Internal resistance of a potato cell
An introduction to the concept of internal resistance, using a more interesting example than a battery.
Apparatus and materials
For each student group
Digital multimeters, 2
Leads, 4 mm, 5
Cells, 1.5 V type C, 4
Resistors of a range of values from 10 ohms to 100 ohms
Crocodile clips, 10 pairs
Health & Safety and Technical notes
Biology teachers should note the 'potato cell' in this experiment refers to a whole potato not an individual potato cell.
a Make your potato cell. Insert the copper and zinc electrodes at either end of the potato. Attach a 4 mm lead to each electrode using a crocodile clip.
b Set up the circuit as shown. Set the resistance substitution box to 4.7 kΩ. This is the load resistance. Record the current and potential difference values in a suitable table.
c Change the load resistance and record the values of current and potential difference. Repeat this process to gather data for a range of load resistances. You will have to change the range of your ammeter. Take care not to confuse amps with milliamps or microamps!
d Plot a graph of V against I. Describe the trend.
1 This experiment can be used for a number of purposes – as an introduction to the concept of internal resistance, an interesting example of internal resistance or an example of a simple cell. If standard resistors are available it is possible to vary the load resistance in smaller steps.
2 The V/I graph line will surprise students who have not been introduced to the concept of internal resistance. Those students familiar with the equation V = Emf – Ir should be able to interpret the data in terms of the internal resistance of the potato cell. However, many students find internal resistance a difficult concept and may find the experiment ‘Internal resistance of a shoe box cell’ a useful support activity.
3 If readings are entered into a spreadsheet it is easy for interested students to plot further graphs, including load resistance/ power dissipated in resistor. Such a graph will show a peak power output when the load resistance is equal to the internal resistance of the cell.
This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.
This experiment was safety-tested in June 2007