Digital Communication

Digital Communication

A digital communication systems offers many advantages to the user, that cannot be achieved with a analog system. Digital communication system may make use of analog links and concepts.

A digital system is a more general case of a binary system. In binary systems, only two signals values can exists. They are often called 0 and 1, but these names represent specific voltages.

The term data is commonly used in digital communication systems. Data is any form of information, that has been put into digital form, so that it can be handled by a digital system. The data itself is measured as bit.(bit is a contraction of the term 'binary digit')

The binary signals are easy to generate and process with digital circuits. These digital circuits are available in the IC form and can generate and process digital data at high speeds.

Principle of Radar

Principle of Radar

Radar works on the principle of 'radio echoes'. The transmitter in a radar, radiates the high power electrical pulses into space. When these pulses are incident on any distant target such as a mountain, ship or aircraft, they get scattered in all directions. The transmitter antenna receives a part of the scattered energy. This transmitter antenna also acts as receiving antenna for receiving pulse. The pulse travels with the speed of light 3*10⁸  ms^-1.In other words, these pulse cover a distance of 300 meters for every micro second. hence by measuring the time taken by the pulse to reach the target and back to the transmitter, the range or distance of the target can be easily determined. To locate the direction of the target, directional antennas are used.

Sky Wave or Ionospheric propagation

Sky Wave or Ionospheric propagation

The ionosphere is the upper portion of the atmosphere, which absorbs large quantities of radiant energy like ultra violet rays, cosmic rays etc., from the sun, becoming heated and ionised. This ionised region contains free electrons, positive and negative ions.

Radio waves in the short wave band, radiated from an antenna at large angles with ground, travel through the atmosphere and encounters the ionised region in the upper atmosphere, Under favourable circumstances, the radiowaves get bent downwards due to refraction from the different parts of the ionised region and again reach the earth at a fer distant point. Such a radio wave is called the sky wave and such a propagation of radio wave is known as sky wave propagation or ionospheric propagation. Long distance radio communication is thus possible through the sky wave propagation.

Nuclear fusion

Nuclear fusion

Nuclear fusion is a process in which two or more lighter nuclei combine to form a heavier nucleus. the mass of the product is always less than the sum of masses of the individual lighter nuclei. The difference in mass is converted into energy. The fusion process can be carried out only at a extremely high temperature of the ode of 10⁷ K because, only at these very high temperatures the nuclei are able to overcome their mutual repulsion. Therefore before fusion, the lighter nuclei must have their temperature raised by several million degrees. The nuclear fusion reactions are known as thermo-nuclear reactions.

Colours of thin films

Colours of thin films

Everyone is familiar with the brilliant colours exhibited by a thin oil film spread on the surface of water and also by a soap bubble. These colours are due to interference between light waves reflected from the top and bottom surfaces of thin filims. When white light is incident on a thin film, the film appers coloured and the colour depends upon the thickness of the film and also the angle of incidence of the light.

Uses of Polaroid

Uses of Polaroid

1. Polaroids are used in the laboratory to produce and analyse plane polarised light.
2. Polaroids are widely used as polarising sun glasses.
3. They are used to eliminate the head light glare in motor cars.
4. They are used to improve colour contrasts in old oil paintings.
5. polaroid films are used to produce three-dimensonal moving pictures.
6. They are used as glass windows in trains and aeroplanes to control the intensity of light. In aeroplane one polaroid is fixed outside the window while the other is fitted inside which can be rotated. The intensity of light can be adjusted by rotating the inner polaroid.
7. Aerial pictures may be taken from slightly different angles and when viewed through polaroids give a better perception of depth.
8. In calculators and watches, letters and numbers are formed by liquid crystal display(LCD) through polarisation of light.
9. Polarisation is also used to study size and shape of molecules.  

Types of Crystals

Types of Crystals

Crystals like calcite, quartz, ice and tourmaline having only one optic axis are called uniaxial crystals.

Crystals like mica, topaz, selenite and aragonite having two optic axes are called biaxial crystals

Polarisation

Polarisation

The phenomena of reflection, refraction, interference, diffraction are common to both transverse waves and longitudinal waves. But the transverse nature of light waves is demonstrated only by the phenomenon of polarisation.

Fluoresence

Fluoresence

When an atomic or molecular system is excited into higher energy state by absorption of energy, it returns back to lower energy state in a time less than 10-5 second and the system is found to glow brightly by 

emitting radiation of longer wavelength.

When ultra violet is incident on certain substance, they emit visible light.

It may be noted that fluorescence exists as long as the fluorescing substance remain exposed to incident ultraviolet light and re-emission of light stops as soon as incident light is cut off. 

