Electrons and protons are the basic charges of ordinary matter. Consequently, the various observations we made in Section 22.1 need to be explained in terms of electrons and protons.

Charge is represented by the symbol $q$ (or sometimes $Q$). A macroscopic object, such as a plastic rod, has charge

where $N_{\text{p}}$ and $N_{\text{e}}$ are the number of protons and electrons contained in the object. An object with an equal number of protons and electrons has no net charge (i.e., $q=0)$ and is said to be electrically neutral.

A charged object has an unequal number of protons and electrons. An object is positively charged if $N_{\text{p}}>N_{\text{e}}$. It is negatively charged if $N_{\text{p}}<N_{\text{e}}$. Notice that an object’s charge is always an integer multiple of $e$. That is, the amount of charge on an object varies by small but discrete steps, not continuously. This is called charge quantization.

In practice, objects acquire a positive charge not by gaining protons, as you might expect, but by losing electrons. Protons are extremely tightly bound within the nucleus and cannot be added to or removed from atoms. Electrons, on the other hand, are bound rather loosely and can be removed without great difficulty. The process of removing an electron from the electron cloud of an atom is called ionization. An atom that is missing an electron is called a positive ion. Its net charge is $q=+e$.

Some atoms can accommodate an extra electron and thus become a negative ion with net charge $q=-e$. A saltwater solution is a good example. When table salt (the chemical sodium chloride, NaCl) dissolves, it separates into positive sodium ions ${\text{Na}}^{+}$ and negative chlorine ions ${\text{Cl}}^{-}$. FIGURE 22.2 shows positive and negative ions.

FIGURE 22.2 Positive and negative ions.

All the charging processes we observed in Section 22.1 involved rubbing and friction. The forces of friction cause molecular bonds at the surface to break as the two materials slide past each other. Molecules are electrically neutral, but FIGURE 22.3 shows that molecular ions can be created when one of the bonds in a large molecule is broken. The positive molecular ions remain on one material and the negative ions on the other, so one of the objects being rubbed ends up with a net positive charge and the other with a net negative charge. This is the way in which a plastic rod is charged by rubbing with wool or a comb is charged by passing through your hair.

FIGURE 22.3 Charging by friction usually creates molecular ions as bonds are broken.