Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Internal resistance of a potato cell

Class practical

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 potato

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.

A circuit Photo courtesy of Mike Vetterlein


Teaching notes

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 


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