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Homework Help: Internal Resistance problem

  1. Mar 20, 2012 #1
    1. The problem statement, all variables and given/known data

    A battery has an internal resistance of 0.50 ohm. A number of identical light bulbs, each with a resistance of 15 ohm, are connected in parallel across the battery terminals. The terminal voltage of the batter is observed to be one-half the EMF of the battery. How many bulbs are connected?

    2. Relevant equations

    V = Vemf - IR

    3. The attempt at a solution

    n = number of light bulbs

    1/(Rb) = (1/15)n
    Rb = 15/n

    I'm not sure what to do at this point..

    V = (1/2)Vemf
  2. jcsd
  3. Mar 20, 2012 #2
    What can be said about the current in the battery and the current in the circuit?
  4. Mar 20, 2012 #3
    The current is the same?

    My thoughts:
    Is the voltage split in half? Because half of the voltage goes through the circuit and the other half is going across the internal resistance?
    So...are the internal resistance and the resistance of the bulbs equal?
  5. Mar 20, 2012 #4

    Because the voltage is split in half (therefore equal) as you say, and the currents are the same, by the equation V=IR the resistances have to be the same.
  6. Mar 20, 2012 #5

    . so that must mean:

    r = R

    15/n = 0.5

    n = 30

    Thanks for helping!
  7. Mar 20, 2012 #6


    User Avatar

    Staff: Mentor

    To be more precise, half the voltage is dropped across the internal resistance and the other half is dropped across the external resistance. Current goes though, potential drops across components.


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