internal resistance revision

Earlier in unit 2, you worked in groups to complete an experiment on internal resistance.  Listed as “Method 2″ on page 8 your printed notes, you built a simple series circuit using a cell, a resistance box and an ammeter.  A voltmeter was connected across the resistance box and you recorded the voltage across (TPD) & current through the resistor as you changed the resistance from 0.5Ω to 1.5Ω in steps of 0.1Ω.  The aim was to draw a graph of your data and use this plot to determine the internal resistance of the cell.

The video below shows the same type of experiment, but uses a potato and two different metals in place of a normal cell.  Watch the video and note the values of I and V each time the resistance is changed – remember to pause the video each time so you can write the results.  Just scroll 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, there is a sheet available to download using the button at the end of this post.  Or you could try printing out a sheet from a graph paper site, use Excel or download the free Calc spreadsheet.

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 units from microamps (uA) to amps (A).

The gradient of your straight line will be a negative number. The gradient is equal to -r, where 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 is equal to the EMF of the cell. (Explanation: on the y-axis, I is zero so TPD = EMF)
  • 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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