COMMUNICATION GADGETSPhysics – Chapter 21

Grade 8th

Module Objectives• Understand the concept of electromagnetic

waves, audio frequency, video frequency and radiofrequency waves.

• Understand the reason of modulation.• Locate frequencies of radio stations .• Draw block diagrams of radio transmitter and

receiver.• Find similarities between radio and mobile

phones.• State advantages of internet.

Introduction• In this chapter we shall study the

fundamentals of modern communication devices like radio, mobiles, television, telephone, etc.

Electromagnetic Waves

There are light waves beyond the red and violet waves of the visible region.These waves cannot be seen by the eye.Eg: the waves of Akashvani,Bangalore are not visible to us, but they are present in the room you are sitting. They can be detected by the radio.

• Definition: • Electromagnetic waves are waves that contain an

electric field and a magnetic field and carry energy.• The electric field and the magnetic fields are

perpendicular to each other, and are also perpendicular to the direction of the wave.

• .

Types of EM waves:

• All EM waves travel with the same speed, ie. ,3 * 10^8 m/s(3 lakh km/s).

• Each EM wave has a different wavelength and frequency.

• EM waves which have very short or very long wavelengths are not visible to naked eye.

RadioRadio is a popular electronic device used to listen music ,speeches and weather bulletins.

A radio device involves 2 things-1.radio transmitter2.Radio receiver

Radio transmitter:

• Music , speeches etc are sound waves, which are produced by vibrations.

• These sound waves are converted to electrical signals. This is called Audio Frequency.

• Microphones convert vibrations to AF.• AF signals are weak. The process of strengthening

the AF signals by electrical devices is called Amplification.

• EM waves carry the strengthened AF signals. These are called the carrier waves or the Radio waves.

• RF waves are produced separately• Each AF wave has a separate carrier wave.in fact

each radio station is identified by the frequency it uses. For eg: Big FM is identified by the frequency 92.7.

• The process of superimposing AF signals on RF signal is called Modulation.

• Modulated waves are further amplified and transmitted by transmitting towers.

• RF wavelengths of the range 10-100 m get reflected by ionosphere.

• Transmitted RF covers large distances, which a major advantage of Radio waves.

BLOCK DIAGRAM OF TRANSMITTER

RADIO RECIEVER• Detector separates RF from AF signals. This

process is called Demodulation.• AF signals are amplified and fed into speaker to

reproduce original sound.• Speaker works in reverse manner of

microphone.

RADIO RECIEVER BLOCK DIAGRAM

TELEVISION

• The image of an object is made to fall on a plate called signal plate using a tv camera.

• This plate consists of many photo cells• Image is scanned as dots and lines by scanner.• Light from objects is converted to video

frequency.• VF signals are amplified and modulated on

RF signals.• Modulated waves are amplified and

transmitted through antenna.• Sound is transmitted as in radio.

BLOCK DIAGRAM

TV RECEIVER• Antenna receives the modulated wave.• The wave is demodulated.• VF signals are fed to the picture tube called

cathode ray tube which produces the original image.

• Sound is reproduced as in radio receiver.

FAX Facsimile transmission is used to send and receive all types of documents. Information is sent over the telephone line. Working of FAX machine:•Document is scanned by light.•Image is converted to electrical signal by phto cells.•Message travels through telephone line and is received by the fax machine on the other end.

• Message is decoded.• A copy of the original document is produced.• Each fax machine has a number which has to

be dialed before sending the message.

TELEPHONETelephone is 2 way cabled communication device.Principle:Sound waves are converted to electric signals by the mouth piece. Signals reach ear piece at the other end.

Cellular telephone exchange system uses computerized techniques.

We knowSTD: subscribers trunk dialingISD: international subscriber dialing

Each transmitting unit has its own code.

Eg: Bangalore’s code= 080The code has to be dialled before dialing the number.

For further reference: https://www.youtube.com/watch?v=LMuwIBez-kM

MOBILES• 2 way radio system. •It consists of both transmitter and receiver.•Consists of portable microchip(sim card) and lithium-ion rechargeable battery.•It has a display and a keypad.Working•Sound waves are converted into radio frequency waves.•These waves travel without any cable and reach the nearby station.•Base station sends radio waves which are detected by the receiver .

• Messages are also transmitted and received through the base station.

• Mobiles use roaming, which is basically interlinking of two networks.

Advantages of mobiles• Communication is possible even when people are

moving.• Communication is possible to and from land line

phones.• Voice messages and text message facilities

available.• Can be used as a mini computer.• Links entire globe by telecommunication using

geostationary satellites.

MOBILE SERVICE THROUGH SATELITE

INTERNETInternet provides access to any kind of information. Information can be downloaded and viewed.Also people can connect to other people through e-mails, social networking sites, chat groups etc.Internet also provides the option of online shopping.