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Electricity

Blazing goIITian

 Joined: 20 Oct 2007 Post: 360
14 Sep 2008 18:59:00 IST
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11
999
doubt regarding value of drift speed
Engineering Entrance , Medical Entrance , AIPMT , JEE Main , AIIMS , JEE Advanced , Physics , 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????

Blazing goIITian

Joined: 20 Oct 2007
Posts: 360
15 Sep 2008 11:37:54 IST
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nd da worst part iz dat both de books hav given their respcetive proofs ! !

plzz help...........

Hot goIITian

Joined: 6 Jan 2008
Posts: 171
15 Sep 2008 19:32:54 IST
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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

Blazing goIITian

Joined: 22 Apr 2007
Posts: 2537
16 Sep 2008 00:44:09 IST
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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 .

Blazing goIITian

Joined: 20 Oct 2007
Posts: 360
16 Sep 2008 09:16:41 IST
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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

Blazing goIITian

Joined: 20 Oct 2007
Posts: 360
16 Sep 2008 09:17:50 IST
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nd  iz it rite 2 ques de credibility of such a reputed book as hc verma  ????????????

Blazing goIITian

Joined: 20 Oct 2007
Posts: 360
16 Sep 2008 09:25:59 IST
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@ ramyani...........

cant v put it diz way..........

Vd = avg drift speed.........

= (inital speed + final speed) /2

= (0 +eEt / m) /2

=1/2 (eE/m)t

= wot mr h c verma thinks....

Cool goIITian

Joined: 28 Apr 2008
Posts: 58
16 Sep 2008 10:14:14 IST
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puja is right..
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!!

Blazing goIITian

Joined: 20 Oct 2007
Posts: 360
16 Sep 2008 16:38:36 IST
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@ islahul_101

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

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

Blazing goIITian

Joined: 21 Dec 2007
Posts: 1025
16 Sep 2008 20:27:12 IST
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in the proof given by u here that is

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

Blazing goIITian

Joined: 21 Dec 2007
Posts: 1025
16 Sep 2008 20:29:33 IST
2 people liked this

in the proof given by u here that is

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

Blazing goIITian

Joined: 20 Oct 2007
Posts: 360
16 Sep 2008 20:38:21 IST
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@ ganesha

bingo......

i think mr hc verma dinks differently here

ncert must b correct.....

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