Q.

A uniformly charged thin spherical shell of radius R carries uniform surface charge density of σ per unit area. It is made of two hemispherical shells, held together by pressing them with force F (see figure). F is proportional to                   [2010]

1 1ε0σ2R2  
2 1ε0σ2R  
3 1ε0σ2R  
4 1ε0σ2R2  

Ans.

(1)

The electrostatic force per unit area, i.e., electrostatic pressure at a point on the surface of a uniformly charged sphere =12ε0E2=σ22ε0

 The force on a hemispherical shell F=σ22ε0×πR2

or, Fσ2R2ε0