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An elementary charge is the magnitude of electric charge associated with a single electron. Similar to time, length or mass, the elementary charge is a fundamental measurement of a fundamental physical constant. The coulomb is the unit of elementary charge in the International System of Units.
The elementary charge is usually represented by e. In order to avoid confusion on the sign, e is normally considered as a positive elementary charge. In other words, protons have a positive e charge, whereas electrons have a negative e charge. The elementary charge is also the electrical charge carried by a proton, however with opposite polarity. The measured value of the elementary charge is approximately (1.602 176 487 ± 0.000 000 040) × 10-19 coulombs or 4.8 × 10−19 statcoulombs in cgs units. The elementary charge was defined before the discovery of quarks, and outside of particle physics, the elementary charge is still considered the smallest electrical charge possible.
The importance of the elementary charge lies in that fact almost all freely existing charged subatomic matter discovered so far, including electrons, have an electric charge equal to the value of the elementary charge or can be expressed by whole number multiples of the value. When it comes to quarks, their charges can be expressed as a fraction such as one-third or two-thirds of the value.