电气英语

1. Switches and Fuses

An electric switch is often on wall near the door of a room. Two wires lead to the

lamp in the room.

The switch is fixed in one of them. The switch can cause a break in

this wire, and them the light goes out .The switch can also join the two parts of the wire

again; then we get a light.

Switches can control many different things .Small switches control lamps and radio

sets

because

these

do

not

take

a

large

current.

Larget

switches

control

electric

fires .Other switches can control electric motors.

2. The Computer Classroom

In some school there is a computer classroom. For example, students can do their

mathematics with a computer writes questions on the screens in front of the students,

and the students answer on their keyboards.

This is part of a lesson with a girl:

Computer (writing on screen) Hello! What is your name and number?

Girl :(pressing buttons) Mary ,208

Compuer :Hello ,Mary!Look at this :

2X+4Y=8(two

x

plus

four

y

equals

eight)

2X-4Y=0(two

x

minus

four

y

equals

nought )

Girl :(after thinking)X=1 Y=2

Computer: No, try again.

Girl: X=2; Y=1

Computer: That

’

s right, Very good, Mary! Now look

…

..

The computer knows Mary .This is not their first lesson .The computer can give Mary the

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right lesson for her, neither too fast, nor too slow .And the computer can do this with

many students at the same time.

Almost Everything

A computer can do a lot of work .It can do thousands of things at high speed .For

example,

the

police

use

computers .The

computers

have

all

the

information

about

traffic, etc. The police can look at this information at any time .If a policeman finds a

car in the country and there is nobody in it or near it ,a computer can help him .Whose

car is it? Where does it come from? He can ask the computer these things. Is there any

information in the computer about the car? Yes ,somebody many miles away lost it two

weeks ago.

ications

In

the

basic

electric

sense,

the

term

“

communications

”

refers

to

the

ion

and

processing

of

information

by

electrical

means.

Radio

communication

was

made

possible

by

the

invention

and

use

the

triode.

It

has

subsequently become even more widespread and refined through the invention and use

of the transistor, integrated circuits and other semiconductor devices.

A modern communications system is first concern with the collation, processing

and storage of information before its transmission. The actual transmission then follows.

Finally we have reception.

In

long-distance

communications,

a

transmitter

is

required

to

process

the

incoming

information

so

as

to

make

it

suitable

for

transmission

and

subsequent

reception.

Constant

When a voltage is applied across the terminals of a circuit with capacitance and

resistance the voltage does not appear across the capacitor instantaneously. It take

time for the plates of a capacitor to reach their full charge.

The

time

for

the

capacitor

to

become

fully

charged

depends

on

the

product

of

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circuit resistance and capacitance. This product, RC, or resistance times capacitance,

is called the time constant of capacitive circuit. The RC time constant gives the time in

seconds

for

the

voltage

to

reach

63%(actually

63.2%)

of

its

maximum

value.

The

greater the time constant, the longer the time for the capacitor to reach its maximum

voltage.

ivity

The current density J in conductor depends on the electric field E and on the nature

of the conductor. In general, the dependence of J on E can be quite complex. For some

materials,

especially,

however,

it

can

be

represented

quite

well

by

a

direct

proportionality. For such materials the ratio of E to J is constant.

We define the resistivity P of a particular material as the ratio of electric field to

cu

P

?

E

J

That is, resistivity is the electric field per unit current density, A

“

perfect

”

conductor

would

have

zero

resistivity

,

and

a

“

perfect

”

insulator

would

have

infinite

resistivity.

Metals and alloys having the lowest resistivityies are the best conductors.

7. Series Circuit

If several electric components, such as resistors are connected so that the current is the

same in every one, the components are

said to be in a series circuit. Consider the

simple series circuit comprising the battery and three resistors. The current I result in

potential difference between the terminals of each resistor.

That is,

V

1

?

R

1

I

V

2

?

R

2

I

< /p>

V

3

?

R

3

I

Clear, the sum of these voltage is equal to the battery emf, of

V

?

V

1

?

V

2

< br>?

V

3

The equation above states that the algebraic sum of potential differences around any

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complete circuit is equal to zero. The equivalent resistance of any number of resistors

connected in series equals the sum of their individual resistance.

e, Resistance and Current

Voltage is the potential difference in an electric circuit. The opposition given by a

conductor or an insulator to the flow of electrons is called resistance.

There are two kinds of current: direct current (DC)and alternating current (AC). A

direct current is a current flowing in a conductor always in one direction. An alternating

current is a current periodically changing its direction of flow.

From ohm

’

s law we know that the current in an electric circuit is equal to the voltage

divided by the resistance. In other words, we get voltage when multiplying the current by

the resistance.

