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a contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 as a unit 2 quantization toggle quantization subsection 2 1 fractional elementary charge 2 2 lack of fractional charges 3 experimental measurements of the elementary charge toggle experimental measurements of the elementary charge subsection 3 1 in terms of the avogadro constant and faraday constant 3 2 jj thomson s electrostatic measurement 3 3 oil drop experiment 3 4 shot noise 3 5 from the josephson and von klitzing constants 3 6 codata method 3 7 tests of the universality of elementary charge 4 see also 5 references 6 further reading toggle the table of contents elementary charge 54 languages العربية беларуская тарашкевіца беларуская български brezhoneg català čeština чӑвашла dansk deutsch esperanto español eesti فارسی suomi français nordfriisk bahasa hulontalo हिन्दी hrvatski magyar հայերեն bahasa indonesia italiano 日本語 ქართული 한국어 lëtzebuergesch lietuvių မြန်မာဘာသာ nederlands norsk nynorsk norsk bokmål polski português română русский sicilianu srpskohrvatski српскохрватски සිංහල slovenčina slovenščina shqip српски srpski sunda svenska தமிழ் ไทย türkçe українська tiếng việt 吴语 中文 isizulu edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia charge carried by one electron elementary charge common symbols e si unit coulomb dimension t i displaystyle mathsf t mathsf i value 1 602 176 634 10 19 c 1 the elementary charge usually denoted by e is the magnitude of the charge on an electron 2 and a fundamental physical constant defined to be exactly e 1 602 176 634 10 19 c 1 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 3 124 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 4 5 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 6 and through the faraday constant at order of magnitude accuracy by johann loschmidt s measurement of the avogadro constant in 1865 as a unit edit elementary charge unit system atomic units unit of electric charge symbol e conversions 1 e in is equal to coulombs 1 602 176 634 10 19 1 ε 0 ℏ c displaystyle sqrt varepsilon _ 0 hbar c natural units 0 30282212088 statc 4 803 204 25 10 10 10 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 7 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 ε 0 is the electric constant and ħ is the reduced planck constant quantization edit see also partial charge all known elementary particles have charges that are integer multiples of the charge on the down quark an observation known as charge quantization 8 thus the charge on the down quark 1 3 e can be considered the fundamental quantum of charge 9 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 8 fractional elementary charge edit 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 edit 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 10 11 it is currently not known why isolatable particles are restricted to integer charges much of the string theory landscape appears to admit fractional charges 12 13 see also anomaly physics anomaly cancellation experimental measurements of the elementary charge edit 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 edit the first determinations of the elementary charge were based on faraday s laws of electrolysis 14 72 if the avogadro constant n a 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 in words the charge of one mole of electrons divided by the number of electrons in a mole equals the charge of a single electron in the late 1800s several physicists including johann josef loschmidt and james clerk maxwell used estimates of the average diameter of the gas molecules to estimate the avogadro number n 15 84 loschmidt s work gave a value of 0 5 10 23 16 today the value of n a has been adopted 2 as a defining constant equal to 6 022 140 76 10 23 mol 1 the value of f could be measured directly using faraday s laws of electrolysis faraday s laws of electrolysis are quantitative relationships based on the electrochemical researches published by michael faraday in 1834 17 in an electrolysis experiment there is a one to one correspondence between the electrons passing through the anode to cathode wire and the ions that plate onto or off of the anode or cathode measuring the mass change of the anode or cathode and the total charge passing through the wire which can be measured as the time integral of electric current and also taking into account the molar mass of the ions one could deduce f 1 in 1874 george johnstone stoney used this method of faraday s second law to give the first estimate of the elementary charge it was too small by a factor of 20 14 in 1901 max planck used his theory of blackbody radiation to estimate the boltzmann constant k b and combined this with a value for the gas constant r to estimate n a via r n a k b displaystyle r n_ mathrm a k_ mathrm b and then applied faraday s second law his value of the elementary charge 4 69 10 10 esu is close to the modern value 4 80 10 10 esu 14 74 jj thomson s electrostatic measurement edit in 1898 j j thomson measured the current in a gas in an electric field exposed to x rays the current i would be i nev where n is the number of ions measured e is the charge on the ions and v their mean velocity the mean velocity in the electric field was previously measured by rutherford thomson had discovered that water droplets would form around the ions if the gas was saturated before the x ray exposure the rate at which the vapor cloud fell gave an estimate of the size of the droplets and the mass of the droplets hitting the bottom of the experimental chamber per unit time gave total flux of water combined this gave an estimate of the number of droplets and thus the number of