Fraunhofer lines

Fraunhofer lines

If the solar spectrum is closely examined, it is found that it consists of large number of dark lines. These dark lines in the solar spectrum are called Fraunhofer lines.Solar spectrum is an example of line absorption spectrum.

The central core of the sun is called photosphere which is at a very high temperature of the order of 14 million kelvin. It emits continuous spectrum. The sun's outer layers called chromosphere. this is at a comparatively lower temperature at about 6000 k. It contains various elements in gaseous state.

When light from the central core of the sun passes through sun's atmosphere ,certain wavelengths are absorbed by the elements present in the chromosphere and the spectrum is marked by dark lines.

By comparing the obsorption spectra of various substance with the Fraunhofer lines in the solar spectrum, the elements present in the sun's atmosphere have been identified.

Phosphorescence

Phosphorescence

There are some substance in which the molecules are excited by the obsorption of incident ultra violet light, and they do not return immediately to their original state. The emission of light continues even after the exciting radiation is removed. This type of delayed fluorescence is called phosphorescence.

Band obsorption spectrum

Band obsorption spectrum

If white light is allowed to pass through iodine vapour or dilute solution of blood or chlorophyll or through certain solutions of organic and inorganic compounds, dark bands on continuous bright background are obtained. The band absorption spectra are used for making dyes.

Line absorption spectrum

Line absorption spectrum

When light from the carbon arc is made to pass through sodium vapour and the examined by a spectrometer, a continuous spectrum of carbon arc with two dark lines in the yellow region is obtained.

Continuous absorption spectrum

Continuous absorption spectrum

A pure green glass plate when placed in the path of white light, absorbs everything except green and gives continuous absorption spectrum

Absorption Spectra

Absorption Spectra

When the light emitted from a source is made to pass through an absorbing material and then examined with a spectrometer, the obtained spectrum is called absorption spectrum. It is the characteristic of the absorbing substance.

               

               Absorption spectra is also of three types

            

               1. Continuous absorption spectrum

               2. Line absorption spectrum 

              

                   and

               3. Band absorption spectrum

Band spectrum

Band spectrum

It consists of a number of bright bands with a sharp edge at one end but fading out at the other end.

Band spectra are obtained from molecules. It is the characteristic of the molecule. calcium or barium salts in a bunsen flame and gases like carbon-di-oxide, ammonia, and nitrogen in molecular state in the discharge tube give band spectra. When the bands are examined with high resolving power spectrometer, each band is found to be made of a large number of fine lines, very close to each other at the sharp edge but shaped out at the other end. Using band spectra the molecular structure of the substance can be studied 

Line spectrum

Line spectrum

Line spectrum are sharp lines of definite wavelengths. It is the characteristic of the emitting substance. It is used to identify the gas. 

Atoms n the gaseous state, i.e. free excited atoms emit line spectrum. The substance in atomic state such as sodium in sodium vapour lamp, mercury in mercury vapour lamp and gases in discharge tube give line spectra. 

Continuous spectrum

Continuous spectrum

It consists of unbroken luminous bands of all wavelengths containing all the colours from violet to red. These spectra depend only on the temperature of the source and is independent of the characteristic of the source.

Incandescent solids, liquids, carbon arc, electric filament lamps etc, give continuous spectra.

Emission spectra

Emission spectra

When the light emitted directly from a source is examined with a spectrometer, the emission spectrum is obtained. Every source has its own characteristic emission spectrum.

The emission spectrum is of three types.

1. Continuous spectrum

2. Line spectrum

3. Band spectrum

Types of Spectrum

Types of Spectrum

When white light falls on a prism, placed in a spectrometer, the waves of different wavelength are deviated to different directions by the prism. The image obtained in the field of view of the telescope consists of the number of coloured images of the slit. Such an image is called a spectrum.

If the slit is illuminated with light from sodium vapour lamp, two images of the slit are obtained in the yellow region of the spectrum. These images are the emission lines of sodium have wave lengths 5896A and 5890A. This is known as spectrum of sodium.

The spectrum obtained from different bodies can be classified into two types

(i) emission spectrum

(ii)obsorption spectrum

Uses of ν-rays

ν-rays

Study of ν rays gives useful information about the nuclear structure and it is used for treatment of cancer

Uses of X-rays

X-rays

(i) X rays are used as diagonistic tools in medicine.

(ii)It is used the crystal structure in solids.

Ultra-violet radiations

Ultra-violet radiations

(i) They are used to destroy the bacteria and for sterilizing surgical instruments.

(ii)These radiations are used in detection of forged documents, finger prints in forensic laboratories.

(iii)They are used to preserve the food items.

