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Thursday, 27 December 2012

Fuse wire


Fuse wire
Fuse wire is an alloy of lead 37% and tin 63%. It is connected in series in an electric circuit. It has high resistance and low melting point. When large current flows through a circuit due to short circuiting, the fuse wire melts due to heating and hence the circuit becomes open. Therefore, the electric appliances are saved from damage.

Wednesday, 26 December 2012

Faraday’s law of electrolysis


Faraday’s law of electrolysis

The factor s affecting the quantities of matter liberated during the process of electrolysis were  investigated by faraday.
First Law: The mass of a substance liberated at an electrode is directly proportional to the charge passing through the electrolyte.
            If an electric current I is passed through the electrolyte for a time t, the amount of charge (q) passed is It. According to the law, mass of substance liberated (m) is
                                    M α q   or m=zit
Where Z is constant for the substance being liberated called as electrochemical equivalent. Its unit is kg C-1.
            The electrochemical equivalent of a substance is defined as the mass of substance liberated in electrolysis when one coulomb charge is passed through the electrolyte.

Second Law: The mass of a substance liberated at an electrode by a given amount of charge is proportional to the chemical equivalent of the substance.
            If E is the chemical equivalent of a substance, from the second law
                                                            mαE 

Tuesday, 18 December 2012

Potentiometer


Potentiometer
The potentiometer is an instrument used for the measurement of potential difference. It consist of a ten meter long uniform wire of manganin or constantan stretched in ten segments, each of one meter length. The segments are stretched parallel to each other on a horizontal wooden board. The ends of the wire are fixed to copper strips with binding screws. A meter scale is fixed on the board, parallel to the wire. Electrical contact with wires is established by pressing the jockey.

Wattmeter


Wattmeter
A wattmeter is an instrument used to measure electric power consumed i.e energy absorved in unit time by a circuit. The wattmeter consists of a movable coil arranged between a pairof fixed coils in the form of a solenoid. A pointer is attached to the movable coil. The free end of the pointer moves over a circular scale. When current flows through the coils, the deflection of the pointer is directly proportional to the power.

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 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 points. 

Saturday, 15 December 2012

Internal resistance of a cell


Internal resistance of a cell
The electric current in an external circuit flows from the positive terminal to the negative terminal of the cell, through different circuit elements. In order to maintain continuity, the current has to flow through the electrolyte of the cell, from its negative terminal to positive terminal.During this process of flow by the electrolyte of the cell. This is termed s the internal resistance of the cell.
A freshly prepared cell has low internal resistance and this increases with ageing.

Electric cells


Electric cells
The starting point to the development of electric cells is the classic experiment by Luige Galvani and his wife Lucia on a dissected frog hung from iron railings with brass hooks. It was observed that, whenever the leg of the frog touched the iron railings, it jumped and this led to the introduction of animal electricity. Later, Italian scientist and genius professor Alessandro Volta named after him consisted of a pile of copper and Zinc dics placed alternately separated by paper and electric bell, it continued to ring, opening a new world of electrochemical cells. His experiment established that, a cell could be made by using two dissimilar metals and a salt solution which reacts with atleast one of the metals as electrolyte.

Monday, 10 December 2012

Applications of superconductors


Applications of superconductors
(i)                 Superconductors form the basis of energy saving power systems, namely the superconducting generators, which are smaller in size and weight, in comparison with conventional generators.
(ii)               Superconducting magnets have been used to levitate trains above its rails. They can be driven at high speed with minimal expenditure of energy.
(iii)             Superconducting magnetic propulsion systems may be used to launch satellites into orbits directly from the earth without the use of rockets.
(iv)             High efficiency ore-separating machines may be built using superconducting magnets which can be used to separate tumor cells from healthy cells by high gradient magnetic separation method.
(v)               Since the current in a suprconducting wire can flow without any change in magnitude it can be used for transmission lines.
       (vi)      Superconductors can be used as memory or storage elements in computers

Supeconductivity


                                                     Supeconductivity

             Ordinary conductors of electricity become better conductor at lower temperatures. The ability of certain metals, their compounds and alloys to conduct electricity with zero resistance at very low temperatures is superconductivity. The materials which exibit his property are called superconductors.
            The phenomenon of superconductivity was first observed by Kammerlingh Onnes in 1911. He found that mercury suddenly showed zero resistance at 4.2K. The first theoretical explaination of superconductivity was given by Bardeen, Cooper and Schrieffer in 195 and it is called the BCS theory.
            The temperature at which electrical resistivity of material suddenly drops to zero and the material changes from normal conductor to superconductor is called the transition temperature or critical temperature Tc. At the transition temperature the following changes are observed:
(i)                 The electrical resistivity drops to zero.
(ii)               The conductivity becomes infinity
(iii)             The magnetic flux lines are excluded from the maerial.

