Electrical Calculations - Basic Mathematics for Water and Wastewater Operators - page 149

To find total current:

E
R

T
T

I =

T

PARALLEL DC CIRCUITS
The principles we applied to solving simple series circuit calculations for determin-
ing the reactions of such quantities as voltage, current, and resistance can be used in
parallel and series-parallel circuits.

p arallel C irCuit C haraCteristiCs
A parallel circuit is defined as one having two or more components connected across
the same voltage source (see Figure 10.18). Recall that a series circuit has only one
path for current flow. As additional loads (resistors, etc.) are added to the circuit,
the total resistance increases and the total current decreases. This is not the case
in a parallel circuit. In a parallel circuit, each load (or branch) is connected directly
across the voltage source. In Figure 10.18, commencing at the voltage source ( E b ) and
tracing counterclockwise around the circuit, two complete and separate paths can
be identified in which current can flow. One path is traced from the source through
resistance R 1 and back to the source, the other from the source through resistance R 2
and back to the source.

v oltage in p arallel C irCuits
Recall that in a series circuit the source voltage divides proportionately across each
resistor in the circuit. In a parallel circuit (see Figure 10.18), the same voltage is pres-
ent across all of the resistors of a parallel group. This voltage is equal to the applied
voltage ( E b ) and can be expressed in equation form as

E b = E R 1 = E R 2 = E Rn

+

E b

R 1 R 2

-

Path 1

Path 2

FIGURE 10.18 Basic parallel circuit.