Matric Physics notes CHAPTER – 18 Electronics

Electronics 

CHAPTER – 18

Definitions

1. Electronics

Electronics is a branch of Physics, which deals with the development of electron emitting devices, their utilization and controlling electron flow in electrical circuits designed for various purposes.

2. Semi Conductor

Substances whose electrical resistance lies between those of conductors and insulators are known as semi-conductors.

3. Doping

Mixing of any tetravalent element into a trivalent or pentavalent element so that its electrical conductivity increases is called dopping.

4. n-Type Substance

A pure semiconductor with a valency of three, doped with a pentavalent element is called n-type semiconductor.

5. p-Type Substance

A pure semiconductor with a valency of three doped with a trivalent element is called n-type semiconductor.

6. Diode

The common boundary of n-type and p-type regions in a semiconductor is called p-n junction diode. It allows the current to flow in only one direction.

7. Forward Biased

If the p-type material of a semi conductor diode is at a positive potential and the n-type material is at a negative potential then the diode is forward biased. It has a very low electrical resistance.

8. Reverse Biased

If the p-type material of a semi-conductor diode is at a negative potential and the n-type material is at a positive potential then the diode is reverse biased. It has a very high electrical resistance.

9. Rectification

The process of conversion of alternating current into direct current is known as rectification.

10. Rectifier

A rectifier is a device that converts Alternating current into Direct current.

11. Transistor

A transistor is a semiconductor, which consists of a thin central layer of one type of semiconductor material sandwiched between two relatively thick pieces of the other type of semiconductor. The central part is known as the base (b) and the pieces at either side are called the emitter (e) and the collector (c).

12. npn Transistor

The npn transistor has a thin piece of p-type substance sandwiched between two pieces of n-type semiconductors.

13. pnp Transistor

The pnp transistor has a thin piece of n-type substance sandwiched between two pieces of p-type semiconductors.

Telegraph

Introduction
A telegraph is a device that is used to send and receive messages between two distant points.

Construction
An electric telegraph consists of a battery that is connected to a buzzer through the tapping key. There is only one wire between the buzzer and the tapping key. The circuit is completed by connecting the other terminal to the ground few feet below. The earth being moist acts as a good conductor.

Working
When the tapping key is pressed, the receiver produces a buzzing sound. The interval between two buzzing sounds can be controlled by the interval between pressing the tapping key. The international Morse Code, which is a combination of dots and dashes is used to send and receive messages with the help of telegraph.

Radio

Introduction
A radio is a device for receiving and sending speech or music over large areas by electromagnetic signals.

Working

1. Transmission: Information is sent out into the atmosphere from a transmitting station. When someone speaks in the microphone at the radio station, sound waves are converted into electrical fluctuating current. This current is converted into high frequency alternating current, which is allowed to pass in the transmitting antenna. The transmitting antenna produces radio waves with fluctuating amplitude. These waves are known as modulated carrier waves.

2. Receiving: When the modulated carrier waves meet a receiving aerial, they generate fluctuating alternating current in it. This AC is converted into DC with the help of a rectifier. An earphone or a speaker is connected to the receiver. The DC energizes the electromagnet of the speaker and causes the diaphragm to vibrate. This produces the sound of same frequency as that at the radio station.

Radar

Introduction
Radar stands for Radio Detection and Ranging. It is used to detect and find out the distance of distant object with the help of radio waves.

Construction
It consists of a transmitter, a receiver and several indicating devices.

Working

1. Transmission: The transmitter generates very high frequency electromagnetic waves in the desired direction with the help of a concave antenna.

2. Receiving: These rays after striking an object are reflected back and are received by the radar antenna. The antenna feeds these rays in the indicating devices.

3. Processing: The indicating devices measure the time taken by the waves to return. They calculate the wave velocity and finally the distance of the object.
Radar waves can penetrate fog, clouds, haze and smoke.

Telephone

Introduction
A telephone is a device by which two persons at distant places can directly talk to each other through electric current carrying wires.

Construction
A telephone system consists of a microphone and a receiver.

1. Microphone: The microphone consists of a diaphragm suspended in front of packing of carbon granules.

2. Receiver: The receiver has an electromagnet and a diaphragm made of magnetic alloy in front of it.

Working

1. Transmission: When someone speaks in front of the microphone, the diaphragm vibrates due to the sound waves. The compressions and rarefactions of the sound waves cause the diaphragm to increase and decrease the pressure on carbon granules. This results in the increment and decrement in the resistance offered by the granules and hence generates fluctuating current.

2. Receiving: At the receiver, the electromagnet receives fluctuating current, which generates a fluctuating magnetic field. The diaphragm in front of the electromagnet also vibrates with different amplitudes and generates sound of same frequency as spoken at the other end.

TV Camera

Working
For the purpose of TV Transmission, TV Camera focuses on object to be televised. The convex lens of the TV Camera produces an image on the thin sensitive plate known as mosaic screen. The mosaic screen is fixed in the camera and has the ability to emit electrons. When light is stronger, more electrons are given out the material making positive at this location. The beam of electrons from the electron gun in the camera tube is meant for scanning the back surface of the mosaic screen along the successive longitudinal lines in it. Special magnetic deflection system achieves this purpose. As soon as the beam hits on an area with high positive charge, few of the negative charges are repelled. If the positive charge is less, more of the electrons are emitted. After the collection of these electrons it is converted into voltage pulse known as video signal. The video signals that have been amplified are utilized to manufacture very high frequency. This frequency is received by a television antenna, which reverses the process and gives us a clear animated picture on the screen.