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مقدمه ای بر الکتریسیته بخش سوم

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Current, Voltage, and Resistance

 

All materials are composed of one or more elements. An

element is a material made up of one type of atom. Elements

are often identified by the number of protons and electrons in

one atom of the element. A hydrogen atom, for example, has

only one electron and one proton. An aluminum atom has 13

electrons and 13 protons. An atom with an equal number of

electrons and protons is electrically neutral.

 

Electrical Charges Electrons in the outer band of an atom can be easily displaced

by the application of external force. When an electron is forced

out of an atom, the atom it leaves behind has more protons

than electrons. This atom now has a positive charge. Atoms or

molecules of a material can also have an excess of electrons,

giving the material a negative charge. A positive or negative

charge is caused by an absence or excess of electrons. The

number of protons remains constant.

Attraction and Repulsion of Electric Charges

The old saying, “opposites attract,” is true when dealing with

 

Electric Charges electric charges. Charged bodies have an invisible electric field

around them. These invisible lines of force cause attraction or

repulsion. When two like-charged bodies are brought together,

their electric fields repel one body from the other. When two

unlike-charged bodies are brought together, their electric fields

The interaction of electrical charges is dependent upon

both the both the amount of each charge and the distance

between charges. The greater the amount of each charge the

more charged objects attract or repel one another. However

this interaction is inversely proportional to the square of the

distance between charges.

 

Current The flow of free electrons in a material from one atom to the

next atom in the same direction is referred to as current and

is designated by the symbol I. The amount of current flowing

is determined by the number of electrons that pass through a

cross-section of a conductor in one second.

Keep in mind that atoms are very small. It takes about 1024

 

atoms to fill one cubic centimeter of a copper conductor. This

means that trying to represent even a small value of current as

electrons would result in an extremely large number.

For this reason, current is measured in amperes, often

shortened to amps. The letter A is the symbol for amps. A

current of one amp means that in one second about 6.24 x 1018

 

electrons move through a cross-section of conductor. These

numbers are given for information only and you do not need

to be concerned with them. It is important, however, to

understand the concept of current flow.

 

attract one body to the other.

Because in practice it is common to find wide variations in the

magnitude of electrical quantities, electrical units often have

metric unit prefixes that represent powers of ten. The following

chart shows how three of these prefixes are used to represent

large and small values of current.

Direction of Current Flow Some sources distinguish between electron flow and current

flow. The conventional current flow approach ignores the flow

of electrons and states that current flows from positive to

negative. To avoid confusion, this book uses the electron flow

 

concept which states that electrons flow from negative to

positive.

Voltage The force required to make electricity flow through a conductor

is called a difference in potential, electromotive force (emf),

or voltage. Voltage is designated by the letter E or the letter V.

The unit of measurement for voltage is the volt which is also

designated by the letter V.

A voltage can be generated in various ways. A battery uses an

electrochemical process. A car’s alternator and a power plant

generator utilize a magnetic induction process. All voltage

sources share the characteristic of an excess of electrons at

one terminal and a shortage at the other terminal. This results in

a difference of potential between the two terminals.

For a direct current (DC) voltage source, the polarity of the

terminals does not change, so the resulting current constantly

flows in the same direction.

Resistance A third factor that plays a role in an electrical circuit is

 

resistance. All material impedes the flow of electrical current

to some extent. The amount of resistance depends upon

the composition, length, cross-section and temperature of

the resistive material. As a rule of thumb, the resistance of a

conductor increases with an increase of length or a decrease of

cross-section. Resistance is designated by the symbol R. The

unit of measurement for resistance is the ohm (W).

 

Resistors are devices manufactured to have a specific

resistance and are used in a circuit to limit current flow and to

reduce the voltage applied to other components. A resistor is

usually shown symbolically on an electrical drawing in one of

two ways, a zigzag line or an unfilled rectangle.

In addition to resistors, all other circuit components and the

conductors that connect components to form a circuit also have

resistance.

The basic unit for resistance is 1 ohm; however, resistance is often

expressed in multiples of the larger units shown in the following

table.

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نوشته شده در : شنبه 18 آذر 1391  توسط : جواد خانی.    نظرات() .

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شنبه 7 مرداد 1396 02:51 ب.ظ
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