Question

Define the SI unit of charge.

Answer 1

Electric charge is one of the basic intrinsic properties of matter. This is the property which is responsible for the force, the matter experiences, when it is inside an electric field. However, we define the electric charge in terms of electric flow instead of electrostatics.

Complete answer:
Let us first understand what we mean by electric charge.
Electric charge is the physical property of matter which is responsible for the force, the matter experiences, when it is put inside an electric field. There are mainly two types of electric charge: positive electric charge and negative electric charge. The direction of force due to an electric field depends on the type of electric charge. However, the magnitude of the force depends on the amount of charge the particle has.

The fundamental value of charge is $1.602176634\text{ }\times \text{ }{{10}^{-}}^{19}~C$. It is the least amount of charge possible for any particle, and any charge transfer must be a multiple of this fundamental value. The fundamental charge is expressed in Coulomb, which is the SI unit of charge.

Hence, we need to understand how the unit “Coulomb” is defined. For that purpose, we look at the flow of electrons, instead of the electrostatics.

We can use the definition of charge like this:
$Q=It$

Where,
I is the current magnitude
Q is the amount of charge flowing through the conductor
t is the time duration of current flow

One Coulomb is defined by the amount of charge that passes through the cross-section of a conductor carrying 1A current in 1 second.

If a conductor is carrying 1 ampere current, then the amount of charge passing through the cross section of the conductor in 1 second is called 1 Coulomb.

This is the universal definition of 1 Coulomb which was proposed in 1946.

Note:
In modern physics however, we can replace the definition with more accurate value. As the fundamental value of charge is $1.602176634\text{ }\times \text{ }{{10}^{-}}^{19}~C$, we can define Coulomb in terms of the fundamental value as well. So, we can say that one coulomb corresponds to the amount of charge for about $6.24\times {{10}^{18}}$ electrons.