After
reading this section you will be able to do
the following:
Identify
Ohm's law and discuss why it is important.
Calculate
the amount of electric current in a circuit using Ohm's law.
Probably the most important mathematical
relationship between voltage, current and resistance in electricity
is something called “Ohm’s Law”. A man named George
Ohm published this formula in 1827 based on his experiments with
electricity. This formula is used to calculate electrical values
so that we can design circuits and use electricity in a useful
manner. Ohm's Law is shown below.
OHM'S LAW
I
= V/R,
I = current,
V = voltage, and R =
resistance
*Depending on
what you are trying to solve we can rearrange it two other
ways.
V
= I x R
R = V/I
*All of these
variations of Ohm’s Law are mathematically equal to
one another.
Let’s look at what Ohm’s
Law tells us. In the first version of the formula, I = V/R, Ohm's
Law tells us that the electrical current in a circuit
can be calculated by dividing the voltage by the resistance. In other words, the current is directly proportional to the voltage and inversely proportional
to the resistance. So, an increase in the voltage
will increase the current as long as the resistance is held constant. Alternately, if the resistance
in a circuit is increased and the voltage does not change, the current will decrease.
The second version of the formula
tells us that the voltage can be calculated if the current and the resistance in a circuit are known. It can be seen from the equation that if either the current or the resistance is increased
in the circuit (while the other is unchanged), the voltage will also have to increase.
The third
version of the formula tells us that we can calculate the resistance in a circuit if the voltage and current are known. If the current is held constant, an increase in voltage will
result in an increase in resistance. Alternately, an increase in current
while holding the voltage constant will result in a decrease in resistance. It should be noted that Ohm's law holds true for semiconductors, but for a wide variety of materials (such as metals) the resistance is fixed and does
not depend on the amount of current or the amount of voltage.
As you can see, voltage, current,
and resistance are mathematically, as well as, physically related
to each other. We cannot deal with electricity without all three
of these properties being considered.
(The symbol for an Ohm
looks like a horseshoe and is pictured after the "100"
in the diagram above.)
Review
Ohm's
Law is used to describe the mathematical relationship between
voltage, current, and resistance.
