In the very last line it is written that but the electric field on the surface of charged conductor is not zero does it m ean that it is Sigma by Epsilon not which may be positive or negative and is the unit vector normal to the surface in the outward direction Electrostatic Shielding In an electrostatic situation if a conductor contains a cavity and if no charge is present inside the cavity then there can be no net charge anywhere on the surface of the cavity This means that if you are inside a charged conducting box you can safely touch any point on the inside walls of the box without being electrocuted This is known as electrostatic shielding Remember the following facts i There is no net charge anywhere on the surface of the cavity i e o 0 The potential inside or on the surface of the cavity The potential inside the material of the conductor The potential on the surface of the conductor V constant i The electric field in or on the surface of the cavity The electric field inside material of the conductor 0 0 E 0 0 E V V V V constant E 0 0 0 0 0 V V constant E 0 E 0 xxxxxx E 040 a O E conducting body zero But the electric field on the surface of a charged conductor is not zero Figure shows an uncharged conductor in an external electric field E 0 The free electrons in the conductor distribute themselves on the surface ao chown in the funcion