Electric Current .
We are familiar with electric current from our very childhood .We make use of current in our day to day activities .One layman once told me current is one kind of fire flowing inside wires. Such kind of conception may be present in many of us. In order to understand what current is we have to understand the basic structure of atoms . Any type of atom contains three elementary particles . These are electron proton and neutron .Electrons and protons are charged particles .electrons are negatively charged and protons positively charged. Protons reside in the central part of atom and they are not movable .Electron circles round the central part (nucleus) in different shells .In some element electrons revolving in the outermost shells are easily detachable and hence they are moveable from one atom to another . In conductor outer most Electrons become practically free to move anywhere randomly .These electrons are called free electrons . So in conductor we have some free electrons moving randomly just like random motion of gas molecules in a container . When current flows in a conducting wire another motion known as drift motion is superimposed on random motion of free electrons due the presence of electric field supplied by battery or other e m f source .Due to drift motion electrons have drift velocity . This drift velocity is not very much only a few centimetre per second . Since electrons are negatively charged they move opposite to the direction of applied field . So to have electric current we need two things one is a substance having free electrons and an electric field across the substance (wire) Metals such as copper ,aluminium and iron etc are all substances with lot of free electrons .To get electric field we need changing magnetic field as changing magnetic field produces electric fields according to Farady's law of induction .Another way to get electric field we need static electric charges as in battery .In battery two conducting plates placed slight distance apart with a chemical substance in between them .The plates become oppositely charged resulting with a generation of electric field .The current generated with the application of electric field across a conductor is proportional to electric field . This is Ohm’s law .As the electrons flow within wire they collide with ions ,atoms and on themselves . Due to these collisions they transfer energy heating the conductor and they feel difficulty in drifting forward . So electrons face resistance as they flow inside conductor. This resistance is a property or characteristics of conductor. It depends on length , Cross sectional area and material nature and temperature etc .A good conductor has less resistance . Our body is also a good conductor of electricity as it contains various ions such as sodium ,potassium etc . When current passes through our body cells will die and death may be it’s consequent result. A conductor’s resistance increases with increase of temperature.In case of insulators resistance is infinite due absence of free electrons . Insulators such as dry wood ,plastic and glass have no free electrons to conduct electricity .But there is a substance called semiconductor that offers resistance in between insulator and conductor. In semiconductor with increase of temperature within a certain range resistance decreases .Silicon and germinium are ideal semiconductor .In modern electronic gadgets we find many application of semiconductors . As temperature is lowered to near absolute zero resistance vanishes ie conductor becomes super conductor. In super conductor current is maintained without any loss of power .Here Ohm’s law is no longer a valid law .Current can be maintained without any power source in super conducting wire
