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XII CLASS PHYSICS Unit 2 TOPIC PAGE 3

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...
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XII CLASS PHYSICS UNIT 2 TOPICS

   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). ...
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Failures of ohm's law

Failures of ohm's law ohm's law is not a fundamental law .in many cases  it fails.  Ohmic conductor : The conductor which obey ohm's law are called ohmic conductor. eg. pure metals, cuso4 solution etc for ohmic conductor; 1.V-I graph is straight line and passes through origin. 2. the direction of current is reverse, if P. D(V)  is reverse. but magnitude of I remains same. 3. for a potential difference,  there is only one value of current. Non ohmic conductor : the conductors which do not obey ohm's law are called non ohmic conductors eg. junction diode, water voltmeter, vaccum tube etc for non ohmic conductor 1.graph B/w V&I is not straight line  i. e V/I not equal to constant. 2.if V-I graph is a straight line it does not passes through the origin. eg. water voltmeter. 3. current can have more value for a given potential difference i. e resistance does not have a constant value. 4.the relation B/w V-I depends upon the sign of V i. e cur...
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