Wednesday, 1 June 2016



1.A 3000-km long cable consists of seven copper wires, each of diameter 0.73 mm, bundled together and surrounded by an insulating sheath. Calculate the resistance of the cable. Use 3x 10-6Ω. cm for the resistivity of the copper. Ans: 3.1x104 Ω

2.The resistivity of seawater is about 25Ωcm . The charge carriers are chiefly Na+ and Cl- ions, and of each there are about 3x1020 / cm3 . If we fill a plastic tube 2 meters long with seawater and connect a 12-volt battery to the electrodes at each end, what is the resulting average drift velocity of the ions, in cm/s? Ans: 2.5x10-5 cm/sec

3. Estimate the average drift speed of conduction electrons in a copper wire of cross section area 2.5 X 10-7m2 carrying a current of 2.7A. Assume the density of conduction electrons to be
9 x 10 28 m3.Ans : 7.5 x10-4 m/s
4. An electron in a hydrogen atom is considered to be revolving around the proton with a velocity 𝑒2/𝑛 in circular orbit of radius 𝑛2/𝑚𝑒2. If I is the equivalent current,express it in m,e,n (n=ℎ2𝜋)
 ANS: I = 𝑚𝑒5/2𝜋𝑛3
5 .What is the momentum acquired by free electron in a wire of length ‘l’ when current ’I’ flow in the wire? Given m=mass of electron and e = charge on electron.EstablIsh the relation between momentum and mobility ANS:. P=𝑚𝐼/𝑛𝑒𝐴
6. Two conducting wires X and Y of same diameter but different materials are joined in series across the battery . If the number density of electrons in X is twice that in Y , find the ratio of drift velocity of electrons of the two wires.
7.A potential difference V is applied across the ends of copper wire of length l and diameter D. What is the effect on drift velocity if
(i) V is doubled (ii) l is doubled (iii) D is doubled.
 Ans:(i) drift velocity doubled (ii) drift velocity halved (iii ) unchanged

8. Estimate the average drift velocity of conduction electron in copper wire of cross sexctional area 1.0 X 10-7m2 carrying the current of 1.5 A. assume the density of conduction of electron 9 X 10 28m-3
A NS:1 mm/sec.