A battery-battery-resistor circuit

  1. 1. The problem statement, all variables and given/known data
    A circuit:
    Battery (1.5v) -- Battery (1.5v) -- Lamp(18 Ohm) and back to the first battery.

    The circuit is producing 0.25 Watt to the lamp. what is the inside-resistance of the batteries?

    2. Relevant equations
    P=VI
    IR=V

    3. The attempt at a solution
    1.5+1.5=3Volt
    P=0.25Watt
    0.25=3*I
    I=1/12
    (1/12)*R=3
    R=36
    we already have 18 Ohms from the lamp so the remining 36-18=18 is the answer.
    WRONG:confused:
     
  2. jcsd
  3. Doc Al

    Staff: Mentor

    You cannot assume that the voltage across the lamp is 3V--not when the batteries have internal resistance. Instead, find an expression for power consumption in the lamp in terms of I and R, and solve for the current.
     
    Last edited: Aug 31, 2007
  4. Oh, I got it now, Thanks!
     
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