on the conductor and the external fields in which it might be placed. We have proved a simple case of this result already: the electric field inside a charged spherical shell is zero. The proof of the result for the shell makes use of the spherical symmetry of the shell (see Chapter ). But the vanishing of electric field in the (charge-free) cavity of a conductor is, as mentioned above, a very general result.
A related result is that even if the conductor FIGURE . The Gaussian surface (a pill box) chosen to derive Eq. ( . ) for electric field at the surface of a charged conductor.
E XAMPLE . FIGURE . The electric field inside a cavity of any conductor is zero. All charges reside only on the outer surface of a conductor with cavity.
(There are no charges placed in the cavity.) is charged or charges are induced on a neutral conductor by an external field, all charges reside only on the outer surface of a conductor with cavity. The proofs of the results noted in Fig. . are omitted here, but we note their important implication.
Whatever be the charge and field configuration outside, any cavity in a conductor remains shielded from outside electric influence: the field inside the cavity is always zero . This is known as electrostatic shielding . The effect can be made use of in protecting sensitive instruments from outside electrical influence. Figure .
gives a summary of the important electrostatic properties of a conductor. Example . (a) A comb run through one’s dry hair attracts small bits of paper. Why?
What happens if the hair is wet or if it is a rainy day? (Remember, a paper does not conduct electricity.) (b) Ordinary rubber is an insulator. But special rubber tyres of aircraft are made slightly conducting. Why is this necessary?
(c) Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion. Why? (d) A bird perches on a bare high power line, and nothing happens to the bird. A man standing on the ground touches the same line