The Voltaic Cell option in SIR POLARITY provides a challenging change of perspective.

You, the instructor, may insert any of seven different metal – metal ion electrodes into the cell, thus creating a choice of 21 different cells, any one of which may have two directions.

All of the previous polarity links occur here, but the difference is that the cell is the source. Now the anode, which involves oxidation – electron loss –, is now pushing electrons out. So it’s negative!

You might note that the anode is always positive if the cell is the load. This is the situation usually encountered in physics courses. Transistors, diodes and discharge tubes (for example, CRT’s) are always loads. It’s only when we start looking inside the source that the apparent anomaly of negative anodes and positive cathodes arises.

This should make it easy to understand what is going on when you go to SIR VOLTA, and measure cell potentials. Here an even larger collection of electrodes is available.

You can measure concatenate networks of cells. For example, determine the potentials and polarities of Ag/Cu and Cu/Cd, then ask the class to predict what you would find in the Ag/Cd cell.

You can build up a set of electrode potentials based on, say, the cadmium cell. The potential differences on such a scale may be used to predict cell potentials just as well as those of the hydrogen scale. So the "standard" potentials are conventional; it’s the differences that are physically meaningful.

Other parts of SIR VOLTA allow you to examine the internal workings of your cells, as in SIR POLARITY. You may vary the electrolyte concentrations and discover the Nernst equation, and do such things as show that the Ag(s)/AgCl(s)/Ag⁺ electrode is just the Ag(s)/Ag⁺ electrode with the silver ion concentration reduced to the value determined by the silver chloride solubility product.

Click here for the "cell convention"

Click here for electrochemical cells

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