A persistent problem in learning introductory chemistry is the apparent discrepancy between "physics electricity" and "chemistry electricity". To the student, "physics electricity" is a matter of volts, positive current, watts, ohms, positive anodes and negative cathodes. "Chemistry electricity" is negative electrons, volts, ions, anodes meaning oxidation and cathodes meaning reduction – and sometimes anodes are positive, sometimes negative. This induces a classic case of cognitive dissonance, and students generally respond in the time-honoured way – they memorize just enough to pass the exam and then forget it.

SIR POLARITY and SIR VOLTA allow you to address this problem. Only one physical principle is required - Coulomb’s law, in the form "like charges repel, opposites attract". With this and a few quite reasonable definitions all can be resolved.

Here is the "Circuit Polarity" section of SIR POLRITY. It shows the basic elements of a DC circuit. Each has two possible polarities, and all the polarities are linked. Initially all the indicators of polarity are masked by question marks. A simple click removes each.

Start with the source. An electric source controls the circuit. It maintains its polarity (plus and minus) whatever current is drawn from it. So what do electrons do? Being negative, they are repelled out of the negative terminal and attracted into the positive one.

For positive current (whatever that may be) the exact opposite is true. So electrons and "current" always go in opposite directions.

(If you dig into Maxwell’s governing equations of electromagnetism, you find that current is the product of charge and velocity, so positive charge going north has exactly the same effect as negative charge going south.)

Now, every load takes on the polarity of the source – positive to positive and negative to negative. (You can bring in impedance here if you wish – but at your peril.)

A meter is treated as a load, so it reasonably
reads positive if its polarity is positive to
positive and negative to negative. If it’s hooked up the other way, it reasonably should read negative, and it does.

A useful drill here is to reset the circuit, which will give you random, concealed source and meter polarities, then reveal any one and ask students what any of the others should be. For example, from the meter reading, deduce the electron flow. A few trials should produce confidence – and that’s all you need to understand the rest of electrochemistry!

Click to apply these ideas to electrochemical cells

Click to see how to discover the law of chemical equilibrium

Click here to return to the home page