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Homework Help: Battery Charger (wattage)

  1. Feb 2, 2008 #1
    1. The problem statement, all variables and given/known data
    http://img2.freeimagehosting.net/uploads/bf073a5eb1.png [Broken]
    http://img2.freeimagehosting.net/uploads/bf073a5eb1.png [Broken]

    Determine the variable resistance R if the 25 Watts to be absorbed by the battery (0.035 ohm resistor and the 10.5 V element).

    2. Relevant equations
    V = IR
    P = IV

    3. The attempt at a solution

    Well I did KVL clockwise current flow
    -13 + 0.02*i + iR + 0.035*i + 10.5 = 0
    2.5 = 0.02i + 0.035i + iR

    Then factoring in 25W=10.5*i
    i = 2.381 A which is close to the answer but doesn't factor in 0.035 and I'm not sure how to do it
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Feb 3, 2008 #2
    The question was to compute R such that you got 25 W dissipated in the 10.5V battery. You computed what the current through this battery must be. Now use that in the equation with i and R you wrote above
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