1. COMMUNICATION GADGETS
Physics – Chapter 21
Grade 8th

2. 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.

3. Introduction
In this chapter we shall study the fundamentals of modern communication devices like radio, mobiles, television, telephone, etc.

4. 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.

5. 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.

6. 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.

7. Radio
Radio is a popular electronic device used to listen music ,speeches and weather bulletins.
A radio device involves 2 things-
1.radio transmitter
2.Radio receiver

8. 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.

9. 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 m get reflected by ionosphere.
Transmitted RF covers large distances, which a major advantage of Radio waves.

10. BLOCK DIAGRAM OF TRANSMITTER

11. 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.

12. RADIO RECIEVER BLOCK DIAGRAM

13. 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.

14. BLOCK DIAGRAM

15. 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.

16. FAX Working of FAX machine:
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.

17. 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.

18. TELEPHONE Telephone 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.

19. We know
STD: subscribers trunk dialing
ISD: international subscriber dialing
Each transmitting unit has its own code.
Eg: Bangalore’s code= 080
The code has to be dialled before dialing the number.
For further reference:

20. 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 .

21. 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.

22. MOBILE SERVICE THROUGH SATELITE

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