Electric potential energy in terms of voltage:
PE = qVVoltage produced by a point charge:
Charge stored on a capacitor:
Q = CVCapacitance of a parallel plate capacitor:
Electricity literally holds the world together. Sure, gravity is pretty important, too, but the primary reason
that the molecules in your body stick together is because of electric forces. A world without electrostatics
would be no world at all.
This chapter introduces a lot of the vocabulary needed to discuss electricity, and it focuses on how to
deal with electric charges that aren’t moving: hence the name, electrostatics . We’ll look at moving
charges in the next chapter, when we discuss circuits.
Electric Charge
All matter is made up of three types of particles: protons, neutrons, and electrons. Protons have an
intrinsic property called “positive charge.” Neutrons don’t contain any charge, and electrons have a
property called “negative charge.”
The unit of charge is the coulomb, abbreviated C. One proton has a charge of 1.6 × 10−19 coulombs.
Most objects that we encounter in our daily lives are electrically neutral—things like couches, for
instance, or trees, or bison. These objects contain as many positive charges as negative charges. In other
words, they contain as many protons as electrons.
When an object has more protons than electrons, though, it is described as “positively charged”; and
when it has more electrons than protons, it is described as “negatively charged.” The reason that big
objects like couches and trees and bison don’t behave like charged particles is because they contain so
many bazillions of protons and electrons that an extra few here or there won’t really make much of a
difference. So even though they might have a slight electric charge, that charge would be much too small,
relatively speaking, to detect.
Tiny objects, like atoms, more commonly carry a measurable electric charge, because they have so
few protons and electrons that an extra electron, for example, would make a big difference. Of course,
you can have very large charged objects. When you walk across a carpeted floor in the winter, you pick
up lots of extra charges and become a charged object yourself ... until you touch a doorknob, at which
point all the excess charge in your body travels through your finger and into the doorknob, causing you to