差距英文Electromagnetic field

Electromagnetic field

The

electromagnetic

field

is

a

physical

field

produced

by

electrically

charged

objects.

It

affects

the

behavior

of

charged

objects

in

the

vicinity

of

the

field

.The

electromagnetic

field

extends

indefinitely

throughout

space

and

describes

the

electromagnetic

interaction. It

is

one

of

the four fundamental forces in the

nature (the others are gravitation, the weak interaction, and the

strong interaction).

The field can be viewed as the combination of an electric field and a magnetic field. The

electric

field

is

produced

by

stationary

charges,

and

the

magnetic

field

by

moving

charges(currents),an

electric

field

will

exist

even

when

there

is

no

current

flowing.

If

current

flowing. If current does flow, the strength of the magnetic field will vary with power consumption

but the electric field strength will be constant. These two are often described as the sources of the

field. The way in which charges and currents interact with the

electromagnetic field is described

by Masxwell

’

s equations and the Lorentz force law.

Electromagnetic

fields

are

present

everywhere

in

our

environment

but

are

invisible

to

the

human eye. Electric fields are produced by the local build-up of electric charges in the atmosphere

associated with thunderstorms. The earth

’

s magnetic field causes a compass needle to orient in a

North-South direction and is used by birds and fish for navigation.

Besides

natural

sources

the

electromagnetic

spectrum

also

includes

fields

generated

by

human-made sources: X-rays are employed to diagnose a broken limb after a sport accident. The

electricity

that

comes

out

of

every

power socket

has

associated

low

frequency

electromagnetic

fields.

And

various

kinds

of

higher

frequency

radiowaves

are

used

to

transmit

information-whether via TV antennas, radio stations or mobile phone base stations.

One

of

the

main

characteristics

which

defines

an

electromagnetic

field

EMF)

is

its

frequency or its corresponding wavelength. Fields of different frequencies interact with the body

in

different

ways.

One can

imagine

electromagnetic

waves

as

series

of

very regular

waves

that

travel

at

an

enormous

speed,

the

speed

of

light. The

frequency

simply

describes

the

number

of

oscillations or cycles per second, while the term wavelength describes the

distance between one

wave and the next. Hence wavelength and frequency are inseparably

intertwined: the higher the

frequency the shorter the wavelength.

A

simple analogy should help to illustrate the concept: Tie a long rope to a door handle and

keep hold of the free end. Moving

it up and then down slowly will generate

a single big wave,

more rapid motion will

generate a whole series of small waves . The length of the rope remains

constant,

therefore,

the

more

waves

you

generate

(higher

frequency)

the

smaller

will

be

the

distance between them (shorter wavelength).

Electric fields exist whenever a positive or negative electrical charge is present. They exert

forces on other charges within the field. The strength of the electric field is measured in

volts per

metre(V/m). Any electrical wire that is charged will produce an associate electric field.

This field

exists even when there is no current flowing. The higher the voltage, the stronger the electric field

at a given distance from the wire.

Electric

fields

are

strongest

close

to

a

charge

or

charged

conductor,

and

their

strength

rapidly diminishes with distance from it.

Conductors such as metal shield them very effectively.

Other materials such as building materials and trees,

provide some shielding capability. Therefore

the electric fields from power lines outside the house are reduced by walls, buildings, and trees.