1. Not finding help here? Sign up for a free 30min 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!

EMF and internal resistance

  1. Feb 28, 2017 #1
    1. The problem statement, all variables and given/known data
    I need to calculate EMF and internal resistance from the given resistance and voltage.
    R = 3 ohms
    U = 3,6 V
    EMF- ?
    I0 - ?
    r - ?

    2. Relevant equations
    I = U/R
    EMF = A/q
    I = EMF/R+r
    I0 = EMF/r

    3. The attempt at a solution
    I = 3,6/3 = 1,2 A
    I = EMF/R+r
    1,2= EMF/3+r
    ^ I hit a wall there, don't know whether to consider EMF to be the same as Voltage or not. Help please.
     
  2. jcsd
  3. Feb 28, 2017 #2

    cnh1995

    User Avatar
    Homework Helper

    Given data is insufficient. Are you sure you posted all the given quantities?
     
  4. Feb 28, 2017 #3
    There is a second measurement which is with a resistance of 5 and a voltage of 3,8. Im not sure if that helps. Also the electric circuit consists of a switch, battery, resistance and an internal voltmeter ( not sure about the internal part but it is on the inside of the circuit and not the outside).
     
  5. Mar 1, 2017 #4

    cnh1995

    User Avatar
    Homework Helper

    Could you state the exact problem statement?
     
  6. Mar 1, 2017 #5
    Circuit consisting of battery, switch, resistance and a voltmeter. We take 2 measurements. One with a resistance of 3 ohms, theres voltage of 3.6 volts. Second with a resistance of 5, theres a voltage of 3.8 volts. Calculate the EMF, internal resistance and short-circuit current(sorry i didnt mention the short circuit current before)
     
  7. Mar 1, 2017 #6

    cnh1995

    User Avatar
    Homework Helper

    Ok. Start by drawing a circuit diagram.
    Calculate current I in both the cases using Ohm's law.
    You know R and I for two measurements. You havd two unknowns Emf and r.
    Form two simultaneous equations with two unknowns from the given measurements and solve them.
     
  8. Mar 1, 2017 #7
    Do you want me to show you the diagram? I cannot show you but i have it drawn. The I in both cases are 1.2 and 0.76 ohms.
    1.2=x:(3+y)
    0.76=x:(5+y) do you mean like this? How to solve them? Do i use the sum method or something?
     
  9. Mar 1, 2017 #8
    Ok i get it now, thanks. I think i can calculate emf and r now from the equations.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: EMF and internal resistance
Loading...