|Name||Symbol||Value||Unit||Expressed in terms of SI base units|
|elementary charge||e||1.602 176 634 × 10−19||C||s A|
The elementary charge, symbol e, is a physical constant. It is the electric charge carried by a single proton or, equivalently, the magnitude of the electric charge carried by a single electron, which has charge −e.
The numerical value of the elementary charge, symbol e, is defined to be exactly 1.602 176 634 × 10−19 when expressed in the unit coulomb, symbol C, or s A.
The elementary charge, e, together with the hyperfine transition frequency of the caesium 133 atom, ΔνCs, forms the basis for the definition of the ampere.
Atoms are composed of subatomic particles called protons, neutrons and electrons. These particles have electric charge of either +e, –e or 0.
|Particle||Charge in e||Charge in SI units|
|electron||-1||-1.602 176 634 × 10−19 C|
|proton||+1||1.602 176 634 × 10−19 C|
The Standard Model of particle physics describes 6 subatomic particles called quarks. These particles have electric charge of either +⅔e or -⅓e. The exact numerical value of the electric charge of these particles expressed in coulombs follows from the definition of the value of e:
|Quark||Charge in e||Charge in SI units|
|up||+⅔||1.068 117 756 × 10−19 C|
|down||-⅓||-0.534 058 878 × 10−19 C|
|charm||+⅔||1.068 117 756 × 10−19 C|
|strange||-⅓||-0.534 058 878 × 10−19 C|
|top||+⅔||1.068 117 756 × 10−19 C|
|bottom||-⅓||-0.534 058 878 × 10−19 C|
A proton is composed of two up quarks of charge +⅔e and one down quark of charge –⅓e, giving a total charge of +1e. A neutron is composed of one up quark of charge +⅔e and two down quarks of charge –⅓e, giving a total charge of 0e.
The elementary charge e corresponds to a coupling strength of the electromagnetic force via the fine-structure constant α = h2/(2c ε0h) where ε0 is the vacuum electric permittivity or electric constant. Some theories predict a variation of α over time. The experimental limits of the maximum possible variation in α are so low, however, that any effect on foreseeable practical measurements can be excluded.