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Homework Help: Battery Problem, Power delivered ,energy dissipated

  1. Oct 28, 2009 #1
    1. The problem statement, all variables and given/known data
    An automotive battery has an emf of 11.1 V. When supplying power to the starter motor, the current in the battery is 20.0 A, and the terminal voltage of the battery is 10.6 V.

    a.)How much power is delivered by the battery due to the chemical reactions within the battery when the current in the battery is 20 A?

    b.)How much of this power is delivered to the starter when the current in the battery is 20 A?

    c.)By how much does the chemical energy of the battery decrease if the current in the starter is 20 A for 6.9 s?

    d.)How much energy is dissipated in the battery during those 6.9 seconds?

    2. Relevant equations

    P=(E)(I);where E is the emf
    Ps=VI; power to the starter V=10.6 in this case.
    E=Pt; t is time P is (E)(I) power
    Q=(P-Ps)t; the energy would be dissipated in the form of Heat (Q)
    r=[tex]\Delta[/tex]V/I; internal resistance inside the battery, delta V would be 11.1-10.6
    Not sure if internal resistance is needed I didn't use it. but it might be needed

    3. The attempt at a solution
    P=11.1*20=222W? or kW? got it wrong so I tried 222^-6 kw wrong
    Then I am just lost, any help would be greatly appreciated
  2. jcsd
  3. Oct 28, 2009 #2
    I really need so type of help.
  4. Oct 28, 2009 #3
    Try drawing a picture, with the physical battery being a box that has two things inside of it: the emf source and the internal resistance. Then connect the box's external terminals to "starter motor." Draw an arrow to show where the given value of current flows. When you use your formula P=EI, you will have to make a choice which voltage is the right one to substitute into it, either the 11.1 V or the 10.6 V.
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