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The elementary charge, usually denoted by e, is a fundamental physical constant, defined as the electric charge carried by a single proton (+ 1e) or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 e. [2] [a]
The CODATA recommended value of a proton's charge radius is 8.4075(64) × 10 −16 m. [54] The radius of the proton is defined by a formula that can be calculated by quantum electrodynamics and be derived from either atomic spectroscopy or by electron–proton scattering.
The quantity of electric charge can be directly measured with an electrometer, or indirectly measured with a ballistic galvanometer. The elementary charge (the electric charge of the proton) is defined as a fundamental constant in the SI. [7] The value for elementary charge, when expressed in SI units, is exactly 1.602 176 634 × 10 −19 C. [1]
Charge number (denoted z) is a quantized and dimensionless quantity derived from electric charge, with the quantum of electric charge being the elementary charge (e, constant). The charge number equals the electric charge ( q , in coulombs ) divided by the elementary charge: z = q / e .
An atom of gold now was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive nuclear charge now was realized to come entirely from a content of 79 protons. Since Moseley had previously shown that the atomic number Z of an element equals this positive charge, it was now clear that Z is identical to the number of ...
The proton radius puzzle is an unanswered problem in physics relating to the size of the proton. [1] Historically the proton charge radius was measured by two independent methods, which converged to a value of about 0.877 femtometres (1 fm = 10 −15 m).
The CODATA recommended value is −e/m e = −1.758 820 008 38 (55) × 10 11 C⋅kg −1. [2] CODATA refers to this as the electron charge-to-mass quotient, but ratio is still commonly used. There are two other common ways of measuring the charge-to-mass ratio of an electron, apart from Thomson and Dunnington's methods.
Measurements in 2017 give the weak charge of the proton as 0.0719 ± 0.0045 . [4]The weak charge may be summed in atomic nuclei, so that the predicted weak charge for 133 Cs (55 protons, 78 neutrons) is 55×(+0.0719) + 78×(−0.989) = −73.19, while the value determined experimentally, from measurements of parity violating electron scattering, was −72.58 .