ncert----pg-98 gives value of drift speed as   

whereas h c verma---pg 173 gives value of drift speed as  

where t = avg  time b/w 2 successive collisions

             E = electric fd intensity

            e = charge on electron

            m= mass of one electron

which one is correct????

v've been told dat d 1st one is correct n its explanation has been given in resnick halliday krane...

if u want d expln then temme......i'll post it

In HcV, The final thing is that V_d = k E  where E is the electric field and k is a constant.

V_d = a t = eEt / m is derived as follows :

Let t₁ , t₂ , ...   t_n  be avearge time intervals between 2 consecutive collisions suffered by a free electron. Velocities acquired by various electrons, moving with an accln   a  are :  

v₁ = u₁ + at₁

v₂ = u₂ + at₂

v₂ = u₂ + at₂ 

..................

v_n = u_n + at_n

Hence average velocity V_d with which an electron drifts under the electric field is given by :

V_d = ( v₁ + v₂ ..................v_n ) / n

=  [ ( u₁ + at₁ ) + ( u₂ + at₂  ) + .......( u_n + at_n ) ] / n

=  [ (  u₁  +  u₂ + ....... u_n  ) / n ]  +  a ( t₁  + t₂   + ....... + t_n ) / n

= 0 + at

[ (  u₁  +  u₂ + ....... u_n  ) / n ] = 0 comes frm the fact that the velocities r random in the beginning i,e, there is no net velocity of electron in one direction.

where, t is the average time interval between 2 consecutive collisions

                                                                = ( t₁  + t₂   + ....... + t_n ) / n   

so, V_d = at = eEt / m

 then the constant V_d = k₁ E

so ultimately there is just a small difference of constant .

hc verma gives.......



s = 1/2 a t *t    ( u = 0 ; as already given above by ramyani.......)



s= dist bw 2 collisons



a = acc= eE/m



t= avg time b/w collisons



s = 1/2 (eE/m)t*t



Vd= s/t



Vd= 1/2 (eE/m)t

nd  iz it rite 2 ques de credibility of such a reputed book as hc verma  ????????????

@ islahul_101



"both books are correct as the drift velocities they hav calculated are different!! "





ddn relly got diz....................