__WORKSHEET#8 CURRENT ELECTRICITY__

__DRIFT VELOCITY,MOBILITY,RESITIVITY & NUMBER DENSITY__
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: 3.1x10**^{4 }**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 𝑒

^{2/}𝑛 in circular orbit of radius 𝑛^{2/}𝑚𝑒^{2}. If I is the equivalent current,express it in m,e,n (n=ℎ2𝜋)**ANS: I =**

**𝑚𝑒**

^{5/}2𝜋𝑛^{3}
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.

**ANS:1:2**

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

^{-7}m^{2}carrying the current of 1.5 A. assume the density of conduction of electron 9 X 10^{28}m^{-3}**A NS:1 mm/sec.**