FORM 2-PHYSICS-STATIC ELECTRICITY
Charge on a Conductor Reside on its Outer Surface
Recognise that charge on a conductor reside on its outer surface
Usually, charges are distributed on the outer surface of conductors of different shapes.
Investigating surface distribution of a charge on conductors
- A proof plane is pressed into contact with the surface at various places of the conductor.
- The charges on the proof plane are then transferred to the electroscope.
- The divergence of the leaf will give a rough measure of the amount of charge transferred and hence surface density of the charge.
Charge on a Conductor is Concentrated on Sharply Curved Surfaces
Show that charge on a conductor is concentrated on sharply curved surfaces
So far we have considered excess charges on a smooth, symmetrical conductor surface. What happens if a conductor has sharp corners or is pointed? Excess charges on a nonuniform conductor become concentrated at the sharpest points. Additionally, excess charge may move on or off the conductor at the sharpest points.
To see how and why this happens, consider the charged conductor. The electrostatic repulsion of like charges is most effective in moving them apart on the flattest surface, and so they become least concentrated there. This is because the forces between identical pairs of charges at either end of the conductor are identical, but the components of the forces parallel to the surfaces are different. The component parallel to the surface is greatest on the flattest surface and, hence, more effective in moving the charge.
The same effect is produced on a conductor by an externally applied electric field, as seen inFigure(c). Since the field lines must be perpendicular to the surface, more of them are concentrated on the most curved parts.
Excess charge on a nonuniform conductor becomes most concentrated at the location of greatest curvature. (a) The forces between identical pairs of charges at either end of the conductor are identical, but the components of the forces parallel to the surface are different. It isF∥that moves the charges apart once they have reached the surface. (b)F∥is smallest at the more pointed end, the charges are left closer together, producing the electric field shown. (c) An uncharged conductor in an originally uniform electric field is polarized, with the most concentrated charge at its most pointed end.
