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Homework Help: Open circuit voltage & EMF help!

  1. Aug 13, 2010 #1
    Any help appreciated with these!

    The problem statement, all variables and given/known data

    1. A battery is made up from a series combination of six 1.2 volt nickel cadmium cells supplying a 10ohm load-

    a) What is the open circuit voltage?
    b) Ignoring internal resistance, what is the current flowing in each cell?

    2. The nickel cadmium cells in Q1 are reconnected in parallel and are connected to the same load-

    a) What is open circuit voltage?
    b) Ignoring internal resistance, what is the current flowing in each cell?

    3. Determine the internal resistance of a carbon cell if the open circuit voltage is 1.70 volts and when a 10 ohm resistor is connected across the battery terminals the voltage falls to 1.26 volts.

    Relevant equations


    The attempt at a solution

    1a- Do i simply use V=IR which would be 6x1.2 = 7.2 V? Is that the correct answer?

    1b- Using I=V/R, is it simply 7.2/10 = 0.72A (current doesn't change through a resistor in a series circuit right?)?

    2a- Do the voltages of each cell coalesce within the parallel circuit or is the total voltage simply V=IR = 1.2V?

    2b- Since I(current) isn't the same through each resistor in a parallel circuit, would it be 0.12A flowing through each cell?

    3- I'm really not sure how to solve for this. Transposing the E(emf)= V - IR(internal) equation, I get R(internal) = (E - V)/I. But I'm not sure which value (ie 1.26 or 1.7V) goes where or how to solve this problem.

    Any help is very much appreciated! Cheers
  2. jcsd
  3. Aug 13, 2010 #2
    1a) 7.2 is correct, but you don't need to use V=IR...
    1b) yes and yes (you can *kind* of think about it as the current having only one route to take. There are better explanations though).

    2a) yes. (ask yourself what the voltage would be if one battery where to stop working?)
    2b) yes.

    3) try http://en.wikipedia.org/wiki/Internal_resistance" [Broken]
    Last edited by a moderator: May 4, 2017
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