SAT Physics Subject Test

Chapter 8 Electric Forces and Fields

Perhaps not surprisingly, you do not need to know a lot of chemistry for the SAT Physics Subject Test. What you do need to know is that the basic components of atoms are protons, neutrons, and electrons. In this chapter we will show you all that you will need to know about these sub-atomic particles, Coulomb’s law, and the electrical field.

ELECTRIC CHARGE

Protons and neutrons form the nucleus of the atom (and are referred to collectively as nucleons), while the electrons keep their distance, swarming around the nucleus. Most of an atom consists of empty space. In fact, if a nucleus were the size of the period at the end of this sentence, then the electrons would be 5 meters away. So what holds atoms together? One of the most powerful forces in nature: the electromagnetic force. Protons and electrons have a quality called electric charge that gives them an attractive force. Electric charge comes in two varieties: positive and negative. A positive particle always attracts a negative particle, and particles of the same charge always repel each other. Protons are positively charged, and electrons are negatively charged. Neutrons are electrically neutral; they have no charge.

Charge It!

Protons: positive
Electrons: negative
Neutrons: neutral

Protons and electrons are intrinsically charged, but bulk matter is not. This is because the amount of charge on a proton exactly balances the charge on an electron. Since most atoms contain an equal number of protons and electrons, their overall electric charge is 0 because the negative charges cancel out the positive charges. Therefore, for matter to be charged, an atom must have unequal numbers of protons and electrons. This can be accomplished by either the removal or addition of electrons (that is, by the ionization of some of the object’s atoms). If you remove electrons, then the object becomes positively charged, and if you add electrons, then it becomes negatively charged. Furthermore, charge is conserved. For example, if you rub a glass rod with a piece of silk, then the silk will acquire a negative charge and the glass will be left with an equalpositive charge. Net charge cannot be created or destroyed. (Charge can be created or destroyed—it happens all the time—but net charge cannot.)

The magnitude of charge on an electron (and therefore on a proton) is denoted e. This stands for elementary charge because it’s the basic unit of electric charge. The charge of an ionized atom must be a whole number times e because charge can be added or subtracted only in lumps of sizee. For this reason we say that charge is quantized. To remind us of the quantized nature of electric charge, the charge of a particle (or object) is denoted by the letter q. In the SI system of units, charge is expressed in coulombs (abbreviated C). One coulomb is a tremendous amount of charge; the value of e is about 1.6 × 10–19 C.