elementary charge

SI defining constant

Name	Symbol	Base units
elementary charge	e	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.
$e = 1.602 \mspace{4mu} 176 \mspace{4mu} 634 \times 10^{-19} \mspace{6mu} \text{C}$

Electric charge

The fixed numerical value of the elementary charge, e, is defined exactly:

$e \mspace{4mu} = 1.602\ 176\ 634 \times 10^{-19} \ \text{C}$

Inverting this relation gives an exact expression for the unit of electric charge, the coulomb, in terms of the elementary charge, e :

$1 \mspace{4mu} \text{C} \mspace{6mu} = \dfrac{1}{1.602 \mspace{4mu} 176 \mspace{4mu} 634 \times 10^{-19}} \mspace{6mu} e$

Electric current

The elementary charge, e, together with the caesium frequency, Δν_Cs, forms the basis for the definition of the unit of electric current, the ampere.

The definition of the ampere implies the exact relation:

$\Delta \nu _{Cs} \mspace{4mu} e \mspace{4mu} = (9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)(1.602 \mspace{4mu} 176 \mspace{4mu} 634 \times 10^{-19}) \ \text{A}$

Inverting this relation gives an exact expression for the ampere in terms of the elementary charge, e, and the caesium frequency, Δν_Cs :

$1 \mspace{4mu} \text{A} \mspace{4mu} = \dfrac{1}{(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)(1.602 \mspace{4mu} 176 \mspace{4mu} 634 \times 10^{-19})} \mspace{4mu} \Delta \nu _{Cs} \mspace{4mu} e\\$

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.