Chapter 21
Ohm’s Law
For many materials and devices (conductors and resisitors, for example), it is found that the greater the
potential difference placed across the device, the greater the resulting current. This is calledOhm’s law, and
may be stated as
VDIR; (21.1)whereVis the potential difference (in volts),Iis the current (in amperes), andRis the resistance (in ohms).
Ohm’s law, like Hooke’s law, is an example of what is called anempirical law: something that is found to
be at least approximately correct in many situations, but is not necessarily always true. This is an important
point: Ohm’s law is not always true! It is just something that is found to work for many things like conductors
and resistors. Ohm’s law doesnotapply in some cases: lamp filaments, diodes, and solar cells, for example.
Such devices are said to benon-ohmic.
Ohm’s law may be considered the most important principle in the analysis of electric circuits. You’ll use
it over and over again as we learn to analyze electric circuits.
21.1 Electric Power
The electric powerPconsumed by a resistor is given by
PDIV; (21.2)wherePis in watts. What this specifically refers to is the rate at which electrical energy is converted to heat.
Commercially made resistors come in several standard power ratings (e.g.^1 / 8 W,^1 / 4 W,^1 / 2 W, 1 W). When
building a circuit, you have to make sure that the product of the current through and voltage across a resistor
does not exceed its power rating.
Using Ohm’s law (Eq. (21.1)) we can write the electric power (Eq. (21.2)) in several equivalent forms:
PDIVDI^2 RD
V^2
R
: (21.3)
You can use any of these to compute the power consumed by a resistor; which one you use depends on which
quantities you know:I,V,orR.