1) Why are the charges on a metal sphere concentrated on the surface ? Why is the electric field inside a metal sphere zero ?

2) Why is the charge on a sphere assumed to be concentrated at the centre ? How is it correct theoretically ?

3) Why is the electric potential inside a charged metallic sphere constant ?

dear pawan,

the electricfield inside the metal sphere is zero( it doesnot depend whther the sphere is placed in electric field or not , the electric field will remain zero there in both cases) therefore there can't be any charge inside the sphere by GAUSS LAW.

electric field inside metal sphere is zero bcoz there is no charge inside the sphere npt even a single unit of charge is present inside to produce electricity .

by shell therom of newton the charge on metal spher can be assumed to be concentared at the center of sphere bcoz electric field produced by charged  metal sphere is same as the electric filed produced by the charge placed at the center of sphere.. it is correct theritically as explained above.......

since electric fielid inside mettalic sphere zero therefore electric potential inside it is constant as if it were not constant then there must be potential gradient resulting in a electric field inside sphere which is not possoble....

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