’

s Law

In 1825 Phm made the discovery that a simple correlation exists between three of

the

basic

electric

quantities,

namely:

resistance,

current

and

voltage.

In

simple

statement, the

current flowing in a circuit is directly proportional to the voltage and

inversely proportional to the resistance. This statement is generally, spoken of as Ohm

’

s

law.

It is common practice to give a shorter form to Ohm

’

s law by using letters, the letter

I meaning current, V----voltage,

and R---- resistance.

Ohm

’

s law can then by written:

I

?

V

R

This

relation

ship

is

considered

to

one

of

the

most

important

in

all

work

with

electricity.

However,

we

should

keep

in

mind

that

this

relations

true

for

metallic

conductors.

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tors

Just as good electrical conductors, such as the metal, are also good conductors of

heat, poor electrical conductors ,such as ceramic and plastic materials, are also poor

thermal

conductors.

The

free

electrons

in

a

metal

carrying

charge

in

electrical

conduction also play an important role in the conduction of heat; hence we expect a

correlation beteen electrical and thermal conductivity.

The discovery that the current density J proportional to the electric field E for a

metallic conductor at constant temperature was made by (1789---1845)and

is called Ohm

’

s law. A material obeying Ohm

’

s law is called an ohmic conductor, or a

liner conductor. If Ohm

’

s law is not obeyed, the conductor is called nonlinear.

ial Difference

To describe the situation of a charge in an electric field, the quantity

“

potential

difference

”

is introduce. The potential

V

AB

between two points A to B is defined as the

ratio of the work that must be done to take a charge q from A to B the value of q :

V

AB

?

W

AB

q

In a uniform electric field, the potential difference between two points is the product

of the field intensity and their separation in a direction parallel to that of the field. A

positive potential difference means that the energy of the charge is greater at B than at

A, while negative potential difference means that is energy less at B than at A.

and Alternating Current

The current that flows steadily in one direction is usually called a direct current. A direct

current is of course usefully. It is the kind of current which is always associated with

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batteries. We know that the electrical system in an automobile and an air plane, the

telegraph, the telephone and the trollv-bus use the direct current. Direct current is also

used to meet some of the industrial requirements.

For industry and many other purposes, however most cities make use of another type of

electric current which flows first in one direction and then in another. It was given the

name of an alternating current. We know that the alternating current is the very current

that makes radio possible.

transformer

One cannot call a transformer a machine, for it has no moving parts. We know the

transformer to be an apparatus that is designed for charging the alternating voltages

and

alternating

currents

by

means

of

magnetic

induction

without

any

change

of

frequency.

One of the great advantages of the alternating current is the ease and efficiency

with which power at low voltage may be transformed into an almost similar amount of

power at high voltage, and vice versa. Using a transformer, it is possible to transmit the

alternating current to very distant places at which the power is required.

Supply

The power supply is essential part of every electronic equipment. In it

’

s simplest

form it may consist of more than a transformer, rectifier and smoothing circuit, but

frepuently

much

more

sophisticated

arrangements

are

required,

especially

in

the

industrial field of computers, digital instruments DC amplifier,etc.

Power supply can be defined as circuits that transform electrical input power, either

AC or

DC ,into

DC ,output power.

This

definition distinguishes power supplies

from

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other

electronic

power

sources

which

are

dealt

with

elsewhere

under

the

following

heading DC to AC inverters, DC to DC converters. The term power supply is commonly

used when referring to an electronic stabilizing circuit.

onic Digital Computers

In general there are two types of digital computers. The first is the special-purpose

digital computer, which performs a fix and preset sequence of calculations. This type of

computer may be constructed more efficiently in that it can be lighter and smaller and

may consume less power,etc., than the general-purpose computer. Because of the

advantages

in

construction,

small

special-purpose

computers

are

used

where

such

factors as weight power consumption, etc.,are critical.

The

second

type

of

computer

is

defined

as

a

general-purpose

digital.

The

sequence of instructions the machine follows is generally read into this type of machine

and stored in the memory of the machine. Since the sequence of operations performed

by

the

general-purpose

digital

computer

may

be

easily

changed,

the

machine

possesses great flexibility, and this is the type of the machine generally used in business

and for scientific computations.

tance

The simplest capacitor consists of a pair of parallel metal plates separated by air or

other insulating material. When a potential difference V is placed across the plates,

each acquires a charge Q of opposite sign to the other, and an electric field E appears

between them. The magnitude of E is directly proportional to the charge Q. Because the

field magnitude E is also proportional to the difference V between the plates, the ratio

between Q and V is a constant for any capacitor. The value of this ratio for a given

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