Group 5 Slide

8/2/2019 Group 5 Slide

1/30

Nguyn Quang MinhLi ng Cng

Nguyn Nht nNguynVTin AnhPhm Vn TrV nh Nht

Group 5

34.5 PRODUCTION OF

ELECTROMAGNETIC

WAVES BY AN ANTENNA


2/30

One cycle in the production of an electro-magnetic wave by anoscillating electric dipole antenna. The red arrows represent

the E field. (B not shown.)

Electron acceleration emit electromagnectic wave


3/30

A pair of metal rods connected to a battery.(a) When the switch is open and no current exists, theelectric and magnetic fieldsare both zero.(b) Immediately after the switch is closed, the rods are being charged (so a currentexists). Because thecurrent is changing, the rods generatechanging electric andmagnetic fields.(c) When the rods are fully charged, thecurrent is zero, theelectric field is a maximum, and the magnetic field is zero.


4/30

Production ofE/M Waves by an Antenna This is a half-wave antenna

Two conducting rods are

connected to a source ofalternating voltage

Thelength ofeach rod isone-quarter of thewavelength of the radiation

to beemitted


5/30

Production ofE/M Waves by an Antenna

The oscillator forces thecharges to accelerate betweenthe two rods

The antenna can be approximated by an oscillating

electric dipole The magnetic field lines form concentriccircles around

the antenna and are perpendicular to theelectric fieldlines at all points

Theelectric and magnetic fields are 90o out of phase atall times

This dipoleenergy dies out quickly as you move awayfrom the antenna


6/30

Production ofem Waves by anAntenna, final

The source of the radiation found far from theantenna is thecontinuous induction of an electricfield by the time-varying magnetic field and the

induction of a magnetic field by a time-varyingelectric field

Theelectric and magnetic field produced in thismanner are in phase with each other and vary as 1/r

The result is the outward flow ofenergy at all times


7/30


8/30

34.6. Waves of the Electromagnetic Spectrum

Nature of Electromagnetic Waves

They areTransverse waves without a medium. (Theycan travel through empty space)

They travel as vibrations in electrical and magnetic

fields. Have some magnetic and someelectrical properties to

them.

Speed ofelectromagnetic waves = 300,000,000

meters/second (Takes light 8 minutes to move from thesun to earth {150 million miles} at this speed.)


9/30

When an electric field changes, so does the

magnetic field. Thechanging magnetic field causestheelectric field to change. When one fieldvibratesso does the other.

RESULT-An electromagnetic wave.


10/30

Electromagnetic Spectrumname for the range ofelectromagnetic

waves when placed in order of increasing frequency

RADIO

WAVES

MICROWAVES

INFRARED RAYS

VISIBLE LIGHT

ULTRAVIOLET RAYS

X-RAYS

GAMMA

RAYS


11/30

RADIO WAVES

A. Have thelongest wavelengths and lowestfrequencies of all theelectromagnetic waves.

B. A radio picks up radio waves through an antenna andconverts it to sound waves.

C. Each radio station in an area broadcasts at adifferent frequency. # on radio dial tells frequency.

D. MRI (MAGNETIC RESONACE IMAGING) Uses Short wave radio waves with a magnet to create an

image


12/30

MRI of the Brain


13/30

AM=Amplitude modulationwaves bounce off ionospherecan pick up stations fromdifferent cities.

(535kHz-1605kHz= vibrate at 535 to 1605 thousand times/second)

+


14/30

FM=Frequency modulationwaves travel in a straight line & through the ionosphere--losereception when you travel out of range.

(88MHz-108MHz = vibrate at 88million to 108million times/second)

+


15/30

Bands of Radio/TV/Microwaves


16/30

MICROWAVES

Microwaveshave the shortest wavelengths andthe highest frequency of theradio waves.

Used in microwave ovens.

Waves transfer energy to the water in the food causing themto vibrate which in turn transfers energy in the form of heatto the food.

Used by cell phones and pagers.

RADAR (Radio Detection and Ranging) Used to find the speed of an object by sending out radio

waves and measuring the time it takes them to return.


17/30

INFRARED RAYS Infrared= below red Shorter wavelength and higher frequency than

microwaves. You can feel thelongest ones as warmth on your skin Heat lamps give off infrared waves. Warm objects give off more heat energy than cool

objects. Thermograma picture that shows regions of different

temperatures in the body. Temperatures arecalculatedby the amount of infrared radiation given off. Thereforepeople give off infrared rays.


18/30


19/30

VISIBLE LIGHT

Shorter wavelength and higher frequency thaninfrared rays.

Electromagnetic waves wecan see.

Longest wavelength= red light Shortest wavelength= violet (purple) light

When light enters a new medium it bends

(refracts). Each wavelength bends a differentamount allowing whitelight to separate into itsvarious colors ROYGBIV.


20/30


21/30

Ultraviolet waves


22/30

2. Application

- Medicine: Ultraviolet waves areeffective

in killing bacteria and viruses, sterilize

equipment, water and air in operating

rooms, treat acne =))

- Sciences: Scientists can study the formation

of stars, about the structure and evolution

of galaxies, about the make up and energy

levels of atoms


23/30

- Forensic: UV is an investigative tool at thecrime scene helpful in locating andidentifying bodily fluids, ejaculated fluids or saliva are detected by high-power UV

- Note: wearing sunglass that do not block UV light is worse for your eyes than

wearing no glass, if you want to wear sunglass, you should buy a high-qualitysunglass or wearing nothing.

3. Ozone(O3)

- Most of the UV light from the Sun

is absorbed by ozone (O3) molecules

in theEarths upper atmosphere, in

a layer called the stratosphere. This

ozone shield converts lethal high-

energy UV radiation to infrared

radiation, which in turn warms the

stratosphere.

- Ozone hole is result of ozone

depletion


24/30

X-rays


25/30

X-rays

1. Introduction

Wavelength: from 10-8 m to 10-12 m.

Source :X-rays was created whenever high-energy

electrons suddenly gave up energy

2. CreateMachines producex-rays by accelerating electrons to extremely high speeds andthen crashing them into a piece of solid materialcalled a target. Theelectronsrapidly slowed down because they collide with atoms in the target, and part oftheenergy is changed into x-rays


26/30

3. Application

- Medicine:X-rays are used in

medicine for medical analysis.

Dentists use them to find

complications, cavities and

impacted teeth. Soft body tissue

are transparent to the waves. Bones also block the rays.

- X-rays are also used in the study of crystal structure

because x-ray wave-lengths are comparable to the atomic separationdistances in solids

- Note :Becausex-rays damage or destroy living tissues and organisms, care must betaken to avoid unnecessary exposure or overexposure


27/30

Gamma rays


28/30

1. Introduction

- Gamma rays areelectromagnetic waves emitted by radioactive nuclei (such as Co

and Cs) and during certain nuclear reactions.- Wavelength: from 10-10 m to 10-14 m


29/30

2. Applications

- Medicine:Gamma rays are used in medicine to kill and treat certain types of

cancers and tumors. Gamma rays passing through tissue of the body produceionization in tissue.

Note: Gamma rays can harm thecells in our body. The rays can also detect brainand cardiovascular abnormalities.

- Science: Gamma rays can be used to detect

beryllium. They also played a very important

role in the development of the atom bomb

- Industry: Gamma rays can be used to examine metalliccastings or welds in oilpipelines for weak points. The rays pass through the metal and darken aphotographic film at places opposite the weak points.


30/30

Group 5 Slide

Documents