Fundamental Engineering Concepts - Mathematics and Statistics for the Environment - page 255

COULOMB'S LAW

Simply put, Coulomb's law points out that the amount of attracting or repelling force that acts
between two electrically charged bodies in free space depends on two things:

• Their charges
• The distance between them

Specifically, Coulomb's law states: Charged bodies attract or repel each other with a force
that is directly proportional to the product of their charges and is inversely proportional to
the square of the distance between them .

Note: The magnitude of electric charge a body possesses is determined by the num-
ber of electrons compared with the number of protons within the body. The symbol
for the magnitude of electric charge is Q , expressed in units of coulombs (C). A
charge of 1 positive coulomb means a body contains a charge of 6.25 × 10 18 . A
charge of 1 negative coulomb (- Q ) means a body contains a charge of 6.25 × 10 18
more electrons than protons.

11.7.2 s imple e leCtriCal C irCuit
An electrical circuit includes the following components (Figure 11.17):

• 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 11.17, or some other means for producing
a voltage. The load that dissipates the energy could be a lamp, a resistor, or some other device (or
devices) 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 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

Conductor (wire)

+

Battery
(EMF)

Load
(resistor)

-

Conductor
(wire)

Control switch

FIGURE 11.17 Simple closed circuit.