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Homework Help: Internal resistance of a cell

  1. May 6, 2012 #1
    1. The problem statement, all variables and given/known data
    A voltmeter is connected across a cell in a circuit containing a switch and a 2Ohm resistor. When the switch is open, the voltmeter reads 12V. When it is closed, the reading is 8V. Find the internal resistance of the cell.

    2. Relevant equations
    I tried to use V=IR - is I constant? does this have to do with voltage drops?

    3. The attempt at a solution
    the solution is 1Ohm. Why is that?
  2. jcsd
  3. May 6, 2012 #2


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    Staff: Mentor

    You're dealing with a DC source (a cell) and resistances, so yes, the current I is constant.
    Yes, this has to do with voltage drops. Can you identify where voltage drops are occurring and what their values are?
    Isn't that what you need to show?

    Have you sketched the circuit as described? With the given information, what expressions can you write for current and potential drops? Can you determine any of the values?
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