Electric charge comes in discrete units; the elementary charge e ≈ 1.6×10⁻¹⁹ C is the smallest unit. Charge is conserved in isolated systems—the total charge cannot be created or destroyed, only transferred between objects.
Start with simple demonstrations: rubbing materials and observing attraction/repulsion. Quantify charge through electrostatic experiments and relate to electron/proton masses.
Electric charge is one of the most fundamental properties of matter, alongside mass. Unlike mass, which is always positive, charge comes in two varieties — positive and negative — and opposite charges attract while like charges repel. The elementary charge e ≈ 1.6×10⁻¹⁹ C is the magnitude carried by a single proton (positive) or electron (negative). All observable charge is an integer multiple of e: you can have 3e or −7e, but never 2.4e. This quantization of charge is a foundational fact of nature that Millikan's oil-drop experiment confirmed in 1909.
The second foundational fact is charge conservation: in any isolated system, the total electric charge — the algebraic sum of all positive and negative charges — never changes. When you rub a glass rod with silk, you don't create charge; you transfer it. The silk gains as many electrons as the glass loses, so the combined system remains neutral. This conservation law is as robust as conservation of energy — no experiment has ever observed a net creation or destruction of charge. In particle physics, charge is conserved even when particles are created or annihilated: a photon (charge 0) can produce an electron and positron (charges −e and +e), and the total remains zero.
It helps to build a microscopic picture. Ordinary matter is made of atoms containing positively-charged protons in the nucleus and negatively-charged electrons in the surrounding cloud. Most objects are electrically neutral because the number of protons and electrons match. When electrons are transferred between objects (as when rubbing materials together), one object becomes negatively charged and the other positively charged by equal amounts. Conductors allow electrons to move freely through the material; insulators hold electrons tightly in place, so charge can only be deposited on the surface.
Understanding that charge is discrete, conserved, and comes in two kinds is the conceptual foundation for everything that follows in electrostatics and circuit analysis. Coulomb's law quantifies the force between charges; electric current is the rate of charge flow; circuits are systems for controlling and exploiting the energy stored in charge separation. Every subsequent concept in electricity and magnetism builds on these two facts: charge is quantized, and charge is conserved.
This is a foundational topic with no prerequisites.
No prerequisites — this is a starting point.