Electric charge is a fundamental property of matter that comes in two types: positive and negative. Protons carry positive charge, and electrons carry negative charge. Like charges repel each other, and opposite charges attract — similar to magnetic poles. Most objects are electrically neutral because they have equal numbers of protons and electrons. An object becomes charged when electrons are added or removed, creating an imbalance.
Rub a balloon on your hair and stick it to a wall. Observe how two charged balloons repel each other. Use a simple electroscope to detect charge. Discuss how rubbing does not create charge — it transfers electrons from one surface to another.
Everything around you — your body, the air, the phone in your hand — is made of atoms, and atoms contain electrically charged particles. Protons in the nucleus carry positive charge, and electrons orbiting the nucleus carry negative charge. In most objects, the number of protons and electrons are exactly equal, so the positive and negative charges cancel out, and the object is electrically neutral.
But electrons are not always locked in place. When you rub a balloon on your hair, electrons transfer from your hair to the balloon. The balloon now has more electrons than protons, giving it a net negative charge. Your hair, having lost electrons, now has a net positive charge. No charge was created — it was simply moved from one place to another. This is the law of conservation of charge: charge can be transferred but never created or destroyed.
The interaction between charges follows a simple rule: like charges repel, opposite charges attract. Two negatively charged balloons push away from each other. A negatively charged balloon and positively charged hair pull toward each other. This is conceptually similar to how magnetic north repels north but attracts south, though the underlying physics is different.
Something surprising is that a charged object can attract a neutral object. When a negatively charged balloon is brought near a neutral wall, the balloon's negative charge pushes electrons in the wall slightly away, leaving the wall's surface locally positive. The nearby positive region is attracted to the balloon more strongly than the distant negative region is repelled, resulting in a net attraction. This process is called polarization, and it is why charged balloons stick to walls even though walls start out neutral.
Electric charge is measured in coulombs (C). A single electron carries a charge of about 1.6 × 10⁻¹⁹ coulombs — incredibly tiny. It takes roughly 6.25 × 10¹⁸ electrons to make one coulomb of charge. Despite the minuscule charge on each electron, the electric forces between charged objects can be enormous, which is what makes static shocks, lightning, and all of electrical technology possible.