Resistance is the ratio of a small potential change, dΔV across an object, to the resulting small change in current, dI through it.
Eric Murray, Fall 2006
You may wish to review the Prelab for Physics 2211 Lab #1, Statistics and Error Analysis.
Resistance is the ratio of a small potential change, dΔV across an object, to the resulting small change in current, dI through it.
R = dΔV/dI
If dΔV
looks strange to you, remember that ΔV is the potential difference across an object, so dΔV is a small change in this potential difference across an object.
If the resistance does not change when the potential changes, then the object is Ohmic, that is, it obeys Ohm's Law. Ohm's Law is essentially Resistance is constant
. In that case
R = ΔV/I or ΔV = IR
Graph (a) illustrates the equivalent relationship I = ΔV/R, to emphasize that the potential difference across an object is the cause of the current flowing through it, not the other way around.
While most of the problems that you work in class will involve Ohmic materials, this is primarily due to the fact that ΔV = IR is such a handy relationship. You should keep in mind that most objects, such as those described by graph (b), are not Ohmic.
You will examine a non-Ohmic object briefly this week. Most of your work, however, will be with resistors, which are devices specifically designed to have a constant resistance over a wide range of potential differences that may be applied across them.