Page 332 - NS-2 Textbook

Basic Electricity

TI,e study of electricity began with the ancient Greeks. pensive. Electrical energy is conveyed at the speed of
They discovered that by rubbing a mineral called amber light through conductors by the free electrons. As the
with a cloth, they could create a mysterious force of at- elech'ical energy passes, each electron moves a very short
traction between the cloth and the amber. They also ob- distance to the neighboring atom, where it replaces one
served that after they rubbed two different ambers with or more electrons by forcing them out of their orbits. The
two different cloths, the two cloths would repel one an- replaced electrons repeat the process in other nearby
other-as strongly as they were ath'acted to amber. TI,ese atoms.
forces were called electric (from the Greek word for Some substances have velY fevv free electrons and
amber), and the cloths and ambers were said to be elec- are therefore poor conductors. These substances} such as
trically climged. rubbel; glass, or dlY wood, are called illsulators. Good
Although the Greeks discovered electric force, they conductors such as \vire are used to cany elech"icityJ and
could not explain it. In fact, it was not until the atomic they are covered by insulating material to prevent the
theory of matter was developed that the true cause of electricity from being diverted from the conductors.
electricity was found. "When scientists discovered that
atoms were composed of negatively charged particles
(elech'ons) that orbit around positively charged particles VOLTAGE
(protons), they could explain electrical charge. Normally
The force that causes electricity to move in a conductor is
there is a balance behveen the negative charge of elec-
called voltage or electmlllotive force (E). TI,ere are six basic
trons and the positive charge of protons. Therefore,
\JI,Tays to generate voltage:
under most conditions, an atom will have no charge. But
if the number of electrons is increased, the atom will be- 1. Frictioll. Voltage can be produced by rubbing
come negatively charged. On the other hand, if electrons hvo materials together. Static electricity is the
are taken mvay, the atom will have a positive charge. most common name for this type. It occurs fre-
Charged atoms are called iOlls. quently in dry climates or on days of low hu-
One of the fundamental laws of electricity is that like midity.
charges repel each other, and unlike charges attract each 2. Pressure. Voltage can be produced by squeezing
other. In the atom, the electrons are held in their orbit by crystals such as natural quartz 01; more usually,
the attractive force between them and the protons in the manufactured crystals. Compressed electrons
nucleus. In the Greeks' experiments with ambel; the tend to move through a crystal at predictable fre-
cloth picked up electrons from the amber, thus becoming quencies. Crystals are frequently used in com-
negatively charged. This left the amber with a positive mlulications equipnlent.
charge-and unlike charges attract one another. 3. Heat. Voltage can be produced by heating the
place where two lmlike metals are joined. The
hot junction ·where the moving electrons from
CONDUCTORS AND INSULATORS
the two different metals meet is called a therlllo-
An electric charge can move through a material if it has a couple. The difference in the temperature of the
large number of free electrolls-that is, electrons that can two metals determines the amOlmt of voltage.
easily move from atom to atom in the material. Sub- As a result, thermocouples are often used to
stances that permit the free motion of a large number of measure and regulate telnperature} as in a ther-
electrons because of their atomic struchu€ are called COll- nl0stat.
ductal'S. Silver, copper, and aluminwn 'v ire, in that order, 4. Light. Voltage can be produced when light strikes
are the best conductors. But copper and ahunlnum ,viTe a photosensitive (light-sensitive) substance. The
are the most commonly used because they are least ex- light dislodges electrons from their orbits around

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