The elementary charge, usually denoted by e, is the magnitude of the charge on an electron and a fundamental physical constant defined to be exactly e = 1.602176634×10−19 C. The elementary charge is a measure of the coupling strength of the electromagnetic force and one of the seven defining constants of the International System of Units (SI). Before being set to a fixed value as part of the 2019 revision of the SI, the elementary charge was viewed as a quantity to be determined experimentally. Robert A. Millikan and Harvey Fletcher's oil drop experiment first directly measured the magnitude of the elementary charge in 1909, differing from the modern accepted value by just 0.6%. Under assumptions of the then-disputed atomic theory, the elementary charge had also been indirectly inferred to ~3% accuracy from blackbody spectra by Max Planck in 1901 and (through the Faraday constant) at order-of-magnitude accuracy by Johann Loschmidt's measurement of the Avogadro constant in 1865.
As a unit
In some natural unit systems, such as the system of atomic units, e functions as the unit of electric charge. The use of elementary charge as a unit was promoted by George Johnstone Stoney in 1874 for the first system of natural units, called Stoney units. Later, he proposed the name electron for this unit. At the time, the particle we now call the electron was not yet discovered and the conceptual distinction between the particle electron and the unit of charge electron was still blurred. Later, the name electron was assigned to the particle and the unit of charge e lost its name. However, the unit of energy electronvolt (eV) is a remnant of the fact that the elementary charge was once called electron. In other natural unit systems, the unit of charge is defined as ε 0 ℏ c , {\displaystyle {\sqrt {\varepsilon _{0}\hbar c}},} with the result that
e = 4 π α ε 0 ℏ c ≈ 0.30282212088 ε 0 ℏ c , {\displaystyle e={\sqrt {4\pi \alpha }}{\sqrt {\varepsilon _{0}\hbar c}}\approx 0.30282212088{\sqrt {\varepsilon _{0}\hbar c}},}
where α is the fine-structure constant, c is the speed of light, ε0 is the electric constant, and ħ is the reduced Planck constant.
Quantization
All known elementary particles have charges that are integer multiples of the charge on the down quark, an observation known as charge quantization. Thus the charge on the down quark, 1/3 e can be considered the fundamental quantum of charge. However down quarks cannot be isolated: they exist only in combination with other quarks to create particles with a single elementary charge, like protons and neutrons. The physical cause of charge quantization is not known and it has no explanation in the Standard Model of elementary particles. Grand unification theories predict charge quantization, but they fail in other ways.
Fractional elementary charge There are two known types of exception to the indivisibility of the elementary charge: quarks and quasiparticles. Quarks, first posited in the 1960s, have quantized charge in multiples of e / 3 {\displaystyle e/3} . However, quarks cannot be isolated; they exist only in groupings, and stable groupings of quarks (such as a proton, which consists of three quarks) all have charges that are integer multiples of e. Quasiparticles are not particles as such, but rather an emergent entity in a complex material system that behaves like a particle. In 1982 Robert Laughlin explained the fractional quantum Hall effect by postulating the existence of fractionally charged quasiparticles. This theory is now widely accepted, but this is not considered to be a violation of the principle of charge quantization, since quasiparticles are not elementary particles.
Lack of fractional charges Paul Dirac argued in 1931 that if magnetic monopoles exist, then electric charge must be quantized; however, it is unknown whether magnetic monopoles actually exist. It is currently not known why isolatable particles are restricted to integer charges; much of the string theory landscape appears to admit fractional charges.
Experimental measurements of the elementary charge The elementary charge as expressed in SI units is exactly defined since 20 May 2019 by the International System of Units. Prior to this change, the elementary charge was a measured quantity whose magnitude was determined experimentally. This section summarizes these historical experimental measurements.
In terms of the Avogadro constant and Faraday constant The first determinations of the elementary charge were based on Faraday's laws of electrolysis. If the Avogadro constant NA and the Faraday constant F are independently known, the value of the elementary charge can be deduced using the formula
e = F N A . {\displaystyle e={\frac {F}{N_{\text{A}}}}.}
… excerpt ends here. Continue reading the full article.
