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Saturday 25 February 2017

Analogue And Digital Electronics

18:46 0

The quantities whose values vary continuously or remain,constant are know as analogue quantities.For example,the temperature of air varies continuous fashion during 24 hours of a day.If we plot a graph between time and temperature recorded at different times,we get a graph.This graph shows that temperature varies continuously with time.Therefore,we can say that temperature is an analogue quantity.Similarly,Time,pressure,distance,etc are analogue quantities.

 The branch of electronics consisting of circuits which process analogue quantities is called analogue electronics.For instant,the public address system is an analogue system in which microphone converts audio or sound into a continuously varying electric potential.This potential is an analogue signal  which is fed into amplifier.Amplifier is and analogue circuit which amplifies signal without changing its shape to such and extent that it can operate a loudspeaker.In this way loud sound is produced by the speaker.Radios,televisions,and telephones are few common devices that process analogue signals. 

 The quantities whose values vary in non-continuous manner are called digital quantities.Digital quantities are expressed in the form of digits or numbers.The branch of electronics which deals with digital quantities is called digital electronics.Digital electronics uses only two digits '0' (zero) and '1' (one) and the whole data is provided in binary form due to which processing of data becomes easy.
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Conductors And Insulators

06:55 0

Conductors:-

 Why do we always use metal wires for conduction of electricity?
Because,they are good conductors of electricity and offer less resistance to the flow of current.But how can they conduct electricity with much easy? Metals like silver and copper have excess amount of free electrons which are not held strongly with any particular atom of metals.These free electrons move randomly in all directions in the metal.When we apply an external electric field these electrons can easily move in a specific direction.This movement of free electrons in particular direction under influence of an external field cause the flow of current in metal wires.The resistance of conductor increases with increase in temperature.This is due to increase in the numbers of collisions of electrons with themselves and with the atoms of metals. 

 Insulators:-

 All materials contain electrons.The electrons in insulator like rubber,however,are not free to move.They are tightly bound inside atoms.Hence current cannot flow though an insulator because there are no free electrons for the flow of current.Insulators have very large value of resistance.Insulators can be easily charged by friction and induced charge remain static on their surface.Other examples of insulators are glass,wood,plastic,fur,silk etc.
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Fuse

00:41 0

A fuse is a safety device that is connected in series with the live wire in the circuit to protect the equipment when excess current flows.It is short and thin piece of metal wire that melts when large current passes through it.If a large,unsafe current passes through the circuit,the fuse melts and break the circuit before the wires become very hot and cause fire.

 Fuse are normally rated as 5 A,10 A,13 A,30 A,etc.

 Following safety measures should be taken while using fuses in household electrical circuits.

 (1) Fuses to be used should have slightly more rating than the current which the electrical appliance will draw under normal condition.For example,for lightning circuit choose a 5 A fuse as the current draw my each lamp is very small about 0.4 A for a 100 W lamp.In such circuit,10 lamps of 100 W can be safely used as the total current drawn is only 4 A which can be calculated using formula P=VI

 (2) Fuses should be connected in live wire so that the appliance will not operate after the fuse has blown.

 (3) Switch OFF the main before changing any fuse. 
Different Types Of Fuse:-


 
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Friday 24 February 2017

Hazards Of Electricity

23:30 0

While electricity becomes part and parcel of our lives,care should be taken to save ourselves from its hazardous effect.Voltage of 50 V and current of 50 mA can be fatal.Major danger of electricity are electric shock and fire.Here we discuss some faults in electrical circuits that may cause electricity hazards.

 Insulation Damage:-
All electrical wires are well insulated with some plastic cover for the safety purpose.But when the electrical current exceed the rated current carrying capacity of the conductor,it can produce excess current that can damage insulation due to overheating of cables.This reason into a short circuit which can severely damage electrical device or persons.

 A short circuit occurs when a current with very small resistance is formed.The low resistance cause the current to be very large.When large appliances are connected in parallel,each addition appliance placed in the circuit reduce the equivalent resistance in the circuit and increases the current through the wires.This addition current might produce enough thermal energy to melt the wiring's insulator which cause a short circuit,or even starts a fire.

 Short circuit can also occurs when the live wire and the neutral wires come in direct contact.

 In order to avoid such situations,the wires carrying electricity should never be naked.Rather they should be covered with good insulator.Such an insulator covered wire is called cable.Constant friction may also remove the insulation from the wire whereas too much moisture also damages the insulation.In such a situation,it is advisable to use cable with two layer of insulation.

 Damp Conditions:-
Dry skin has a resistance of 100,00 ohms or more! But under damp condition (wet environment) resistance of human skin is reduced drastically to few hundred ohms.Therefore,never operate any electrical appliance with wet hands.Also keep the switches,plugs,sockets and wires dry.
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Investigating The properties Of Electrons

19:28 0
An electron gun is used to investigate the properties of electron beam.The electrons are produced by the thermionic emission from a tungsten filament heated by 6V supply.A high positive potential is applied to cylindrical anode (+).The electrons are accelerated to a high speed and pass through the hole of the anode in the form of a fine beam of electrons.The whole set up is fitted in an evacuated glass bulb.

