The elementary charge, usually denoted e,^{[1]} is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive charge". It has a measured value of approximately 1.602176487(40)×10^{ −19} coulombs.^{[2]} In cgs, the value is 4.80320427(12)×10^{ −10} statcoulombs.^{[3]} In the system of atomic units as well as some other systems of natural units, e functions as the unit of electric charge, i.e. e = 1 in those unit systems.
The magnitude of the elementary charge was first measured in Robert Millikan's famous oildrop experiment in 1909.
Charge quantization is the statement that every stable and independent object (meaning an object that can exist independently for a prolonged period of time) has a charge which is an integer multiple of the elementary charge e: A charge can be exactly 0, or exactly e, –e, 2e, etc., but not, say, ^{1}⁄_{2} e, or –3.8e, etc. (This statement must not be interpreted to include quarks or quasiparticles, since neither quarks nor quasiparticles possess the ability to exist on their own for prolonged periods of time. Quarks have charges that are integer multiples of ^{1}⁄_{3} e.)
This is the reason for the terminology "elementary charge": it is meant to imply that it is an indivisible unit of charge.
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There are two known sorts of exceptions to the indivisibility of the elementary charge: quarks and quasiparticles.
All known elementary particles, including quarks, have charges that are integer multiples of ^{1}⁄_{3} e. Therefore, one can say that the "quantum of charge" is ^{1}⁄_{3} e. In this case, one says that the "elementary charge" is three times as large as the "quantum of charge".
On the other hand, all isolatable particles have charges that are integer multiples of e. (Quarks cannot be isolated, except in combinations like protons that have total charges which are integer multiples of e.) Therefore, one can say that the "quantum of charge" is e, with the proviso that quarks are not to be included. In this case, "elementary charge" would be synonymous with the "quantum of charge".
In fact, both terminologies are used.^{[4]} For this reason, phrases like "the quantum of charge" or "the indivisible unit of charge" can be ambiguous, unless further specification is given. On the other hand, the term "elementary charge" is unambiguous: It universally refers to the charge of a proton.
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
(In other 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 practice, this method is not how the most accurate values are measured today. Instead of calculating e from F and N_{A}, in fact N_{A} is calculated from F and e, while e is measured independently by a different method.^{[5]} Nevertheless, this is a legitimate and still quite accurate method, and experimental methodologies are described below:
The value of F can 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.^{[6]} In an electrolysis experiment, there is a onetoone correspondence between the electrons passing through the anodetocathode 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 timeintegral of electric current), and also taking into account the molar mass of the ions, one can deduce F.^{[5]}
The value of the Avogadro constant N_{A} was first approximated by Johann Josef Loschmidt who, in 1865, estimated the average diameter of the molecules in air by a method that is equivalent to calculating the number of particles in a given volume of gas.^{[7]}
Today the value of N_{A} can be measured at very high accuracy by taking an extremely pure crystal (in practice, often silicon), measuring how far apart the atoms are spaced using Xray diffraction or another method, and accurately measuring the density of the crystal. From this information, one can deduce the weight (in grams) of a single atom; and since the molar mass is known, the number of atoms in a mole can be calculated.^{[5]}
A famous method for measuring e is Millikan's oildrop 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.^{[8]}
Any electric current will be associated with noise from a variety of sources. One of these sources, shot noise, is due to the fact that the current is not a smooth continual flow, but rather consists of discrete electrons which pass through one at a time. By carefully analyzing the noise of a current, the charge of an electron can be calculated. This method can give a value of e accurate to a few percent.^{[9]}
Currently, the most 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.^{[10]}
The Josephson constant is
(where h is Planck's constant). It can be measured directly using the Josephson effect.
The von Klitzing constant is
It can be measured directly using the quantum Hall effect.
From these two constants, the elementary charge can be deduced:
