2.1 Electric Charges

Electric Charge Basics

Electric charge is a fundamental property of matter, existing as positive or negative. Charge is quantized — it always comes in integer multiples of the elementary charge, e = 1.6 × 10⁻¹⁹ C, carried by protons (+e) and electrons (−e). Like charges repel; opposite charges attract. Objects with equal numbers of protons and electrons are neutral.

Conservation of Charge

Total electric charge in an isolated system is always conserved — it cannot be created or destroyed, only transferred between objects. Charge can move via conduction (direct contact) or induction (redistribution without contact, due to a nearby charged object). In both processes, the total charge before and after remains constant.

Conductors vs. Insulators

In conductors (e.g., metals), electrons move freely, allowing charge to redistribute quickly across the material. In insulators (e.g., rubber, glass), electrons are bound tightly to atoms, so charge stays localized where it's placed. This distinction explains phenomena like charging by friction, induction, and grounding.

Key terms

Elementary charge (e)

1.6 × 10⁻¹⁹ C, the smallest unit of free charge, carried by a proton (+e) or electron (−e).

Quantization of charge

Electric charge exists only in integer multiples of the elementary charge e.

Conservation of charge

Total charge in an isolated system stays constant; charge can be transferred but not created or destroyed.

Conduction (charging)

Transfer of charge between objects through direct physical contact.

Induction (charging)

Redistribution of charge on an object caused by a nearby charged object, without direct contact.

Try an AP-style question

An object has a charge of −3.2 × 10⁻¹⁹ C. What does this mean?

1. It has 3.2 extra electrons
2. It has 2 more protons than electrons
3. It has 2 more electrons than protons
4. It has lost 2 electrons