IIT JEE , AIEEE Forum - Physics , Electricity - a problem with potential.... again i want to be taught something

biki

please read everything carefully to understand what i m unable to understand ...

The field inside a hollow CONDUCTING sphere is zero as there is no charge enclosed by the sphere.

Now consider the second case. Let the spherical hollow conductor be concentric with another charged non-conducting sphere of charge Q? placed inside. Now the field inside the hollow sphere is not zero as the Gaussian sphere through the empty space encloses a chrage which is not zero but equal to Q?.

Now I have seen in books like Pradeep?s Fundamental Physics ? 2

that they write the potential for a hollow conducting sphere of charge Q at different points as ?

1) outside (r>R) at distance r = kq/r

2) at (r=R) the surface = kq/R

3) inside (r<R) the sphere also = kq/R (field is zero so V=constant)

But in problems where there are two concentric spheres having radii R2 and R1 and charges Q2 and Q1 and outer one is hollow (say) and the inner one is solid (say), and we are required to find the potential of the inner sphere. What the books do is ??

They take the sum of the potentials due to charges Q1 and Q2 at the surface of the inner sphere of radius R1.

So potential at the surface of the inner sphere {by statement (2)} = k(Q1)/(R1)

But for the outer hollow sphere (radius R2 and charge Q2) {by statement (3)} they write potential at R1 due to Q2 = k(Q2)/(R2) which is in fact the potential at surface of outer sphere due to its own charge. This would have been equal to potential at R1 (inside the hollow sphere) if the field inside were zero.

How can potential at R1 be equal to potential at R2 due to the charge Q2 in this case. The field is not zero in the empty part now!!!! Is it ??? Does it not enclose a charge Q1 now ???

srini

Hey,

good illustration.lets see if i have understood what you are asking...

when we say field inside a conductor is zero (i have got this doubt a hundred times)one of my prof said we mean inside the material not necessarily the area enclosed(like one of the jee questions with cavity)

Here, they are saying the field is zero because the and equal and opposite charge is induced on the inner surface of the big sphere so using guass law..the net charge is still zero which means the field is zero,hence over here they are adding

.

srini

Hey,

you seem to have posted the same question.

biki

sorry u dint get my ques.
i also kno that field inside the material is zero for its a conductor.

tell me sumthing......
pot^n at the surface = pot^n at any point inside for a hollow conducting sphere is true becoz the field is zero inside and so pot^n is constant.

But in the given problem .....
how can the pot^n due to outer sphere at the surface of inner sphere(placed inside the outer sphere) be equal to pot^n at the surface of the outer sphere???
Here field inside (in the empty space) is not zero na ???

i m not talking about the material of the sphere ... i kno its zero there

srini

hey,

i am not sure but here since charges are induced in the inner surface...it is a conductor hence field does become zero

biki

i very well kno that charges are induced on the inner surface...

this is because when we consider a gaussian surface through the material of the hollow conductor enclosing a charge q say, we should get the total charge enclosed by the gaussain sphere = 0 as field is zero in the material of the conductor. So we take -q to be induced on the inner surface which makes field INSIDE THE MATERIAL of the hollow conductor= 0...

sister I M NOT ASKING THIS......... PLEASE READ IT CAREFULLY
especially 4rm the part where 1) 2) 3) points are written...

i m asking why the pot^n due to outer sphere at the surface of inner sphere(placed inside the outer sphere) be equal to pot^n at the surface of the outer sphere???
Here field inside (IN THE EMPTY SPACE) is not zero na ???

ramyani

Here field inside (IN THE EMPTY SPACE) is zero.

ramyani

u may luk here

http://www.goiit.com/posts/list/electricity-gaus-s-law-in-hcvsolved-example-26608.htm

priyesh

field inside the empty space is zero

bhupesh

If Electric field is not zero the potential can not remain constant from outter to inner surface . There is shell theorem which is applicable to any kind of spherical surface and with the help of which we can replACE any spherical surface bya point charge at it centre so you know how to find out the potentil at a point due to a point charge . You know supper postion priniciple

Let me know if anything left to understand.

biki

field inside is zero !!!!!!!!!! ...

howz that true??

Here, the field in the empty space inside the hollow sphere is not zero as the Gaussian sphere through the empty space encloses a chrage which is not zero but equal to Q.

ramyani

the potential inside a thin shell ( charged ) is same everywhere.

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