1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Internal resistance question

  1. Apr 13, 2007 #1
    1. The problem statement, all variables and given/known data
    Hi I've tried to solve this question but I keep getting stuck at a certain point, it probably very simple but any tips would be greatly appreciated

    The EMF of a cell is measured using a potentiometer and is found to be 1.51V. When a voltmeter, which has a resistance of 3.5k Ohms is connected across the cell, it reads 1.49V. What is the internal resistance of the cell. What current flows through the voltmeter?
    If a resistor of resistance 50 Ohms is connected across the cell, what current flows through it?

    2. Relevant equations
    Kirchhoff's rules

    3. The attempt at a solution

    I said so far:
    (internal resistance)*I = 1.51
    1.49= I1*(3.5*10^3) - 1.51
    & found I1 current flowing through voltmeter = 8.6*10^-4A but i dont know what to do from here and i think this might be wrong anyway...:frown:
  2. jcsd
  3. Apr 13, 2007 #2
    Are you sure its "3.5k" ohms?
    Because that would be 3500ohms.
  4. Apr 13, 2007 #3
    If you use kirchoff rule and consider the two resistances: where v is the ideal voltage (that is under no load of the emf)

    and V-R(int)i=0.02 solve for R, is that what you got?
  5. Apr 13, 2007 #4
    i dont understand,how do you solve that equation when you dontknow what V is, do i take 1.51V as the value for V in that equation?
  6. Apr 13, 2007 #5
    yes thats the ideal voltage measured under no current. The 1.49 is with the Ir drop due to current across int resistance.
  7. Apr 13, 2007 #6
    and i just use the current value i have already calculated?
    ok i think i follow now,thanks for the help
  8. Apr 13, 2007 #7
    I'm a little unclear on your first eqn, if those don't work use the two I posted, remember its 2 unknowns and two equations tho the error from using just the 3500 Ohm resistor to compute i will likely be negligible.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook