R = V/I is the correct definition of "resistance" for a
non-reactive two-terminal network. Non-reactive two-terminal devices that
exhibit a linear I vs V relationship obey "Ohm's Law". Resistors that you
can buy at Radio Shack are pretty good Ohm's Law devices. The best way to
determine if some other device (like a particular kind of light bulb) is
an Ohm's law device is to measure the I vs V curve.
It is important to point out to students that V = IR is not Ohm's
Law (it's just the definition of resistance). The linear I vs V curve is
Ohm's Law....
Mark (just an old radio amateur that tires of all the nit picking that
seems to pass for pedagogy these days.....)
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866