Drift Speed of Electrons In a wire
Two wires X and Y have the same resistivity, but their cross sectional areas in the ratio 2:3 and lengths in the ratio 1:2 They are first connected in series and then in parallel to a dc source Find out ratio of drift speeds of electrons in two wires for the two cases. case i) Series In series combination current I through each resistor will be the same Current I= neAV d n=number of electrons per unit volume e=Charge of an electron A=Area of cross section of wire V d = Drift speed V d = I / (neA) V d1 = I / (neA 1 ) --------------(1) V d2 = I / (neA 2 ) --------------(2) Note that 'ne' is same for both the wires since the resistivity same means same material From 1) and 2) V d1 / V d2 = A 2 / A 1 ie Ratio of drift speed is 3:2 case ii) Parallel In Parallel combination potential ' v 'through each resistor will be the same Current I= neAV d By ohms Law v =I R ie v = (neAV d) R V d = v / RneA and R=ΡL/A R 1 =ΡL 1 /A