electric charge

electric charge. Electric charge is a fundamental property of subatomic particles that gives rise to electromagnetic interactions. It is quantified and conserved, manifesting as either positive or negative, with like charges repelling and opposite charges attracting. The study of electric charge is central to classical electromagnetism, quantum electrodynamics, and countless technological applications.

Definition and basic properties

Electric charge is an intrinsic property of matter, carried by particles such as the positively charged proton and the negatively charged electron. The photon, the force carrier for the electromagnetic force, is itself neutral. The quark, a constituent of protons and neutrons, carries fractional charge, but is confined within hadrons. The property of charge is invariant under Lorentz transformation, a cornerstone of special relativity. The fundamental unit of charge is carried by the electron, a concept established through experiments like the oil-drop experiment conducted by Robert Millikan. The positron, the antiparticle of the electron discovered by Carl David Anderson, carries a positive charge of equal magnitude. The algebraic sum of charges in an isolated system is described by the principle of charge conservation.

Units and measurement

The International System of Units (SI) unit for electric charge is the coulomb, defined in terms of the fixed numerical value of the elementary charge and the Avogadro constant. Historically, charge was measured using instruments like the gold-leaf electroscope and the torsion balance, famously used by Charles-Augustin de Coulomb to formulate Coulomb's law. Precise modern measurements often involve counting elementary charges in phenomena like the quantum Hall effect or using single-electron devices. The ampere, the SI base unit for electric current, is defined such that the coulomb is one ampere-second. The franklin is a unit of charge in the centimetre–gram–second system of units (CGS-ESU). The statcoulomb is another CGS unit related to electrostatic interactions.

Conservation and quantization

The law of charge conservation states that the net electric charge of an isolated system remains constant, a principle linked to gauge invariance in quantum field theory. This conservation law is a result of Noether's theorem applied to the symmetry of the electromagnetic field. Electric charge is also quantized, meaning it appears as integer multiples of the elementary charge, a fact explained by the standard model of particle physics. The quark model allows for fractional charges, but these are never observed in isolation due to color confinement within particles like the proton and neutron. Experiments at facilities like the Large Hadron Collider continue to test the limits of charge conservation and quantization. The positron emission tomography (PET) scanner relies fundamentally on the creation and annihilation of charged particle-antiparticle pairs, demonstrating these principles.

Interactions and forces

Stationary charges interact via the electrostatic force, described quantitatively by Coulomb's law. Moving charges create magnetic fields, and the combined phenomena are described by Maxwell's equations, which unified the work of James Clerk Maxwell, Michael Faraday, and André-Marie Ampère. The full relativistic description of the force on a charged particle is given by the Lorentz force law. At the quantum level, the interaction between charged particles is mediated by the exchange of virtual photons, as described by quantum electrodynamics (QED), a theory developed by figures like Richard Feynman and Julian Schwinger. The strength of the electromagnetic interaction is characterized by the fine-structure constant. In plasma physics, the collective behavior of charged particles is governed by magnetohydrodynamics.

In matter and materials

In bulk matter, electric charge manifests as electric current when charges flow, a principle harnessed in all electrical circuits and devices from the light bulb to the transistor. Materials are classified as conductors, insulators, or semiconductors based on their ability to allow charge movement, a field pioneered by scientists at Bell Labs. The triboelectric effect describes charge transfer through friction, while induction describes charge redistribution in a conductor due to a nearby charged object. In electrochemistry, such as within a voltaic pile or a lithium-ion battery, chemical reactions drive the separation of charge. The piezoelectric effect, used in devices like the quartz clock, generates charge in response to mechanical stress. The behavior of charge in superconductors, like those studied in the context of the BCS theory, exhibits zero electrical resistance and the Meissner effect. Category:Electromagnetism Category:Physical quantities Category:Concepts in physics