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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)