3.1 Electric Charge

Charge is fundamental and conserved

Electric charge is a fundamental property of matter, existing as two types: positive and negative. The conservation of charge states that charge can neither be created nor destroyed, only transferred between objects. The elementary charge, e = 1.6 × 10⁻¹⁹ C, is the smallest unit; all charge is quantized in integer multiples of e.

Protons, electrons, and net charge

Atoms contain positively charged protons and negatively charged electrons. An object's net charge depends on the imbalance between protons and electrons — gaining electrons makes it negative, losing electrons makes it positive. Neutral objects have equal numbers of protons and electrons.

Charging methods

Objects become charged through friction (rubbing transfers electrons), conduction (direct contact transfers charge), and induction (a charged object rearranges charges in a nearby conductor without contact). In all cases, total charge in an isolated system is conserved.

Key terms

Elementary charge (e)

1.6 × 10⁻¹⁹ C — the smallest unit of charge, carried by one proton or electron.

Conservation of charge

Total electric charge in an isolated system remains constant; charge is only transferred, never created or destroyed.

Charging by conduction

Transferring charge between objects through direct physical contact.

Charging by induction

Rearranging charges in a nearby object using a charged object, without direct contact.

Quantization of charge

All charge exists in whole-number multiples of the elementary charge e.

Try an AP-style question

In a metal wire, which particles actually move to carry current?

1. Protons
2. Positive ions
3. Electrons
4. Neutrons