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Homework Help: What is the resistance?

  1. Oct 27, 2008 #1
    1. The problem statement, all variables and given/known data
    trrttA battery has an emf of 15.0 V. The terminal voltage of the battery is 11.6 V when it is delivering 20.0 W of power to an external load resistor R.
    (a) What is the value R?
    (b) What is the internal resistance of the battery?

    2. Relevant equations

    3. The attempt at a solution
    I'm sure how to start this problem. I'm sure I get rolling if I can just get some momentum started. Please Help.
  2. jcsd
  3. Oct 27, 2008 #2
    Hopefully these equasions will help. If you get stuck just reply.

    V = Voltage (Volts)
    I = Current (Amps)
    R = Resistance (Ohms)
    P = Power (Watts)

    V = IR (Ohm's Law)

    I = V/R (Ohm's Law Rewritten)

    P = VI

    P = V2/R

    P = I2R
    Last edited: Oct 27, 2008
  4. Oct 27, 2008 #3
    Keep in mind, the internal resistance of the battery can be modeled as a resistor in series with an ideal voltage source.
  5. Oct 27, 2008 #4


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    Homework Helper

    Draw a circuit diagram with the external and internal resistor.
    You have a voltage divider there don't you?

    And you know Power = I2R
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