Chapter-3|Current Electricity| NCERT 12th Physics: Temperature Dependence of Resistivity/ Resistance: Resistivity of a metal conductor is given by  

Chapter-3|Current Electricity| NCERT 12th Physics: Colour Code for Carbon Resistors: The value of resistance used in electrical and electronic circuits vary over a wide range. These resistances are usually carbon resistances and a colour code is used to indicate the value of resistance. Carbon resisters are compact, inexpensive and are used in electronic circuits.  

Chapter-3|Current Electricity| NCERT 12th Physics: Resistivity of Various Materials: It is defined as the resistance of a unit length with unit area of cross section of the material of the conductors. Its SI unit is ohm-metre. Dimensional formula is [ML³T⁻³A⁻²].  

Chapter-3|Current Electricity| NCERT 12th Physics: Mobility: It is defined as the magnitude of drift velocity(Vd) of charge per unit electric field applied.  

Chapter-3|Current Electricity| NCERT 12th Physics:   Relation Between Drift Velocity and Electric Current: We consider a conductor of length l and A be the uniform area of cross-section. Volume of conductor V= Al If n be the number of free electron per unit volume, then Total number of free electron in conductor= nAl If e

Chapter-3|Current Electricity| NCERT 12th Physics: Drift Velocity: It is defined as the average velocity with which the free electrons in a conductor get drifted towards positive end of the conductor under the influence of an electric field applied across the conductor. It is denoted by Vd. The Drift Velocity of electron is of the order

Chapter-3|Current Electricity| NCERT 12th Physics: Limitations of Ohm’s Law: The devices which do not obey Ohm’s law are called non ohmic devices, such a vacuum tube. The relation V/I=R is valid for both ohmic and non ohmic devices. For ohmic conductors value of R is constant. For non ohmic devices, value of R is not

Chapter-3|Current Electricity| NCERT 12th Physics: Effect of Temperature on Resistance: Experimentally it was observed that on increasing temperature from T to T+dT, Resistance changes from R₀ to R₀+dR, then   For metals the value of α is positive, therefore resistance of the metal increases with rise in temperature. For insulators and semiconductors the value of α is negative, therefore

Chapter-3|Current Electricity| NCERT 12th Physics: Ohm’s Law: It is stated that the current I flowing through a conductor is always directly proportional to the potential difference V across the ends of the conductor, provided that the physical conditions (temperature, mechanical strain, etc) are kept constant. Note:  The value of a resistance of a conductor depend