Page 193 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch06_139-176.indd Page 170 9/1/10 9:41 PM user-f465
tiL12214_ch06_139-176.indd Page 170 9/1/10 9:41 PM user-f465 /Users/user-f465/Desktop

monthly.” This is another safety device called a ground-fault

interrupter (GFI), which offers a different kind of protection

from a fatal shock. Fuses and circuit breakers are designed to
protect circuits from overheating that might occur from over-
loads or short circuits. It might occur to you that when a fuse
or circuit breaker trips, there is already a problem somewhere
in the circuit from a heavy load. If a person is in the circuit,
she or he may have already received a fatal jolt. How does a
GFI react fast enough to prevent a fatal shock? To answer that
question, we must first review a few basics about household

circuits.
A household circuit always has two wires, one that is carry-
ing the load and one that is the neutral system ground. Th e load-
carrying wire is usually black (or red), and the system ground is
FIGURE 6.40 This is the so-called “polarized” plug, with one usually white. A third wire, usually bare or green, serves as an
prong larger than the other. The larger prong can fit only in the
larger slot of the outlet, so the smaller prong (the current-carrying appliance ground. Normally, the currents in the load-carrying
wire) always goes in the smaller slot. This is a safety feature and system ground wires are the same. If a short occurs, some
that, when used correctly, results in the switch disconnecting the of the current is diverted to the appliance ground or, worse yet,

current-carrying wire rather than the ground wire. through a person with wet feet. The GFI monitors the load-

carrying and system ground wires. If any difference is detected,
the GFI trips, opening the circuit within a fraction of a second.
Have you ever noticed a red push-button on electrical This is much quicker than the regular fuse or general circuit

outlets where you live or where you have visited? Th e button breaker can react, and the difference might be enough to prevent

is usually on outlets in bathrooms or outside a building, places a fatal shock. The GFI, which can also be tripped by a line surge

where a person might become electrically grounded by stand- that might occur during a thunderstorm, is reset by pushing in
ing in water. Usually, there is also a note on the outlet to “test the red button.

SUMMARY

Th e first electrical phenomenon recognized was the charge produced A flow of electric charge is called an electric current (I). A cur-

by friction, which today is called static electricity. By the early 1900s, rent requires some device, such as a generator or battery, to maintain
the electron theory of charge was developed from studies of the atomic a potential diff erence. The device is called a voltage source. An electric

nature of matter. These studies led to the understanding that matter circuit contains (1) a voltage source, (2) a continuous path along which

is made of atoms, which are composed of negatively charged electrons the current flows, and (3) a device such as a lamp or motor where work

moving about a central nucleus, which contains positively charged pro- is done, called a voltage drop. Current (I) is measured as the rate of fl ow

tons. The two kinds of charges interact as like charges produce a repellant of charge, the quantity of charge (q) through a conductor in a period
force and unlike charges produce an attractive force. An object acquires of time (t). The unit of current in coulomb/second is called an ampere

an electric charge when it has an excess or deficiency of electrons, which or amp for short (A).

is called an electrostatic charge. Current occurs in a conductor when a potential diff erence is
A quantity of charge (q) is measured in units of coulombs (C), the applied and an electric fi eld travels through the conductor at nearly the
18

charge equivalent to the transfer of 6.24 × 10 charged particles such speed of light. The electrons drift very slowly, accelerated by the electric
as the electron. Th e fundamental charge of an electron or proton is fi eld. Th e fi eld moves the electrons in one direction in a direct current
–19
1.60 × 10 coulomb. Th e electrical forces between two charged objects (dc) and moves them back and forth in an alternating current (ac).
can be calculated from the relationship between the quantity of charge Materials have a property of opposing or reducing an electric

and the distance between two charged objects. The relationship is current called electrical resistance (R). Resistance is a ratio between the
known as Coulomb’s law. potential diff erence (V) between two points and the resulting current (I),
A charged object in an electric fi eld has electric potential energy or R = V/I. Th e unit is called the ohm (Ω), and 1.00 Ω = 1.00 volt/1.00 amp.
that is related to the charge on the object and the work done to move The relationship between voltage, current, and resistance is called

it into a field of like charge. Th e resulting electric potential diff erence Ohm’s law.

(V) is a ratio of the work done (W) to move a quantity of charge (q). Disregarding the energy lost to resistance, the work done by a voltage
In units, a joule of work done to move a coulomb of charge is called source is equal to the work accomplished in electrical devices in a circuit.
a volt. Th e rate of doing work is power, or work per unit time, P = W/t. Electrical
170 CHAPTER 6 Electricity 6-32