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The coulomb was originally defined, using the latter definition of the ampere, as 1 A × 1 s. [4] The 2019 redefinition of the ampere and other SI base units fixed the numerical value of the elementary charge when expressed in coulombs and therefore fixed the value of the coulomb when expressed as a multiple of the fundamental charge.
Electrons have an electric charge of −1.602 176 634 × 10 −19 coulombs, [80] which is used as a standard unit of charge for subatomic particles, and is also called the elementary charge. Within the limits of experimental accuracy, the electron charge is identical to the charge of a proton, but with the opposite sign. [ 83 ]
Thus, an object's charge can be exactly 0 e, or exactly 1 e, −1 e, 2 e, etc., but not 1 / 2 e, or −3.8 e, etc. (There may be exceptions to this statement, depending on how "object" is defined; see below.) This is the reason for the terminology "elementary charge": it is meant to imply that it is an indivisible unit of charge.
Charge is quantized: it comes in integer multiples of individual small units called the elementary charge, e, about 1.602 × 10 −19 C, [1] which is the smallest charge that can exist freely. Particles called quarks have smaller charges, multiples of 1 / 3 e , but they are found only combined in particles that have a charge that is an ...
Charge number or valence [1] of an ion is the coefficient that, when multiplied by the elementary charge, gives the ion's charge. [2]For example, the charge on a chloride ion, , is , where e is the elementary charge.
A proton by definition carries a charge of exactly 1.602 176 634 × 10 −19 coulombs. This value is also defined as the elementary charge. No object can have a charge smaller than the elementary charge, and any amount of charge an object may carry is a multiple of the elementary charge.
The constants listed here are known values of physical constants expressed in SI units; that is, physical quantities that are generally believed to be universal in nature and thus are independent of the unit system in which they are measured.
1 Coulomb's law. 2 Electric field ... if magnetic fields or electric currents do exist, ... Quantization of charge, the charge units carried by electrons or protons.