Current Electricity


Current Electricity

The branch of physics deals with the study of motion of electric charges is called
current electricity. In  an charged metallic conductor at rest, some (not all) electrons are continually moving randomly through the conductor because they are very loosely attached to the nuclei. The thermodynamic internal energy of the materials sufficient to liberate the outer electrons from individual atoms,enabling the electrons to travel through the material. But the net flow of charge at any point is zero. Hence, there is zero current. The external energy necessary to drive the free electrons in a definite direction is called electromotive force (emf). The emf is not a force ,but it is the work done in moving a unit charge from one end to the other. The flow of free electrons in a conductor constitutes electric current 

Thursday, 6 December 2012

Relation between electric field and potential


Relation between electric field and potential

Let the small distance between A and B be dx. Work done in moving a unit positive charge from A to B is dV=E.dx.
         The work has to be done against the force of repulsion in moving unit positive charge towards the charge +q. Hence,
dV= -E.dx
E= -dV/dx
         The change of potential with distance is known as potential dradient, hence the electric field is equal to the negative gradient of potential.
         The negative sign indicates that he potential decreaces in the direction of electric field.The unit of electric intensity an also be expressed as Vm-1

Electric Potential


Electric Potential

Let a charge q be placed at a point O. A and B are two points in the electric field. When a unit positive charge is moved from A to B against the electric force, work is done. This work is the potential difference between these two points.
         The potential difference between two points in an electric field is defined as the amount of work done in moving a unit positive charge from one point to the other against the electric force.
         The unit potential difference is volt. The potential difference between two points is 1 volt if 1 joule of work is done in moving 1 coulomb of charge from one point to another against the electric force.
         The electric potential in an electric field at a point is defined as the amount of work done in moving unit positive charge from infinity to that point against the electric forces.

Wednesday, 7 November 2012

Lightning conductor


Lightning conductor

           This is a simple device used to protect tall buildings from the lightning.

It consists of a long thick copper rod passing through the building to ground. The lower end of the rod is connected to a copper plate buried deeply into the ground. A metal plate with number of spikes is connected to the top end of the copper rod and kept at the top of the building.

            When a negatively charged cloud passes over the building, positive charge will be induced on the pointed conductor. The positively charged sharp points will ionize the air in the vicinity. This will partly neutralize the negative charge of the cloud, thereby lowering the potential of the cloud.  The negative charges that are attracted to the conductor travels down to the earth. Thereby preventing the lightning stroke from the damage of the building

Tuesday, 6 November 2012

Van de Graaff Generator


Van de Graff Generator



           In 1929 Robert J. Van de Graff designed an electrostatic machine which produces large 
           electrostatic potential difference of the order of the order of 107 V

                       The working of Van De Graff Generator is based on the principle of electrostatic 
           induction and action of points.
           
                      A hollow metallic sphere A is mounted on insulating pillars as shown in fig. A pulley  B 
          is mounted at the centre of the sphere and another pulley C is mounted near the bottom. A belt
          made of silk moves over the pulleys. The pulley C is driven continuously by an electric motor.
          Two comb-shaped conductors D and E having number of needles, are mounted near the pulleys. 
          The comb D is maintained at a positive of the order of 104  is connected to the inner side of the 
           hollow metal sphere.

                      Because of the high electric field near the comb D, the air gets ionised due to action of points,
           the negative charges in air move towards the needles and positive charges are repelled on towards the
          belt, moves up and reaches near the comb E.

                      As a result of electrostatic induction, the comb E acquires negative charge and the sphere 
           acquires positive charge. The acquired positive charge is distributed on the outer surface of the sphere.
           The high electric field at the comb E ionize the air. Hence, negative charges are repelled to the belt,  
          neutralises the positive charge on the belt passes over the pulley. Hence the descending belt will
          be left uncharged.
                    
                      Thus the machine , continuously transfer the positive charge to the sphere. As a result, the 
          potential of the sphere keeps increasing till it attains a limiting value(maximum). After this stage
          on more charge can be placed on the sphere, it stars leaking to the surrounding due to ionisation
          of the air

                      The leakage of charge from the sphere can be reduced by enclosing it in a gas filled steel
          chamber at very high pressure

                      The high voltage produced in this generator can be used to accelerate positive ions ( protons,
          deuterons) for the purpose of nuclear disintegration

                              

Monday, 5 November 2012


Microwave oven

            It is used to cook the food in a short time. When the oven is operated, the microwaves 
are generated, which in turn produce a non-uniform oscillating electric field. The water molecules in the food which are the electric dipoles are excited by an oscillating torque. Hence few bonds in the water molecules are broken, and heat energy is produced. This is used to cook food.

