1.1 Electric Charge

Charge is quantized and conserved

Electric charge comes in two types, positive and negative, and is measured in coulombs (C). All charge is quantized — it exists in integer multiples of the elementary charge, e = 1.6 × 10⁻¹⁹ C, carried by protons (+e) and electrons (−e). In any isolated system, total charge is conserved: it can be transferred but never created or destroyed.

Like and unlike charges interact

Like charges repel, and opposite charges attract. This underlies Coulomb's Law, which describes the force between two point charges. Charge is an intrinsic property of matter, independent of an object's motion or position, and the net charge of an object is the sum of all its constituent charges.

Conductors vs. insulators

In conductors, charges (usually electrons) move freely, allowing charge to redistribute across the material. In insulators, charges are bound and stay localized. This distinction explains phenomena like charging by induction (redistributing charge without contact) versus charging by conduction (direct transfer of charge through contact).

Key terms

Elementary charge (e)

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

Charge quantization

All charge exists as integer multiples of the elementary charge e.

Conservation of charge

Total electric charge in an isolated system is constant; charge is transferred, not created or destroyed.

Conductor

A material in which charges move freely, allowing redistribution across the material.

Insulator

A material in which charges are bound and remain localized, not moving freely.

Try an AP-style question

Two identical metal spheres carry +6 μC and −2 μC. They touch and separate. What is the charge on each?

1. −2 μC
2. +2 μC
3. +4 μC
4. +3 μC