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I know that the electric field inside a perfect conductor is zero as all the charges resides on its surface to minimize its energy but if it's true then why we are still getting the electric field inside a solid sphere (having radius $R$) $E\propto r$, where $r$ is the radius of the Gaussian sphere inside the solid sphere $(i.e. r < R)$?

Why it is not zero?

Please Help me!!

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    $\begingroup$ You should include some references or try to explain you question better. If the solid sphere is a conductor you cannot have electric field inside of it. The reason for this is that, if there was any electric field, the charges inside the sphere would move. This would keep going until a static situation is achieved. This is not true if the sphere is non-conducting. $\endgroup$ Commented Jan 3, 2021 at 12:08

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(a) On a macroscopic scale (that is considering only electric fields averaged over distances spanning many atoms) the electric field in a conductor in electrostatic conditions is zero everywhere. This must be the case because if there were a resultant electric field it would exert forces on the mobile charge carriers and they would be moving, that is we wouldn't have electrostatic conditions!

(b) The solid sphere in which E is radial and its magnitude is proportional to $r$ is not a conducting sphere, but a sphere made of a non-conducting material with a uniform charge density (carrying a uniform charge per unit volume).

(c) This electric field would force charge to the surface of the sphere, if the non-conducting sphere could be made to conduct, so all the charge would finish up on the surface, and there would be zero charge and zero field inside the sphere. [We could deduce the zero field inside by using Gauss's law and the symmetry of the surface charge distribution, but it is not necessary to use this argument, as we know from (a) above that, once there is no more movement of charge there will be no field inside the conductor.]

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The electric field inside a conductor is 0 (at static equilibrium) because the charges are free to move (delocalised electrons). Although charges still want to minimize electric potential energy in insulators, they are unable to move, so an electric field can be maintained.

For a non-conducting sphere with a uniform charge density, the electric field inside is proportional to radius. This can be shown by either gauss' law or newton's shell theorem.

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As you mentioned The electric field inside a conducting sphere is zero as the charges are present on the surface. So when you consider a gaussian surface inside the sphere, it does not enclose any charge. But in case of a solid sphere the gaussian surface does enclose charge due to which electric field is present.

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