(iv)They help to find the structure of atoms.

Visible light

Visible light

Visible light emitted or reflected from objects around us provides information about the world. The wavelength range of visible light is 4000 A to 8000A.

Infra red waves

Infra red waves

(i) Infrared lamps are used in physiotheraphy.

(ii) Infrared photographs are used in weather forecasting.

(iii) As infrared radiations are not obsorbed by air, thick fog, mist etc, they are used to take photograph of long distance objects.

(iv) Infrared obsorption spectrum is used to study the molecular structure.

Microwaves

Microwaves

Due to their short wavelengths, they are used in radar communication system. Microwave ovens are an interesting domestic application of these waves 

Radio waves

Radio waves

These waves are used in radio and television communication systems. AM band is from 530kHz. Higher frequencies up to 54 MHz are used for short waves bands.

Television waves range from 54 MHz to 890 MHz. FM band is from 88 MHz to 108 MHz. Cellular phones use radio waves in ultra high frequency (UHF) band.

Pointer type moving galvanometer

Pointer type moving galvanometer

The suspended coil galvanometers are very sensitive. They can measure current of the order of 10-8 ampere. Hence these galvanometers have to be carefully handled. So, in the laboratory, for experiments like Wheatstone's bridge, where sensitivity is not required, pointer type galvanometers are use. In this type of galvanometer, the coil is pivoted on ball bearings. A lighter aluminium pointer attached to the coil moves a scale when current is passed. The restoring couple is provided by a spring.

Electric current

Electric current

The current is defined as the rate of flow of charges across any cross sectional area of the conductor. If a net charge q passes through any cross section of a conductor in time t, then the current I=q/t, where q is in coulomb and t is in second. The current I is expressed in ampere. If the rate of flow of charge is not uniform, the current varies with time and the instantaneous value of current is given by,

i = dq/dt

Current is a scalar quantity. The direction of conventional current is taken as the direction of flow of positive charges or opposite to the direction of flow of electrons.

Applications of secondary cells

Applications of secondary cells

The secondary cells are rechargeable. They have very low internal resistance. Hence they can deliver a high current if required. They can be recharged a very large number of times without any deterioration in properties . These cells are, two wheelers, trucks etc. The state of charging these cells is, simply monitoring the specific gravity of the electrolyte. It should lie between 1.28 to 1.12 during charging and discharging respectively

Leclanche cell

Leclanche cell

A Leclanche cell consists of a carbon electrode packed in a porous pot containing manganese dioxide and charcol powder. The porous pot is immersed in a saturated solution of ammonium chloride (electrolyte) contained in an outer glass vessel. A zinc rod is immersed in electrolytic solution.

At the zinc rod, due to oxidation reaction Zn atom is converted in to Zn++  ions and 2 electrons.Zn++  ions reacting with ammonium chloride produces zinc chloride and ammonia gas.

            i.e          Zn++ ( + ) 2  NH4 Cl   ----->2  NH3    +  ZnCl 2      +  2H+  + 2e-

The ammoniagas escapes. The hydrogen ions diffuse through the pores of the porous pot and react with manganese dioxide. In this process the positive charge of hydrogen ion is transferred to carbon rod.When zinc rod and carbon rod are connected externally, the two electrons from the zinc rod move towards carbon and neutralizes the positive charge. Thus current flows from carbon to zinc.

Leclanche cell is useful for supplying intermittent current. The emf of the cell is about 1.5 V, and it can supply a current of 0.25 A.

Daniel cell

Daniel cell

Daniel cell is a primary cell which cannot supply steady current for a long time. It consists of a copper vessel containing a strong solution of copper sulphate. A zinc rod is dipped in dilute sulphuric acid contained in a porus pot is placed inside the copper sulphate solution.

A zinc rod reacting with dilute sulphuric acid produces zn++ ions and 2 electrons.

Zn++ ions pass through the pores of the porus pot and reacts with copper sulphate solution, producing cu++ ions. The cu++ ions deposit on the copper vessel. When Daniel is connected in a circuit , the two electrons on the zinc rod pass through the external circuit and reach the copper vessel thus neutralizing the copper ions. This constitutes an electric current from copper to zinc. Daniel cell produces an emf of 1.08 volt.

Right hand palm rule

Right hand palm rule

The coil is held in the right hand so that the fingers point in the direction of the current in the windings. The extended thumb, points in the direction of the magnetic field.

