Finding the maximum power delivered from a baterry

  1. 1. The problem statement, all variables and given/known data

    A battery of emf [insert symbol for emf here] and internal resistance is hooked up to a variable "load" resistance R. If you want to deliever the maximum possible power to the load, what resistance should you choose?(You can't change the emf and r, of course).

    2. Relevant equations

    Possibly V=[insert symbol for emf here]-Ir

    3. The attempt at a solution

    I think what I should do is differentiate the Power with respect to the variable load R and sint dP/dR =0

    P=V^2/R=([insert symbol for emf here]-Ir)^2/R
  2. jcsd
  3. tiny-tim

    tiny-tim 26,054
    Science Advisor
    Homework Helper

    Hi pentazoid! :wink:

    (what's "sint"? :confused:)
    Looks good! :smile:
  5. nrqed

    nrqed 3,048
    Science Advisor
    Homework Helper

    This is correct but the current I depends on R as well so this not yet in a useful form.

    I think it's simpler to use [tex] P = R I^2 [/tex] with [tex] I = {\cal E} /(R+r) [/tex]. Now differentiate wrt R, set to zero and solve for R.
  6. I'm trying to solve this problem but I'm having a hard time figuring out why I = (emf)/(r+R). Isn't V = emf when the conductivity is infinite? How is it infinite (or very big) in this case if the resistance is r.
  7. cepheid

    cepheid 5,194
    Staff Emeritus
    Science Advisor
    Gold Member

    To be honest, I don't know what you're talking about.

    Kirchoff's voltage law (KVL) says that the sum of the voltages around a closed loop in a circuit has to equal zero (this is just the conservation of energy, when you think about it). In other words, the voltage drop across the two resistors has to be equal to the voltage across the battery terminals (which is just the emf). Everything is in series, so current through the circuit is the total voltage over the total resistance, hence I = ε/(r+R).
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