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 of battery

  1. Jun 12, 2008 #1
    1. The problem statement, all variables and given/known data

    a battery with unknown internal resistance and open circuit potential (emf) is connected toa 3 ohm external resistor. the terminal potential is found to be 60 volts. when the same battery is connected to a 48 ohm resistor, the terminal potential is 96 volts. what is the internal resistance of the battery?

    2. Relevant equations

    electric potential V = IR where I is current, R is resistance

    current I = epsilon/R where epsilon is emf

    3. The attempt at a solution

    I_1 = V_1/R_1 = 60/3 = 20 ampere

    I_2 = V_2/R_2 = 96/48 = 2 ampere

    now that is as far as i got, to find R internal, do i use a similar formula to R = V/I where V and I are the change in them?

    how does the emf come into play?
  2. jcsd
  3. Jun 12, 2008 #2
    First, your analysis about the current is wrong--you never took into account the internal resistance of the battery. In actuality, the current is I = V / (R_int + R), where R is the value of the resistor and R_int is the internal resistance of the battery. Anyways, this is the wrong approach.
    The key equation is the "voltage divider" equation: V = V_bat [R/(R + R_int)]. In this case, V is the voltage, or potential difference, between a point in the circuit just above the added resistor and the ground. Now, your given two cases but there are two unknowns in each case: (1) the battery's voltage and (2) the internal resistance. Are any of these unknowns constant? If so, what can we do with our two equations obtained from both cases?
    I noticed that you posted a lot of questions about circuits. If your willing to learn, and take the time, I recommend downloading a SPICE application--with it you can build circuits for most homework problems and obtain the desired quantity and check your work accordingly. Good luck!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook