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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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Physics unit 2 page 2 class XII

                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...
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