Electric Charge · aiwiki.page

Electric charge is a fundamental property of particles and matter that determines their participation in electromagnetic interactions. It occurs in two forms, conventionally called positive and negative. Charges of the same sign repel, while charges of opposite signs attract under electrostatic conditions. Charge is usually represented by $q$ or $Q$ and measured in coulombs. Although physicists commonly speak of “a charge” to mean a charged particle or object, charge itself is a property rather than a substance. (openstax.org)

Charge in matter

The electron carries negative charge, while the proton carries positive charge of equal magnitude. The neutron has zero net electric charge. In an electrically neutral atom, the number of electrons equals the number of protons in its nucleus. An atom or group of atoms with an imbalance between these charges is an ion: losing electrons produces a positive ion, while gaining electrons produces a negative ion. Ordinary objects become charged when positive and negative contributions no longer balance. (openstax.org)

Neutrality does not mean that an object contains no charged constituents. Rather, its total positive and negative charges sum to zero. Their spatial distribution may nevertheless change in response to nearby charged objects. This redistribution helps explain why a charged object can attract an electrically neutral one: opposite charges can become closer to the external charge than like charges, producing a net attractive force. (openstax.org)

Units and quantization

In the International System of Units (SI), the unit of charge is the coulomb, symbol C, with $1\ \mathrm{C}=1\ \mathrm{A\,s}$ . The elementary charge, $e$ , is the positive magnitude of the electron’s charge and has the exact value

e=1.602176634\times10^{-19}\ \mathrm{C}.

The SI definition of the ampere fixes this numerical value. Consequently, an electron carries $-e$ , and a proton carries $+e$ ; one coulomb corresponds to approximately $6.24\times10^{18}$ elementary charges. (physics.nist.gov)

Charge is quantized rather than arbitrarily divisible at the particle level. For ordinary objects whose net charge results from gaining or losing electrons,

Q=ne,

where $n$ is a positive or negative integer, or zero. Because $e$ is extremely small, macroscopic charge is often treated as continuously distributed, an approximation suitable for many calculations. (openstax.org)

An important qualification comes from quarks. Up-type quarks carry $+2e/3$ , and down-type quarks carry $-e/3$ ; their antiparticles carry opposite charges. Quarks are not observed as isolated free particles, but are confined within composite particles. Thus, the elementary charge should not be described without qualification as the smallest charge carried by any constituent of matter. (scoollab.web.cern.ch)

Forces and fields

For two stationary point charges in vacuum, Coulomb’s law gives the force magnitude:

F=\frac{1}{4\pi\varepsilon_0} \frac{|q_1q_2|}{r^2},

where $r$ is their separation and $\varepsilon_0$ is the vacuum permittivity. The force acts along the line joining the charges. For several charges, the total force is the vector sum of the individual contributions, a principle called superposition. Extended charge distributions can be treated by adding contributions from small elements of charge. (openstax.org)

An electric field describes the electrical influence at each position. A particle of charge $q$ in a field $\mathbf E$ experiences the electric force $q\mathbf E$ . If it moves with velocity $\mathbf v$ through a magnetic field $\mathbf B$ , the combined Lorentz force is

\mathbf F=q(\mathbf E+\mathbf v\times\mathbf B).

Maxwell’s equations describe how charges and currents generate electromagnetic fields and how those fields evolve. They connect the behavior of charged matter with electromagnetic waves. (openstax.org)

Conservation and current

Electric charge is conserved: the algebraic total remains constant in a system through whose boundary no charge passes. Charging by rubbing or contact transfers charge between objects rather than creating net charge. Conservation also applies when particles are created or destroyed. For example, an electron and a positron can annihilate into electrically neutral photons, with zero total charge both before and after the process. (openstax.org)

Electric current measures the rate at which charge passes through a surface:

I=\frac{dQ}{dt}.

Conventional current follows the direction in which positive charge would move. In a metal, the mobile carriers are electrons, whose drift direction is opposite to conventional current. In other media, including electrolytes, both positive and negative ions can transport charge. Current therefore need not imply that the conducting material has a substantial net charge. (openstax.org)

Charging and material behavior

An electrical conductor contains charges able to move relatively freely, whereas an electrical insulator strongly restricts their movement. Charge deposited on a conductor redistributes; on an insulator, it can remain localized. In electrostatic equilibrium, the electric field inside the conducting material is zero, and excess charge resides on its surface. These distinctions concern charge mobility, not whether the material contains charged particles. (openstax.org)

Objects may acquire net charge through contact or through electrostatic induction combined with a path for charge transfer, such as grounding. Bringing a charged object near an isolated conductor redistributes its charges but does not, by itself, alter its total charge. If the conductor is grounded during the process, electrons may enter or leave; removing the ground connection before withdrawing the inducing object can leave a net charge behind. (openstax.org)