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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.