Environmental Engineering Reference
In-Depth Information
I
T
= 20 A
I
1
= 10 A
I
2
= ?
R
1
R
2
100 V
FIGURE 10.23
Illustration for Example 10.19.
I
T
I
1
I
2
I
3
120 V
R
1
15 ohms
R
2
15 ohms
R
3
12 ohms
FIGURE 10.24
Illustration for Example 10.20.
I
T
=
I
1
+
I
2
I
2
=
I
T
-
I
1
= 20 amperes - 10 amperes = 10 amperes
The current in branch
R
2
, then, is 10 amperes.
■
Example 10.20
Problem:
A parallel circuit consists of two 15-ohm and one 12-ohm resistors across
a 120-volt line (see Figure 10.24). What current will flow in each branch of the circuit
and what is the total current drawn by all the resistors?
Solution:
There is a 120-volt potential across each resistor. Using Equation 10.13,
apply Ohm's law to each resistor:
V
R
120
15
I
== =
8
amperes
1
1
V
R
120
15
I
== =
8
amperes
2
2
V
R
120
12
I
== =
10
amperes
3
3
p
arallel
C
irCuit
r
esistanCe
Unlike series circuits, where total resistance (
R
T
) is the sum of the individual resis-
tances, in a parallel circuit the total resistance is
not
the sum of the individual resis-
tances. In a parallel circuit, we can use Ohm's law to find total resistance:
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