Electric lines of force


Electric lines of force

The concept of field lines was introduced by Michel Faraday as an aid in visualizing electric and magnetic fields.
Electric line of force is an imaginary straight or curved path along which a unit positive charge tends to move in an electric field.
The electric field due to simple arrangements of point charges 

Properties of lines of forces for charges:

1.    Lines of force start from positive charge and terminate at negative charge.

2.    Lines of force never intersect.

3.    The tangent to a line of force at any point gives the direction of the electric field (E) at that point.

4.    The number of lines per unit area, through a plane at right angles to the lines, is proportional to the magnitude of E. This means that, where the lines of force are close together, E is large and where they are far apart, E is small.

5.    Each unit positive charge give rise to 1/Є0 lines of force in free space. Hence number of lines of force originating from a point charge q is N=q/ Є0 in free space.



Basic properties of electric charges


Basic properties of electric charges

Quantisation of electric charge:
            The fundamental unit of electric charge (e) is the charge carried by the electron and its unit is coulomb. E has the magnitude 1.6* 10-19 C
            In nature, the electric charge of any system is always an integral multiple of the least amount of charge. It means that the quantity can take only one of the discrete set of values. The charge, q=ne where n is an integer

Conservation of electric charge:
            Electric charges can neither be created nor destroyed. According to the law of conservation of electric charges, the total charge in an isolated system always remains constant. But the charges can be transferred from one part of the system to another, such that the total charge always remains conserved. For example, Uranium(92 U 238) can decay by emitting an alpha particle(2He4 nucleus) and transforming to thorium      (90  Th 234)
                       
                        92U238 --------------> 90Th234 + 2He4

Total charge before decay= +92e, total charge after decay = 90e =2e. Hence, the total charge is conserved. i.e. it remains constant.

Additive nature of charge:
            The total electric charge of a system is equal to the algebraic sum of electric charges located in the system. For example, if two charged bodies of charge +2q, -5q are brought in contact, the total charge of the system is -3q

Sunday, 4 November 2012

Two kinds of charges



Two kinds of charges


        If the glass is rubbed with a silk cloth, it acquires positive charge while the silk cloth acquires an equal amount of negative charge.

        If an ebonite rod is rubbed with fur, it becomes negatively charged, while the fur acquires equal amount of positive charge. This classification of positive and negative charges was termed by American scientist, Benjamin Franklin.

        Thus, charging a rod by rubbing does not create electricity, but simply transfers or redistributes the charges in a material.

Electrostatics


Electrostatics

Electrostatics is the branch of Physics, Which deals with static electric charges or charges at rest. The charges in a electrostatic field are analogous to masses in a gravitational field. These charges have forces acting on them and hence posses potential energy. The ideas are widely used in many branches of electricity and in the theory of atom

Electrostatics – Frictional electricity

              In 600 B.C., Thales, a Greek Philosopher observed that, when a piece of amber is rubbed with fur, it acquires the property of attracting light objects like bits of paper. In the 17th century, William Gilbert discovered that, glass, ebonite etc, also exhibit this property, when rubbed with suitable materials.

             The substances which acquire charges on rubbing are said to be ‘electrified’ or charged. These terms are derived from the Greek word electron, meaning amber. The electricity produced by friction is called frictional electricity. If the charges in a body do not move, then, the frictional electricity is also known as Static Electricity

Friday, 2 November 2012

Resistance & conductivity


          
           Resistance & conductivity


The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend up on the material out of which the wire is made. Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as


                                                      R=(ρL)/A

                                                      ρ=Resistivity
                                                      L=Length
                                                      A=Cross sectional area

The factor in the resistance which takes in to account the nature of the material is the resistivity. Although it is temperature dependent, it can be used at a given temperature to calculate the resistance   of the wire of given geometry.

The inverse of resistivity is called conductivity. There are contexts where the use of conductivity is more      convenient

                              Electrical Conductivity= σ =(1/ρ)                                                                                                                                                                                

Wednesday, 31 October 2012

How to memorize color code of resistor

How to memorize color code?

         Students can easily memorize the color code by easy  method

                                                           "B B ROY Great Britain Very Good Wife"

             
B        
Black
0
B
Brown
1
R
Red
2
O  
Orange
3
Y  
Yellow  
4
Great
Green
5
Britain
Blue
6
Very
Violet
7
Good
Grey
8
Wife
White
9

   

Tuesday, 30 October 2012

Color Code Table

         
     
                          Color Code Table

Color
First Band digit
Second Band digit
Third Band multiplier
Tolerance
 Black
0
0
100=1

Brown
1
1
101=10
1%
Red
2
2
102=100
2%
Orange
3
3
103=1000
3%
Yellow
4
4
104=10000
4%
Green
5
5
105=100000

Blue
6
6
106=1000000

Violet
7
7
107=10000000

Grey
8
8
108=100000000

White
9
9
109=1000000000

Gold
-
-
-
5%
Silver
-
-
-
10%
None
-
-
-
20%