Rub a balloon on your hair and it sticks to the wall. The force at work is electric, and it starts inside atoms. As you know from protons, neutrons and electrons, protons carry positive charge and electrons carry an equal amount of negative charge.
Key idea
An object's net charge is (number of protons) − (number of electrons), counted in units of one electron's charge. Objects become charged by gaining or losing electrons: gain electrons and become negative, lose electrons and become positive. The protons stay put in the nuclei.
Why it works
Protons are locked inside the nucleus at the center of each atom, held by strong nuclear forces. The outer electrons are much easier to move, so when two materials rub, some electrons cross from one to the other. Charge is never made or destroyed: whatever negative charge the balloon gains, your hair loses, so the hair ends up positive by exactly the same amount.
Worked example: Count the charge
An oxygen atom has 8 protons. It gains 2 electrons. What is its net charge?
- Protons: 8. Electrons: $8 + 2 = 10$.
- Net charge: $8 - 10 = -2$ (in units of the electron's charge).
Answer: $-2$
Worked example: A balloon and hair
A balloon rubbed on hair gains 5 million extra electrons. What happens to the hair?
- Charge is conserved: the electrons came from the hair.
- The hair loses 5 million electrons, so it has 5 million more protons than electrons: positive.
Answer: The hair becomes positive by the same amount
Common mistake
Thinking an object becomes positive by gaining protons. Protons are stuck in nuclei; in everyday charging, only electrons move. A positive object has lost electrons. Check: "positive" means fewer electrons than protons.
Build atoms and ions particle by particle in the Science Lab: Build an Atom.