I am tasked with finding the internal resistance and voltage of a battery. I am given the following:

A simple circuit consists of a real battery and a resistor in series. When the resistor's resistance is 35-Ohms, a voltmeter reads 9V. The resistor is then swapped out for one with a resistance of 14-Ohms and the voltmeter reads 8.1V

I'm not sure how I can use this information to find the internal resistance and voltage of the battery.

Since I have values for two situations and two unknowns I think I need to relate the values in such a way that I have two equations and can then solve for the needed values. I tried using the loop rule and some of the fundamental formulas for current/resistance etc. to set up a system of equations since that's the only method I've seen that seems potentially useful but it doesn't line up correctly.

Any tips/hints?

  • 1
    $\begingroup$ Make a plot of voltage as a function of current. $\endgroup$
    – user137289
    Nov 9, 2018 at 20:40

1 Answer 1


Smells of homework. But a real battery is a ideal battery (of voltage V) and a resistor, R. The question says that you apply a load of 35 $\Omega$ (which would be in series with R) and measure 9 V across it. Then you apply a load of 14 $\Omega$ and the voltage is now 8.1 V What are the values of V & R?

  • $\begingroup$ Thats what I'm trying to figure out. I can't figure out how to relate the variables to the knows in such a way that I can solve for the former. $\endgroup$ Nov 9, 2018 at 22:45
  • $\begingroup$ Do the plot as @Pieter says. Fit a line. (or derive the equation) What's the voltage when the current is zero? That's your V. Using this V and the information in either of the results, calculate R. $\endgroup$
    – D Duck
    Nov 10, 2018 at 11:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.