A Description of Coulombs in the International System of Units

Electric charge is measured in coulombs (C), which is the standard unit in the International System of Units (SI). A 1-ampere (A) current carries 1 C of electricity in one second (s). This amount is equal to the electrical charge of about 6.24 x 1018 electrons or protons, which is equivalent to 6.24 quintillion particles.

The coulomb is a derived unit in the SI standard and is constructed from the base units of ampere and second. Prior to 2018, the SI standard was based on seven base units, and the derived units were constructed from them. Now, the standard is based on seven defining constants, and all units can be constructed from them. However, the concept of base and derived units is still retained in the SI standard.

One of the seven constants is the elementary charge, which is the electric charge carried by a single electron or proton. The elementary charge is fixed at 1.602176634 x 10-19 C. Electrons and protons carry the same amount of charge, but electrons are negative, and protons are positive.

By fixing the elementary charge at a specific value, the coulomb is also fixed at a specific number of electrons or protons that make up 1 C of charge. The formula Q = n x e can be used to calculate this number, where Q represents the amount of charge in coulombs, and n refers to the number of electrons or protons. Dividing both sides of the equation by e gives the number of particles in a coulomb as approximately 6.24 x 1018.

6,241,509,070,000,000,000

When we talk about electric current in SI base units, we use the Coulomb (C) for electric charge, while the Ampere (A) is the standard unit of electric current. It's important to understand that 1 C equals 1 A ⋅ 1 s or C = A ⋅ s. Alternatively, we can say that an Ampere is equal to 1 Coulomb divided by 1 second, or A = C / s. So, if a circuit has a current of 1 A, that means that 1 C of charge flows through a point in the circuit every second.

To be more specific, an Ampere is defined as "the electric current that corresponds to the flow of 1 / (1.602176634 ⋅ 10-19) elementary charges per second." Meanwhile, a Coulomb is the amount of electric charge carried by a current of 1 Ampere in 1 second.

Before the update to the SI in 2018, the Ampere was defined based on the force between two current-carrying conductors using vacuum magnetic permeability at 4π ⋅ 10−7 henries per meter.

The force between charged objects, whether attracting or repelling, depends on the product of their charges in Coulombs and the distance between them. If polarities are the same (either both negative or both positive), the Coulomb force between the objects is repulsive. However, if the polarities are opposite (one positive, one negative), the Coulomb force is attractive. Additionally, the force decreases proportionally with the square of the distance between the objects' charge centers.

For further reference, other important terms to be familiar with in relation to electric circuits include resistance, Ohm's law, reactance, admittance, susceptance, and Henry.