Atomic view of charges

These properties of charges are readily interpreted in terms of the following model of the atom.

  

Figure 9.1: Basic model of the atom

In this model there are some number of protons and neutrons in the nucleus at the center - the protons have positive charge and the neutrons are electrically neutral. Orbiting around the nucleus are electrons, which are negatively charged. Usually atoms have equal numbers of protons and electrons, which means that they are electrically neutral, and therefore normally no electric forces are present. However, when one rubs two objects another, it is possible to transfer some of the outer electrons from one object to another; the protons and neutrons, being about 1800 times more massive than the electrons and bound together in the nucleus, don't move. The object that lost the electrons thus has a deficiency of electrons, leaving it positively charged, and the object that gained the electrons is negatively charged. These two charged objects will thus attract each other because of the electric force between them.

It is also easy to see in this model why charge is quantized in terms of some basic unit - charge arises because of a transfer of electrons, and so all charged objects will have charge equal to an integral number of the basic charge on an electron. This charge, measured in units called Coulombs (C), is given by

which is a very small charge compared to typical charges we usually encounter. What this means is that most charged objects involve the transfer of an enormous number of electrons.

We also can see why in this picture charge is never created nor destroyed (ie, is conserved), as to charge an object means simply to transfer electrons from one object to another. In this process one does not create nor destroy electrons, but just move them.