An atom usually has an equal number of protons and electrons. When this condition exists, the atom is electrically neutral because the positively charged protons exactly balance the negatively charged electrons. However, under certain conditions, an atom can become unbalanced by losing or gaining an electron. When an atom loses or gains an electron, it is no longer neutral. It is either negatively or positively charged, depending on whether the electron is gained or lost. Thus, in an atom, a charge exists when the number of protons and electrons is not equal.
Under certain conditions, some atoms can lose a few electrons for short periods. Electrons in the outer orbits of some materials, especially metals, can be easily knocked out of their orbits. Such electrons are referred to as free electrons, and materials with free electrons are called conductors. When electrons are removed from the atom, the atom becomes positively charged because the negatively charged electrons have removed, creating an unbalance condition in the atom.
An atom can just as easily acquire additional electrons. When this occurs, the atom becomes negatively charged.
Chargers are, thus, created when there is an excess of electrons or protons in an atom. When one atom is charged and unlike charge is in another atom, electrons can flow between the two. This electron flow is called electricity.
An atom that has lost or gained an electron is considered unstable. A surplus of electrons in an atom creates a negative charge. A shortage of electrons creates a positive charge. Electric charges react to each other in different ways. Two negatively charged particles repel each other. Positively charged particles also repel each other. Two opposite charges attract each other. The law of electric charges states that like charges repel and unlike charges attract. Figure 1 shows an illustration of the law of electric charges.
All atoms tend to remain neutral because the outer orbits of electrons repel other electrons. However, many materials can be made to acquire a positive or negative charge by some mechanical means, such as friction. The familiar crackling when a hard rubber comb is run through hair on a dry winter day is an example of an electric charge generated by friction.