CHARGE RESIDE ON THE SURFACE OF

On a charged insulator the charges are localized at places where they are supplied. But, in case of a conductor the charges given to any point immediately spread themselves over the surface. Extra charges do not reside inside the volume of a conductor. Three famous experiments show this.

(i)                         BIOT’S EXPERIMENTS

Take a metal sphere mounted on an insulated stand. Charge it with +ve or –ve charge. Then take two metallic hemispheres which exactly fit onto a sphere and are provided with insulating handles as shown in the figure 32.11. Now fit them over the sphere enclosing it completely. Bring this set near the electroscope. The leaves will diverge. When the hemispheres are separated and tested they are found to possess charge. But the sphere does not show any charge on it. This clearly shows that charges spread out over the outer surface and don’t reside in the interior of the conductor.

(ii)                     FARADAY’S BUTTERFLY- NET EXPERIMENTS

A brass ring is mounted on an insulating stand and conical muslin net is attached to the ring. Two silk threads are tried to the cone so that it can be turned in or out, as desired (fig, 32,12). Give some charges to the net and test for the charge inside and outside. It is found that the charge is entirely on the outer side and no charges are found inside.

Now turn the net inside out by pulling the outer thread, and test for the charge. It is found that the charges are present on the outer side.

(iii)                  FARADAY CASE

Faraday built metal cage supported on an insulating base. He entered the cage and sat on the seat which was insulated from its base. It was charged to very high potential. But he could not detect any charge inside the cage even though the outer surface was sparking into air.

These experiments confirm that charges don’t reside into a hollow charged conductor. This assigns can be tested by taking a hollow charged spherical shell with a hole on it and inserting a proof plane inside it.