Electric Charge

Pull a load of laundry from the dryer and you’ll notice socks clinging to shirts, crackling sounds when you peel pieces apart, and maybe a small spark when you touch the metal door. That’s not magic — it’s electric charge in action.

When two different materials rub together (like clothes tumbling in a dryer), electrons transfer from one surface to the other. One material ends up with extra electrons (net negative charge); the other is missing some (net positive charge). The resulting attraction between opposite charges is what makes your socks cling to your shirts.

This chapter is the foundation for everything in electrostatics, circuits, and magnetism. Get the basics of charge — what it is, how it moves, how things become charged — and the rest builds straightforwardly on top.

What Is Electric Charge?

Electric charge is a fundamental property of matter, just like mass. There are exactly two types: positive and negative. Protons carry positive charge; electrons carry negative; neutrons carry none. Like charges repel; opposite charges attract.

Quantization of Charge

Charge comes in discrete packets. You can’t have half an electron’s worth of charge. The smallest unit of free charge is the elementary charge:

The SI unit of charge is the coulomb (C). One coulomb is an enormous amount of charge — roughly $6.25 \times 10^{18}$ electrons. In everyday static electricity (like the laundry example), you’re typically dealing with nanocoulombs ($10^{-9}$ C) or microcoulombs ($10^{-6}$ C).

Conservation of Charge

Charge is never created or destroyed — only transferred. Rub a balloon on your hair: the balloon gains $-5$ nC of charge, your hair gains $+5$ nC. Total system charge is still zero. (Just like before.)

Conductors vs. Insulators

• Conductors (metals, saltwater, plasma) have electrons that can move freely throughout the material. Touch a charged conductor and the charge spreads over the entire surface. Metals conduct because their outer electrons are delocalized in a “sea” of shared electrons.
• Insulators (rubber, glass, plastic, dry wood) hold electrons tightly in place. Charge placed on an insulator stays where you put it — it doesn’t spread. This is why you can charge one end of a plastic rod and the other end stays neutral.
• Semiconductors (silicon, germanium) sit in between. Their conductivity can be tuned by adding impurities (doping) — that’s the basis of every chip in every electronic device. Not a major MCAT topic, but you should know the category exists.

Three Methods of Charging

Charging by Friction (Triboelectric)

Rub two different materials together → electrons transfer from one to the other. Which material gains electrons depends on the triboelectric series. Classic example: rubbing a glass rod with silk pulls electrons off the glass (glass → positive, silk → negative). This is also what’s happening when you shuffle across carpet on a dry day and shock yourself on a doorknob — friction with the carpet has charged you.

Charging by Contact (Conduction)

Touch a charged object to a neutral conductor. Charge flows between them until they reach the same potential. Both objects end up with the same sign of charge. If a negatively charged rod touches a neutral metal sphere, electrons hop from rod to sphere, and now both are negative.

Charging by Induction

Bring a charged object near (but not touching) a neutral conductor. Charges in the conductor rearrange — opposite charges drift toward the nearby object, like charges drift to the far side. Now ground the conductor (connect it to Earth) while the charged object is still nearby — the repelled charges drain off into the ground. Remove the ground, then remove the charging object, and the conductor is left with a net charge opposite to the original object.

Polarization (of Insulators)

Even insulators respond to nearby charges. When a charged rod is brought near a neutral insulator, the electrons within each atom or molecule shift slightly, creating tiny induced dipoles. The material isn’t truly charged, but the near side becomes slightly attracted to the rod. This is why a charged balloon sticks to a neutral wall, why a charged comb attracts small bits of paper, and why dust collects on TV screens.