# elementary charge

##### SI defining constant
Name Symbol Value Unit Expressed in terms of SI base units
elementary charge e 1.602 176 634 × 10−19 C s A

### Definition

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.

### Particle physics

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 neutron 0 0 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.

### Nature

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.