Charging of a body
Charging can be done by two methods; 1. Conduction 2. Induction
Ordinarily, matter contains equal number of protons and electrons. A body can be charged by transfer of electrons or redistribution of electrons.
Charging by conduction
The process of charging from an already charged body can happen either by conduction or induction.
Conduction from a charged body involves transfer of like charges. A positively charged body can create more bodies which are positively charged but the sum of the total charge on all positively charged bodies will be the same as the earlier sum.
Charging by induction
Induction is a process by which a charged body accomplishes creation of other charged bodies, without touching them or losing its own charge.
When a positively charged rod is brought near a plate, the free electrons are attracted by the +ve charge and move near to the rod.
Thus the portion nearer to rod becomes negatively charged and the portion farther from the rod becomes positively charged.
Further movement of electrons are repelled by the -ve charge already on the surface near the rod and finally this movement is stopped.
If the other end is grounded the +ve charge goes to the ground and, after removal of the rod, the plate has net –ve charge.
Conductors and insulators
We know that in some materials the charges flow easily. Such materials are called conductors. Atoms have equal number of protons and electrons which make them electrically neutral. When several atoms of a conductor unite, some of the outermost electrons do not remain attached to the atom and are free to move.
There are very few or no free electrons in an insulator. It causes restrictions in the movement of –ve charge and hence the movement of charge in insulators is restricted.
Semiconductors are in between conductors and insulators. They behave as insulators at low temperature but some of the electrons become free at higher temperatures and they start conducting to an extent.