Question

When finding Electric Field and Electric potentials of concentric spheres, we usually mark the charge on the inner sphere on the outher sphere .So when we are going to find the Electric potential on a point in between the two spheres , Do we have to take the charge of the inner sphere that we previously marked on the outer spehere?​

Answer 1

Answer:

Application of Gauss law of electrostatics: Electric Field Due To Two Thin Concentric Spherical Shells

Let us again discuss another application of Gauss law of electrostatics that is Electric Field Due To Two Thin Concentric Spherical Shells:-

Consider charges +q1 and +q2 uniformly distributed over the surfaces of two thin concentric metallic spherical shells of radii R1 and R2­ respectively

In order to determine the electric field E at a point P, distant r from the centre O by using Gauss’s law ,draw a concentric Gaussian spherical surface of radius r through P. Due to symmetric distribution of charge,the mgnitude of E at all points on the Gaussian surface will be same and radially outward in direction (Try to make figure yourself).

Thus, for any area element dS taken on the gaussian surface,the field vector E and area vector dS both are parallel, therefore,

E.dS=EdS cos 00=EdS

Hence, the flux through the entire Gaussian surface will be

Φ=∫E.dS=∫EdS=E∫dS

But ∫dS=4πr2

Φ=E(4πr2) (1)

According to Gauss’s law for electrostatics

Φ=q/ε0 (2)

By comparing equation (1) and (2),we get

E(4πr2)=q/ε0

Or E=q/4πε0r2 (3)

Three cases arises here:-

(i) Electric Field at a Point Inside the inner Shell of Two Thin Concentric Spherical Shells(r<R1­):-

In this case there would be no charge within the Gaussian surface.Therefore,according to Gauss ‘s law q=0.Thus,E=0 at a point inside the inner shell.

(ii) Electric Field at a Point Between the two Shells of Two Thin Concentric Spherical Shells(R1<r<R2):-

In this case,consider that the point P at which the electric field is to be determined lies between R1 and R2.Therefore, the net charge enclosed by the Gaussian sphere is only the charge residing on inner shell,that is q1 alone. Therefore, from equation (3)

E= q1­/4πε0r2

(iii) Electric Field at a point outside the outer shell of Two Thin Concentric Spherical Shells:-

In this situation ,consider that the point P lies outside the outer shell at a distance r from the centre O,such that r>R2.In this case the net charges enclosed by the Gaussian surface will be the sum of charges on both shells,that is,

Q=q1+q2

Thus from equation(3),

E=q1+q2/4πε0r2

If two spherical shells have charges equal and opposite that is, if one shell has +q charge and the other have =q .In this situaion,inside the inner spherical shell and outside the outer shell the electric field is zero.