Physics

Mobility (u) : The conductivity of a substance is due to its mobile charge carrier ( electrons in case of metals,  +ve and -ve ions in case of electrolyte and electron & holes in case of semiconductors) mo bilty of the charge carrier is defined as the magnitude of Drift velocity per unite electric field.     u= vd/E where vd is drift velocity and E is electric field   In case of conductor or metals  sigma =( ne)×e×(nh) In case of semiconductor  sigma =( ne) ×e× (ue) +( nh) ×e×( nh)  where (ne)  , e  & (nh)  are number of electrons per unit volume ,  electric charge & electron mobility respectively. 

page 4 relation between current & drift velocity consider a conductor of length(l)  &  cross sectional area (a)   volume of the conductor =A×l  if n be the number of free electron per unit volume then total number of free electrons=n×A×l total charge in the conductor q= n×A×l×e (due to all free electron)  now time taken by electron to cross the length (l) of the conductor    t=l/vd      where vd is drift velocity of the electrons I =q/t or I = (n×e×A×l×vd) /l or I =n×e×A×vd since n, A, e are constant. so I { vd  hence current flowing in a conductor is directly proportional to the drift velocity.  Deduction of ohm's law the drift velocity is given by     vd =(e×E×T) /m  =(e×v×T) /ml also I= n×e×A×vd  =( n×e×A×e×v×T) /ml or v/l= (m×l) /n×e^2×T×A at fixed temp. m, l, n, e, T & A are all constant for a given conductor  therefore v/l =constant=R, this is ohm's...

page 3 Drift velocity(vd) :every metal has a larger no of free electrons. they move randomly inside the metal about velocities 10^5m/s. however these velocities are destributed randomly in all directions.  there is no net direction of motion .so their average  velocity is zero. vectors(u1, u2, u3........ .. un)  are the randomly velocities of n free electrons then average velocities are     vectors(u1+u2+u3+... . +un) /n=o hence their is no net flow of electron in any direction & no current flow. when  a potential difference is applied across a conductor an electric field is set up at every point inside the conductor andeach electron experiences a force in the opposite direction of electric field and move with an acceleration. as the electrons accelerates they collide with a very small velocity(10^-5)   oppositeto electric field is called drift velocity hence drift velocity is the average velocity of the free electrons with which th...

                page 2     resistance:  it is the property of a conductor by which it oppose the flow of charge or current through the conductor . R=V/I   hence resistance of conductor is defined as the ratio of the potential difference applied across the conductor to the current.  S.I unit:  ohm  hence resistance of a  is conductor is said to be 1 ohm if 1 ampere current flowing through it after applying 1 volt potential difference.  dimension formula: [ML^2T^-3A^-2]  specific resistance or resistivity:  the resistance of a conductor is directly proportional length (l)  of conductor and inversely proportional to its cross sectional area (a).   R{l/a  or R=pl/a where p is constant called resistity or specific resistance of the material of the conductor.  it's value depend on the nature of the material of the conductor...

   unit 2 electric current & ohm's law     page1 Electric current: the rate of flow of change through any cross section of a conductor is called electric current. Electric current=charge/time  if charge q flows through a wire in time t,  then I=q/t if charge dq flows through a wire in time dt, then I=dq/dt Direction of current: As electron move lower potential to higher potential. example: water flow from a higher level to lower level. therefore conventionally, the direction of current is taken along the direction of +ve charge know as conventional current. the direction of flow of electrons gives the direction of electronic current which is opposite to the magnitude and direction. electric current has both magnitude and direction ,but it is not a vector quantity,  because law of vector addition is not valid to add the current S. I unit :  s.i unit for current is coulomb per second which is called ampere (A). ...