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9., I3D, connection. They are and they are widely used in electrical, LEGENDS' (MGDN) STUDY MATERIAL-Il PUC PHYSICS (NCERT)-Current Electricity, L, Derive an expression for conductivity of a material in terms of relaxatio, time., ne, Derive o =, UI, PA, In the figure,, 2, is the length of the cylindrical metallic wire,, A is the area of cross-section of the wire,, V is the potential difference applied across the wire,, E is the electric field set up in the wire,, Vd is the drift velocity of the electrons,, I is the current in the wire,, We have J = o E, %3D, I, But J = –, O =, AE, But I = n A Vae.., AE, ƏPAU, Eet, But Va =-, 1., ne, In the above relations,, J is the current density, o is the conductivity of the wire, is the number of electrons per unit volume,, is the relaxation time, e is the charge of an electron,, m is the mass of an electron., NOTE, Resistor; It is a material of known resistance or It is a component of known resistance., Wire wound resistors., These are made by winding the wire of alloys such as nichrome, manganin an, constantan; They are used in resistance boxes., Carbon resistors : Theses are the resistors made up of carbon., Carbon resisters are cylindrical in shape and provided with two wires for electncal, connection. They are cheap and compact. Therefore, they are widely used in elecu, circuits. Carbon resisters are cylindrical in shape. Their length is few centimeters ae, diameter is few millimeters. Because of small size of resistor, colour code is used to ma, resistance value,