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www.GOALias.blogspot.comPhysicsFIGURE 2.18 The electric field inside acavity of any conductor is zero. Allcharges reside only on the outer surfaceof a conductor with cavity. (There are nocharges placed in the cavity.)is charged or charges are induced on a neutralconductor by an external field, all charges resideonly on the outer surface of a conductor with cavity.The proofs of the results noted in Fig. 2.18 areomitted here, but we note their importantimplication. Whatever be the charge and fieldconfiguration outside, any cavity in a conductorremains shielded from outside electric influence: thefield inside the cavity is always zero. This is knownas electrostatic shielding. The effect can be madeuse of in protecting sensitive instruments fromoutside electrical influence. Figure 2.19 gives asummary of the important electrostatic propertiesof a conductor.FIGURE 2.19 Some important electrostatic properties of a conductor.70EXAMPLE 2.7Example 2.7(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 ofaircraft are made slightly conducting. Why is this necessary?(c) Vehicles carrying inflammable materials usually have metallicropes touching the ground during motion. Why?(d) A bird perches on a bare high power line, and nothing happensto the bird. A man standing on the ground touches the same lineand gets a fatal shock. Why?Solution(a) This is because the comb gets charged by friction. The moleculesin the paper gets polarised by the charged comb, resulting in anet force of attraction. If the hair is wet, or if it is rainy day, frictionbetween hair and the comb reduces. The comb does not getcharged and thus it will not attract small bits of paper.