Deflection Of Electrons By Electric Field:-
We can set up electric field by applying a potential difference across two parallel metal plates placed horizontally separated by some distance.When an electron beam passes between the two plates ,it can be seen that the electrons are deflected toward the positive plate.The reason for this is that electrons are attracted by the positive charges and are repelled by the negative charges due to force F=qE,where 'q' is the electron charge and 'E' is the electron field due to plates.The degree of deflection of electrons from their original direction is proportional to the strength of the electric field applied.

 Deflection Of Electrons By Magnetic Field:-
Now we apply magnetic field at right angle to the beam of electrons by using a horseshoe magnet.We will notice that the spot of the electrons beam on the screen is getting deflected from its original direction.Now change the direction of horseshoe magnet.We will see that spot on the fluorescent screen is getting deflected in the opposite direction.  
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Thermionic Emission

18:52 0

In the 1850's physicists started to examine the passage of electricity through a vacuum by putting two electrodes in a sealed vacuum tube.Some kind of rays were emitted from the cathode or the negative electrode.These rays were called cathode rays.J.J Thormson in 1897 observed the deflection of cathode rays by both electric and magnetic fields.From these deflection experiments,he concluded that cathode rays must carry a negative charge.These negative charged particles were given the name electrons.

 The process of emission of electrons from the hot metal surface is called thermionic emission. Metals contain a large number of free electrons.At room temperature electrons can not escape the metal surface due to attractive forces of nucleus.If the metal is heated to a high temperature,some of the free electrons may gain sufficient energy to escape the metal surface. 

 Thermionic emission can also be produced by electrically heating a fine tungsten filament.Typically values of voltage and current used are 6V And 0.3A respectively.
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The Measurement Of Current

07:58 0
For this purpose we use different electrical instruments to detect the current in the circuit.Galvanometer and ammeter are some common examples of current measuring instruments.
Galvanometer is very sensitive instrument and can detect small current in a circuit.A current of few milliamperes is sufficient to cause full scale deflection in it.While making the connections polarity of the terminals of galvanometer should be taken into consideration.Generally the terminal of galvanometer with red colour shows the negative polarity while that of with black colour shows the negative polarity.An ideal galvanometer should have very small resistance to pass maximum current in the circuit.

 After the suitable modification galvanometer can be converted into an ammeter.A large current of the range such as 1A or 10A can be measured by mean of ammeter.Like galvanometer,ammeter is also connected in series,so the current flowing in the circuit also passes through the ammeter.

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Conventional Current

07:38 0
Before the idea of free electrons which constitute current in metal,it was through that current in conductors flow due to the motion of positive charges.Therefore this convention is still in use.We can understand the concept of the conventional current from the following analogies.

 We know that when the end of heated copper wire are at different temperature heat energy flow from the end higher temperature to the end at lower temperature.Similarly when a conductor is connected to the battery,it pushes charge to flow current from higher potential to the lower potential.The flow of current continues as long as there is a potential difference.

Conventional Current Is Defined As:
CURRENT FLOWING FROM POSITIVE TO NEGATIVE TERMINAL OF BATTERY DUE TO FLOW OF POSITIVE CHARGES IS CALLED CONVENTIONAL CURRENT.

 Conventional current produces the same effect as the current flowing from negative terminal to the positive terminal due to flow of negative charges.
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Thursday 23 February 2017

Electric Current

22:53 0

Most of the electric current around us us bound in neutral atoms.It is not easy to overcome the electrostatic force of attraction between the nuclei and electrons in at atom.How,ever in metals some electrons are not tightly bond to the nuclei and are free to move around randomly.They have weak force between them and nucleus.Similarly in solutions some positive and negative charges can freely move around randomly.When such free charges are exposed to an external electric field,they move in specific direction and thus constitute current.

 Electric current is produce due to the flow of either positive charge or negative charge or both of charges at same time.In metals the current is produced only due to the flow of free electrons I.E negative charges.In case of electrolyte its molecules in aqueous solution dissociate among positive and negative ions.So the current in electrolyte is produced due to the flow of bot positive and negative charges.

 "THE RATE OF FLOW OF ELECTRIC CHARGE THROUGH ANY CROSS SECTIONAL AREA IS CALLED CURRENT"

 If the Q is passing through any cross sectional area in time t,then current I flowing through it will be given by
CURRENT=CHARGE/TIME

I=Q/T
SL unit of current is Amperes (A).

 If a charge of one coulomb passes through a cross sectional area in one second then the current is one ampere.Smaller units of current are Milli Amperes (mA),Micro Ampere (uA),which are defined as:
1mA=10^-3 A

1uf=10^-6 A
Battery is one of the source of current.The electrochemical reactions inside a battery separates positive and negative electric charges.The separation of charges setup potential difference between the terminals of the battery.When we connect a conducting wire across the terminals of battery,the charges can move from one terminal to the other due to the potential difference.The chemical energy of the battery changes to electrical potential energy.The electrical potential energy decreases as the charges move around the circuit.This electrical potential energy can be converted to other useful forms of energy such as Heat Energy,Light Energy,And Sound Energy.It is only the energy which changes form but the number of charge carries and the charge on each carrier always remain the same i.e charge carries are not used up.Instead of electrical potential energy we use the term electric potential which is potential energy per unit charge.
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