Electric Charge
Electric charge is a fundamental, conserved property of matter that causes it to experience and exert electromagnetic forces, occurring in two types (positive and negative) and quantised in integer multiples of the elementary charge e.
Curious
Rub a balloon on your hair and it sticks to a wall — electric charge, built up by friction, is pulling it there. Every atom is built from charged particles: negatively charged electrons orbiting a positively charged nucleus (protons and neutral neutrons). Normally these charges balance exactly, making atoms electrically neutral overall — but strip away or add even a single electron, and the atom becomes charged, ready to push or pull on other charges nearby.
Charge comes in exactly two types, arbitrarily labelled positive and negative by Benjamin Franklin in the 18th century. Same-sign charges repel; opposite-sign charges attract. This simple rule, combined with how strong the force is (covered by Coulomb's law), explains an enormous range of everyday phenomena, from static cling to lightning.
Exploring
Two properties of electric charge are foundational to all of electromagnetism:
| Property | Statement |
|---|---|
| Quantisation | Charge exists only in integer multiples of the elementary charge, e = 1.602176634 × 10⁻¹⁹ C — no smaller "fraction" of charge has ever been observed as a free particle |
| Conservation | Total electric charge in an isolated system never changes — charge can move from one object to another, but the total amount is always conserved exactly |
Worked example: A charged rod has gained 5 million extra electrons through friction. Its net charge is Q = −(5×10⁶)×(1.602176634×10⁻¹⁹ C) = −8.01 × 10⁻¹³ C — an almost immeasurably tiny fraction of a coulomb, illustrating just how enormous the elementary charge count is in even modest amounts of everyday static electricity.
Deep Dive
Robert Millikan's celebrated oil-drop experiment (1909–1913) provided the first precise measurement of the elementary charge e, by suspending charged oil droplets in a controlled electric field and measuring the field strength needed to balance gravity exactly. Millikan found that every droplet's charge was always an exact integer multiple of a single smallest value — direct, precise experimental confirmation of charge quantisation, and a Nobel Prize-winning result (1923).
Conservation as a symmetry: Charge conservation, like energy and momentum conservation, has a deep justification via Noether's theorem: it follows from a mathematical symmetry of quantum electrodynamics called U(1) gauge invariance — the fact that the theory's predictions are unchanged under a specific class of transformations of the electromagnetic field's mathematical description. This is a considerably more fundamental justification than simply observing that charge appears to be conserved in every experiment performed to date.
In particle physics, electric charge is one of a small number of conserved quantum numbers used to classify and constrain which particle interactions are physically allowed — for example, a process that would change total charge is forbidden regardless of what other conservation laws it might satisfy.
Concept Relationship Map
Requires: none (foundational entry point)
Enables: Coulomb's law, electric field
Commonly confused with: fractional quark charges appearing to violate quantisation (quarks are never free particles)
Sources
- Millikan, R.A. (1913). Physical Review, 2(2), 109–143.
- Griffiths, D.J. (2017). Introduction to Electrodynamics, 4th ed. Cambridge University Press, Ch. 1–2.
- NIST CODATA 2022 — elementary charge value. See Constants Reference.