    Since r2 and r3 are parallel, this can be combined into a resistor of resistance r2r3/(r2 + r3). Since r2 is known, I went ahead and wrote that as 14r3/(14+r3).

    Now, this resistor is in series with r1, so I can find the equivalent resistance to be

    2 + [14r3/(14+r3)]

    Now, I can call the current due to the battery

    i = V/R = V/(2 + [14r3/(14+r3)])

    Allowing me to use the dissipation formula for r3

    P = r3[V/(2 + [14r3/(14+r3)])]^2

    Now I figured it would be a matter of differentiating this function and setting it equal to zero, but the derivative is unreasonably complex.

    Any ideas?

    Attached Files:

  2. jcsd
  3. Sep 30, 2012 #2


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    Staff: Mentor

    Take another look at the problem statement; You want to maximize the power dissipated in R3, not in the whole network.
  4. Sep 30, 2012 #3
    I don't understand what you mean. I don't know the current through the battery and represented the equivalent resistance as a function of r3, allowing me to show the current of the battery as a function of r3 (with V constant) allowing me to create a dissipation due to r3 function (P=i^2(r3)).

    I really suck at this electrical engineering crap. What i goes into P=i^2(R)? And how do I find it?
  5. Sep 30, 2012 #4


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    Staff: Mentor

    You want the power dissipated by R3 as a function of R3. that way you will be in a position to maximize that function.

    That means you need to find either the voltage across R3 or the current through R3 so that you can write the power as I2R3 or V2/R3.
  6. Sep 30, 2012 #5
    Ok, so the current through r3 isn't the current supplied by the battery?

    Also, you said I either need the current through r3 or the potential difference across r3, but don't I need one to know the other?
  7. Sep 30, 2012 #6


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    Staff: Mentor

    Nope, because the battery also supplies current to R2 which does not flow through R3.
    If you know one you can find the other via Ohm's law, but either one solved in terms of the circuit component values will do. You want to write KVL and/or KCL equations to solve for either the potential across R3 or the current through R3.
  8. Sep 30, 2012 #7
    I'm going to do some more reading. Thanks for your guidance!
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