Electricity and Magnetism

Foundations of Electric Charge, Electrostatic Forces, and Fields

Electric charge is a fundamental property of matter that comes in two types: positive and negative. At the atomic level, protons carry a positive electric charge, while electrons carry a negative electric charge. A core rule governing these charges is that like electric charges repel each other, and opposite electric charges attract each other. The standard SI unit used to measure electric charge is the coulomb (C). In any physical process, the law of conservation of charge applies, stating that the total electric charge in an isolated system remains constant.

Materials differ greatly in how freely charges can travel across them. Electrical conductors allow electric charge to move easily through them, and in metals, these mobile charges are electrons. In contrast, electrical insulators strongly resist the flow of electric charges. Objects can acquire a net charge through distinct processes. Charging by conduction involves the direct transfer of charge through physical contact between objects. Alternatively, charging by induction gives an object a net charge without direct contact with the charging body, typically by grounding the object while a nearby charge polarizes it.

Charged objects exert influences across space governed by Coulomb's law. This law states that the magnitude of the electrostatic force between two point charges is directly proportional to the product of the magnitudes of their charges and inversely proportional to the square of the distance between them. An electric field exerts an electrostatic force on any charged particle placed within it. When mapping these forces, electric field lines point away from positive charges and toward negative charges.