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 '18 at 20:40

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. $\endgroup$ – user3776749 Nov 9 '18 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 '18 at 11:10

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