Electricity - Physical & Chemical
John S. Martin, Dept. of Chemistry, The University of Alberta
1. Coulomb's Law: like charges repel, unlike attract.
2. A DC circuit must be complete; no charge accumulation.
3. Positive and negative charge may both carry "current"; positive charge going one way is electromagnetically (if not chemically) the same as negative charge going the other way.
4. Conventional current is positive, but (see #3) it may be carried by electrons, holes, anions or cations..
5. The source maintains a positive charge at one terminal and negative at the other, so the source determines the polarity of the circuit. It pushes electrons out of its negative terminal and attracts them into its positive terminal; the opposite applies to conventional current.
6. Every load takes on the polarity of the source: positive to positive and negative to negative.
7. A meter is a load. If hooked up appropriately, positive to positive of the source, it reads positive; otherwise it reads negative (if it can).
8. Anode means oxidation and electrons released; cathode means reduction and electrons absorbed. If the cell is a load, this means anode positive and cathode negative, which is the usual physics and electronics convention. Watch out - the anode of a voltaic cell (source) is negative!
9. If you know the polarity, sense or direction of any element in a circuit, you can deduce that of every other element, using reasoning based on the principles above - in particular, Coulomb's law.
10. The cell convention (anode on the left) is a question. So, for that matter, is the convention of writing a chemical equation with a directional arrow. You are asking, "does it go this way?" a question to be answered using the Gibbs free energy change, equilibrium constant or cell potential.
Simulations and Interactive Resources for Instructors
These are interactive simulations, animations and illustrations of chemical phenomena, designed to be used in class by means of a projection device. SIR Polarity, SIR Faraday and SIR Volta are particularly useful for illustrating the principles of electrochemistry described herein. The two illustrations below are taken from SIR Polarity.
You may remove the question marks in the first display and replace them by directional symbols by clicking on them. For example, if you click on the meter you'll see its reading (positive or negative). Then you may ask for any of the other items; for example, the polarity of the source or the direction of electron flow.
Once your students are adept at the analysis of simple circuits, you may introduce them to more and more complex electrochemical ones.
In the second example, the process at the left-hand electrode has been revealed: it's losing mass. So what are the meter reading, the current direction, the directions of migration of the ions, etc.?
Of course the key is to recognize that chromium metal is being oxidized to Cr3+ ions; this releases electrons and reveals the direction of electron and current flow in the circuit.
The SIRs may be obtained from the Journal of Chemical Education: Software, University of Wisconsin-Madison, 1101 University Avenue, Madison WI USA 53706-1396. Ask for the General Chemistry CD-ROM, Sixth Edition, Special Issue No. 16. For more information check the JCES pages at the back of each issue of the Journal of Chemical Education, or visit their web site, http://jchemed.chem.wisc.edu/JCESoft.html
. You may also visit our site, http://www.fsj.ualberta.ca/chimie/lt.html which also has a link to the JCES site.