If the voltage were unknown and we
knew the current and resistance: I = 0.5 A,
and R = 12 ohms, then:
E = I × R or 0.5 A × 12 Ω = 6 V
If the resistance were unknown and the
voltage and current were given as: I = 0.5 A,
E = 6 volts, then:
R = or = 12 Ω
If you have difficulty remembering
this equation in its three forms, the sim-
ple memory device shown in Figure 4-4
may help.
Place your finger over the unknown
quantity and observe what it equals. For
example: Put your finger over E, the
answer is I × R. Put your finger over I, the
answer is:
Put your finger over R, the answer is:
The purpose of the memory device is
to make it easier to remember how to use
Ohm’s law. The best way to learn Ohm’s
law is to practice its use.
You must remember that when using
Ohm’s law, E, I, and R must be in volts,
amperes, and ohms, respectively. Study
Figures 4-5 and 4-6. Frequently current is
given in milliamperes, which is:
of an ampere or 0.001 A
You must convert to amperes before
using the equation. Studying the following
examples will help you to do this:
1 ampere = 1000 milliamperes
0.5 ampere = 500 mA
0.1 amp = 100 mA
50 mA = 0.05 amp
500 mA = 0.5 amp
10 mA = 0.01 amp
1 mA = 0.001 amp
1
1000
E
I
E
R
6 V
0.5 A
E
I
Chapter 4 Ohm’s Law
37
Georg Simon Ohm (1787–1854)
Ohm was educated at the University
of Erlangen and became a professor of
physics at Munich in 1849. Ohm devel-
oped the law for which he is best known.
However, it should be noted that he
received little acknowledgment for this
achievement for 20 years. He also did
much pioneering research on the human
ear and how different sounds or frequen-
cies are broken down by the different
parts of the inner ear.
History Hit!
–
+
E = 6 volts R = 12 ohms
Figure 4-3.
The current equals 0.5 amperes.
E
I R
Figure 4-4.
A memory device for Ohm’s law.