Not only should we think of Ohm's law as delta-V = IR, we should also
remember that the real "meat and potatoes" of Ohm's law is that R
should be constant. Or, better stated, that current through a device
is a linear function of the potential difference across the device.
If the current through device is not a linear function of the potential
difference, then we say the device is non-ohmic... it does not follow
Ohm's law. This sometimes doesn't make sense to students (and
teachers) because they interpret Ohm's law as an algebraic equation,
and the algebra still works for non-ohmic devices simply by calculating
a new resistance for each measured potential difference and current.
But Ohm's law is not simply an algebraic equation. It is a statement
that current is a linear function of potential difference. Materials
and devices for which that is true (or nearly true) are called ohmic,
and those for which it is not true are called non-ohmic.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817