Secondary cells

Secondary cells

The advantage of secondary cells is that they are rechargeable. The chemical reactions that take place in secondary cells are reversible. The active materials that are used up when the cell delivers current can be reproduced by passing current through the cell in opposite direction. The chemical process of obtaining current from a secondary cell is called discharge. The process of reproducing active materials is called charging. The most common secondary cells are lead acid accumulator and alkali accumulator

Primary cell

Primary cell

The cells from which the electric energy is derived by irreversible chemical actions are called primary cells. The primary cells is capable of giving an emf, when its constituents, two electrodes and a suitable electrolyte are assembled together. The two main primary cells, namely Daniel cell and Leclanche cell. These cells cannot be recharged electrically

Electrostatic shielding

Electrostatic shielding

It is a process of isolating a certain region of space from external field. It is based on the fact that electric inside a conductor is zero

During a thunder accompanied by lighting, it is safer to sit inside a bus than i open ground or under a tree. The metal body of the bus provides electrostatic shielding, where the electric field is zero. During lightning the electric discharge passes through the body of the bus

Dielectrics

Dielectrics

A dielectric is an insulating material in which all the electrons are tightly bound to the nucleus of the atom. There are no free electrons to carry current . Ebonite, mica and oi are few example of dielectrics. The electron are not free to move under the influence of the external field 

Comparison of emf and potential difference

Comparison of emf and potential difference

1.The difference of potentials between the two terminals of a cell in an open circuit is called the electromotive force(emf) of  a cell. The difference in potentials between any two points in a closed circuit is called potential difference.

2. The emf is independent of external resistance of the circuit, whereas potential difference is proportional to the resistance between any two points.

Lenz's law

Lenz's law

The Russian scientist H.F.Lenz in 1835 discovered a simple law giving the direction of induced current produced in a circuit. Lenz's law states that the induced current produced in circuit always flows in such a direction that it opposes the change or cause that produces it.

If the coil has N number of turns and φ is the magnetic flux linked with each turn of the coil then, the total magnetic flux linked with the coil at any time is Nφ

e= -d(Nφ)/dt

e= -Ndφ/dt

= -N(φ1 - φ2)/t

Fleming’s right hand rule

Fleming’s right hand rule

The forefinger, the middle finger and the thumb of the right hand are held in the three mutually perpendicular directions. If the forefinger points along the direction of magnetic field and the thumb is along the direction of motion of the conductor, then the middle finger points in the direction of the induced current. This rule is also called generator rule.

Choke coil

Choke coil

A choke coil is an inductance coil of very small resistance used for controlling current in an a.c. circuit. If a resistance is used to control current, there is wastage of power due to joule heating effect in the resistance. On the other hand there is no dissipation of power when a current flows through a pure inductor.

Induction motors

Induction motors

Eddy currents are produced in a metallic cylinder called rotor, when it is placed in a rotating magnetic field. The eddy current initially tries to decrease the relative motion between the cylinder and the rotating magnetic field. As the magnetic field continues to rotate, the metallic cylinder is set in to rotation. These motors are used fans.

Electro magnetic brakes

Electro magnetic brakes

A metallic drum is coupled to the wheels of a train. The drum rotates along with the wheel when the train is in motion. When the brake is applied, a strong magnetic field is developed and hence, eddy currents are produced in the drum which oppose the motion of the drum. Hence, the train comes to rest.

Speedometer

Speedometer

In a speedometer, a magnet rotates according to the speed of the vehicle. The magnet rotates inside an aluminium cylinder(drum) which is held in position with the help of hair springs. Eddy currents are produced in the drum due to the rotation of the magnet and it opposes the motion of the rotating magnet. The drum inturn experiences a torque and gets deflected through a certain angle depending on the speed of the vehicle. A pointer attached to the drum moves over a calibrated scale which indicates the speed of the vehicle.

Induction furnace

Induction furnace

In an induction furnace, high temperature is produced by generating eddy currents. The material to be melted is placed in a varying magnetic field of high frequency. Hence a strong eddy current is developed inside the metal. Due to the heating effect of the current, the metal melts.

Dead beat galvanometer

Dead beat galvanometer

When current is passed through a galvanometer, the coil oscillates about its mean position before comes to rest. To bring the coil to rest immediately, the coil is wound on a metallic frame. Now, when the coil oscillates, eddy currents are set up in a metallic frame, which opposes further oscillations of the coil. This inturn enables the coil to attain its equilibrium position almost instantly. Since the oscillations of the coil die out instantaneously, the galvanometer is called dead beat galvanometer

Electric heating device

Electric heating device

Electric iron, electric heater, electric toaster are some of  the appliances that work on the principle of heating effect of current. In these appliances, Nichrome which is an alloy of nickel and chromium is used as the heating element for the following reasons.

1.      It has high specific resistance

2.      It has high melting point

3.      It is not easily oxidized