Electrical Calculations - Basic Mathematics for Water and Wastewater Operators - page 133

10 Electrical Calculations

Basic electrical conversion example problems and solutions were presented in
Chapter 2. This chapter provides a more in-depth discussion of typical direct cur-
rent (DC) electrical calculations because many state licensure examinations include
basic, practical electrical problems. Alternating current (AC) is not presented here
because, for the water/wastewater operator, electrical knowledge beyond Ohm's law
and basic circuit analysis is not required. Only the very basics are presented here, as
operators who take these exams are required to perform only very basic electrical
computations. Moreover, licensure aside, many operators are required to perform
basic electrical maintenance and installations that may involve basic computations.
It should be pointed out, however, that only qualified electricians should perform
electrical work of any type.

ELECTRICAL CIRCUITS AND POWER
An electrical circuit includes

• An energy source—a source of electromotive force (emf) or voltage, such

as a battery or generator

• A conductor (wire)

• A load

• A means of control

The energy source could be a battery, as shown in Figure 10.1, or some other means
of producing a voltage. The load that dissipates the energy could be a lamp, a resistor,
or some other device that does useful work, such as an electric toaster, a power drill,
a radio, or a soldering iron. Conductors are wires that offer low resistance to current;
they connect all of the loads in the circuit to the voltage source. No electrical device
dissipates energy unless current flows through it. Because conductors, or wires, are
not perfect conductors they heat up (dissipate energy), so they are actually part of
the load. For simplicity, however, we usually think of the connecting wiring as hav-
ing no resistance, as it would be tedious to assign a very low resistance value to the
wires every time we wanted to solve a problem. Control devices might be switches,
variable resistors, circuit breakers, fuses, or relays.

o hM ' s l aW
Simply put, Ohm's law defines the relationship between current, voltage, and resis-
tance in electric circuits. Ohm's law can be expressed mathematically in three ways:

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