12.1 Conductors and insulators

Conductors and insulators

All atoms are electrically neutral i.e. they have the same amounts of negative and positive charge inside them. By convention, the electrons carry negative charge and the protons carrypositive charge. The basic unit of charge, called the elementary charge, e , is the amount of charge carried by one electron.

The charge on a single electron is $q_e=\mathrm{1,6}x{10}^{-19}\mathrm{C}$ . All other charges in the universe consist of an interger multiple of this charge (i.e. $\mathrm{Q}={\mathrm{nq}}_e$ ). This is known as charge quantisation.

All the matter and materials on earth are made up of atoms. Some materials allow electrons to move relatively freelythrough them (e.g. most metals, the human body). These materials are called conductors .

Other materials do not allow the charge carriers, the electrons, to move through them (e.g. plastic, glass).The electrons are bound to the atoms in the material. These materials are called non-conductors or insulators .

If an excess of charge is placed on an insulator, it will stay where it is put and there will be a concentration of charge inthat area of the object. However, if an excess of charge is placed on a conductor, the like charges will repel each otherand spread out over the outside surface of the object. When two conductors are made to touch, the total charge on them is shared between thetwo. If the two conductors are identical, then each conductor will be left with half of the total charge.

Electrostatic force

The electrostatic force determines the arrangement of charge on the surface of conductors. This is possible because charges can move inside a conductive material. When we placea charge on a spherical conductor the repulsive forces between the individual like charges cause them to spread uniformly over thesurface of the sphere. However, for conductors with non-regular shapes, there is a concentration of charge near the point or pointsof the object. Notice in [link] that we show a concentration of charge with more $-$ or + signs, while we represent uniformly spread charges with uniformly spaced $-$ or + signs.

This collection of charge can actually allow charge to leak off the conductor if the point is sharp enough. It is for this reasonthat buildings often have a lightning rod on the roof to remove any charge the building has collected. This minimises thepossibility of the building being struck by lightning. This “spreading out” of charge would not occur if we were to placethe charge on an insulator since charge cannot move in insulators.