current (I), and resistance (R). A thorough
understanding of the use of Ohm’s law will
help you to understand how any circuit
operates.
If you do not completely understand
Chapter 2, perhaps now is a good time to
review it once again. You will remember
that an electric current was caused to flow
in a conductor when a force or voltage was
applied to the circuit. Figure 4-1 shows a
simple circuit using a battery as a voltage
or potential difference source.
R represents the resistance in the circuit
and I stands for “intensity” of the current.
E or V represents electromotive force.
As the voltage of battery (B) is fixed
and the resistance of the circuit is fixed, a
definite value of current will flow in the
circuit. (Note the direction of current flow
as indicated by the arrows.)
If the voltage were increased to twice
the value, as in Figure 4-2, then the current
would also increase to twice its former
value. As the voltage increases, the current
increases. As the voltage decreases, the
current decreases. A mathematician would
say that the current and voltage are in direct
proportion to each other.
The current flowing in these circuits
also depends on the resistance of the
circuit. If we increase the resistance to twice
its value, the current is cut in half. We may
conclude that as the resistance increases,
the current decreases. As the resistance
decreases, the current increases. Again,
mathematically speaking, the current is in
inverse proportion to the resistance.
Georg Simon Ohm, the German
scientist, proved this relationship to be true
in his experiments. The law is named in his
honor. Ohm’s law is stated as:
I =
where,
I = current in amperes
E = voltage in volts
R = resistance in ohms
By simple algebra, the formula may be
changed to read:
R = or E = IR
One may readily see that if any two
quantities are known in a circuit, the
third quantity can be found. Referring to
Figure 4-3, notice that values have been
assigned to E and R.
The current is easily computed by
Ohm’s law:
I = or I = = 0.5 A
6 V
12 Ω
E
R
E
I
E
R
36
Electricity
–
+
Source
B
(Battery)
Connecting wire
E = 6 volts
Control
SW (Switch)
Load
R
(Resistor)
Figure 4-1.
A simple electrical circuit. Note connection of
elements that make up circuit.
–
+
B
E = 12 volts R
Figure 4-2.
As voltage is increased, the current increases.
Math Manipulation!