ions n then the current measured could be converted to an estimate of the elementary charge 18 406 oil drop experiment edit main article oil drop experiment a famous method for measuring e is millikan s oil drop experiment a small drop of oil in an electric field would move at a rate that balanced the forces of gravity viscosity of traveling through the air and electric force the forces due to gravity and viscosity could be calculated based on the size and velocity of the oil drop so electric force could be deduced since electric force in turn is the product of the electric charge and the known electric field the electric charge of the oil drop could be accurately computed by measuring the charges of many different oil drops it can be seen that the charges are all integer multiples of a single small charge namely e the necessity of measuring the size of the oil droplets can be eliminated by using tiny plastic spheres of a uniform size the force due to viscosity can be eliminated by adjusting the strength of the electric field so that the sphere hovers motionless shot noise edit main article shot noise any electric current will be associated with noise from a variety of sources one of which is shot noise shot noise exists because a current is not a smooth continual flow instead a current is made up of discrete electrons that pass by one at a time by carefully analyzing the noise of a current the charge of an electron can be calculated this method first proposed by walter h schottky can determine a value of e of which the accuracy is limited to a few percent 19 however it was used in the first direct observation of laughlin quasiparticles implicated in the fractional quantum hall effect 20 from the josephson and von klitzing constants edit another accurate method for measuring the elementary charge is by inferring it from measurements of two effects in quantum mechanics the josephson effect voltage oscillations that arise in certain superconducting structures and the quantum hall effect a quantum effect of electrons at low temperatures strong magnetic fields and confinement into two dimensions the josephson constant is k j 2 e h displaystyle k_ text j frac 2e h where h is the planck constant it can be measured directly using the josephson effect the von klitzing constant is r k h e 2 displaystyle r_ text k frac h e 2 it can be measured directly using the quantum hall effect from these two constants the elementary charge can be deduced e 2 r k k j displaystyle e frac 2 r_ text k k_ text j codata method edit the relation used by codata to determine elementary charge was e 2 2 h α μ 0 c 2 h α ε 0 c displaystyle e 2 frac 2h alpha mu _ 0 c 2h alpha varepsilon _ 0 c where h is the planck constant α is the fine structure constant μ 0 is the magnetic constant ε 0 is the electric constant and c is the speed of light presently this equation reflects a relation between ε 0 and α while all others are fixed values thus the relative standard uncertainties of both will be same tests of the universality of elementary charge edit particle expected charge experimental constraint notes electron q e e displaystyle q_ text e e exact by definition proton q p e displaystyle q_ text p e q p e 10 21 e displaystyle left q_ text p e right 10 21 e by finding no measurable sound when an alternating electric field is applied to sf 6 gas in a spherical resonator 21 positron q e e displaystyle q_ text e e q e e 10 9 e displaystyle left q_ text e e right 10 9 e by combining the best measured value of the antiproton charge below with the low limit placed on antihydrogen s net charge by the alpha collaboration at cern 22 antiproton q p e displaystyle q_ bar text p e q p q p 10 9 e displaystyle left q_ bar text p q_ text p right 10 9 e hori et al 23 as cited in antiproton proton charge difference listing of the particle data group 24 the particle data group article has a link to the current online version of the particle data see also edit committee on data of the international science council references edit 1 2 3 4 2022 codata value elementary charge the nist reference on constants units and uncertainty nist may 2024 retrieved 2024 05 18 1 2 newell david b tiesinga eite 2019 the international system of units si nist special publication 330 gaithersburg maryland national institute of standards and technology doi 10 6028 nist sp 330 2019 s2cid 242934226 the international system of units pdf v4 01 9th ed international bureau of weights and measures jun 2026 isbn 978 92 822 2272 0 millikan r a 1910 the isolation of an ion a precision measurement of its charge and the correction of stokes s law science 32 822 436 448 doi 10 1126 science 32 822 436 fletcher harvey 1982 my work with millikan on the oil drop experiment physics today 35 6 43 47 doi 10 1063 1 2915126 klein martin j 1 october 1961 max planck and the beginnings of the quantum theory archive for history of exact sciences 1 5 459 479 doi 10 1007 bf00327765 issn 1432 0657 s2cid 121189755 g j stoney 1894 of the electron or atom of electricity philosophical magazine 5 38 418 420 doi 10 1080 14786449408620653 1 2 foot r joshi g c lew h volkas r r december 30 1990 charge quantization in the standard model and some of its extensions modern physics letters a 05 32 2721 2731 doi 10 1142 s0217732390003176 issn 0217 7323 q is for quantum by john r gribbin mary gribbin jonathan gribbin page 296 web link preskill j 1984 magnetic monopoles annual review of nuclear and particle science 34 1 461 530 bibcode 1984arnps 34 461p doi 10 1146 annurev ns 34 120184 002333 three surprising facts about the physics of magnets space com 2018 retrieved 17 july 2019 schellekens a n 2 october 2013 life at the interface of parti...