Vriti Edustore
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Electricity
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????
Comments (11)
In HcV, The final thing is that Vd = k E where E is the electric field and k is a constant.
Vd = a t = eEt / m is derived as follows :
Let t1 , t2 , ... tn 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
..................
vn = un + atn
Hence average velocity Vd with which an electron drifts under the electric field is given by :
Vd = ( v1 + v2 ..................vn ) / n
= [ ( u1 + at1 ) + ( u2 + at2 ) + .......( un + atn ) ] / n
= [ ( u1 + u2 + ....... un ) / n ] + a ( t1 + t2 + ....... + tn ) / n
= 0 + at
[ ( u1 + u2 + ....... un ) / 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 + ....... + tn ) / n
so, Vd = at = eEt / m
then the constant Vd = k1 E
so ultimately there is just a small difference of constant .
my teacher has been a student of hc verma and he told that the difference is that hc verma has given the explanation on the base of drift velocity theory where the average drift velocity is important!!
so both books are correct as the drift velocities they hav calculated are different!!
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
how is v=s/t
this formula can be used only if accleration is 0
i mean the full formula is s=ut+1/2at^2
s=1/2at^2 u=0
= 1/2*eE/m*t^2
v^2=u^2+2as
u=0 and a=Ee/m s =1/2*eE/m*t^2
solving we get
again
v=eE/m
i think ncert is right
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