measuring internal resistance

Warning: I am expecting you to do more than just read this text.  Please do try plotting the graph and finding the properties of the cell.

We did an experiment in class (it is listed as “Method 2″ in your printed notes) where you built a simple series circuit using a cell, a variable resistor and an ammeter.  A voltmeter was connected across the series resistor and you recorded the voltage across (TPD) & current through the resistor as you changed its resistance.

The video below shows the same type of experiment, but uses a potato and two different metals in place of normal cell.  Watch the video and note the values of I and V each time the resistance is changed – remember you can pause the video or go back if you miss any.

Now plot a graph with current along the x-axis and TPD along the y-axis.  If you don’t have any sheets of graph paper handy, try printing out a sheet from this graph paper website or use Excel or the free OpenOffice.org Calc spreadsheet.  You have probably guessed that I prefer to draw graphs by hand, though.

Draw a best-fit straight line for the points on your graph and find the gradient of the line.  When calculating gradient, remember to convert the current value from microamps (uA) to amps (A).

The gradient of your straight line will be a negative number. The gradient is equal to -r, where r is the internal resistance of the potato cell used in the video.

You can obtain other important information from this graph…

  • Extend your best fit line so that it touches the y-axis.  The value of the TPD where the line touches the y-axis (on the y-axis, I is zero so TPD = EMF) is equal to the EMF of the cell.
  • Now extend the best-fit line so that it touches the x-axis, the current at that point is the short-circuit current – this is the maximum current that the potato cell can provide when the variable resistor is removed from the circuit altogether and replaced with just a wire.

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