CHARGES ON CONDUCTORS
Charges on conductors situation (in which there is no net motion of point within a conductor is zero and that any excess located entirely on its surface (Fig. 23-20a). But what if (Fig. 23-20b)? If there is no charge within the as A (which lies completely within the material the net charge on the surface of the cavity must be zero,
as shown But now we let the ball touch the inner wall (Fig. Xz: at the ball becomes, in effect, part of the cavity surface. The situation Fig. 23-20b if Gauss’s law is correct, the net charge on the . Thus the ball must lose all its charge. Finally, we pull the indeed lost all its charge . performed in the nineteenth century by the English scientist metal with a lid, and it is called Faraday’s experiments were carried out in the eighteenth century by and Joseph Priestley in England, although with much less the validity of Gauss’s law and therefore of Coulomb’s because Coulomb’s experimental method, using a of charges, was not very precise it is very difficult to the electrostatic force with great precision by direct force experiments like Faraday’s test the validity of Gauss’s law with much greater precision. experiment is shown in Fig. 23-23. The details of aren’t important; its job is to place charge on the outer. The inner box with a dial is a sensitive elector meter motion of extremely small amounts of charge between the Gauss’s law is correct, there can never be any charge on the ere. If so, there should be no flow of charge between is being charged and discharged. The fact that no flow is sensitive confirmation of Gauss’s law and therefore of the experiment is limited mainly by the elector meter, Experiments have shown that the exponent 2 from precisely 2 by more than 10-16• So there is no than exactly 2 Faraday’s icepail experiment is used in a Van de Graaff _ 23-24). The charged conducting ball of Fig. 23-22 is continuously carries charge to the inside of a conducting away to the outside surface of the shell. As a result, the electric field around it can become very large very rapid used as an accelerator of charged particles and for .”A the basis for electrostatic shielding. Suppose we have a that we want to protect from stray electric fields. We surround the instrument with a conducting
. and ceiling of the room with a conducting material such electric field redistributes the free electrons in the on the outer surface in some regions and a net negative ‘This charge distribution causes an additional electric field point inside the box is zero, as Gauss’s law says it must the box also alters the shapes of the field lines near the setup is often called a Faraday cage. The same physics. places to be in a lightning storm is inside an automobile; 1:. the charge tends to remain on the metal skin of the is produced inside the passenger compartment.
point to point on the surface. We will show in Chapter 24 direction of E is always perpendicular to the surface. To find a relation between a at any point on the surface of the electric field at that point, we construct a small cylinder (Fig. 23-26). One end face, with area A, the other lies just outside. The electric field is zero at all Outside the conductor the component of E perpendicular to der is zero, and over the end face the perpendicular positive, the electric field will point O to f the conductor negative, the field will point inward and EJ. will be through the surface. The charge enclosed within the from Gauss’s law, and We can check this with the results we have obtained for surfaces. We showed in Example 23-8 that the field magni oppositely charged conducting plates also equals a/£o. In is the same at all distances from the plates, but in all increasing distance from the surface.