SI Unit of Electric Charge: All You Must Know
Find out what the SI unit of electric charge is when getting started with electrostatics. We already know charge has a scalar nature with signs that are either positive or negative, and the only way to measure it is using this unit accepted everywhere.
Coulomb, or the C symbol, is the SI Unit of electric charge. If you remember your previous year in Class 11, this unit is derived and not a base one. You learnt it in Units and Measurement Class 11 notes.
When we go through the electric charge Class 12, this SI unit, Coulomb, gives us a consistent way to measure and compare charges so that we can calculate related quantities, namely electric current and strength of electric field.
Maybe this is an overlooked topic for many, but let’s get you clear with its derivation from its base unit, Ampere. With that, we can be sure about solving contextual numericals when it comes to using Coulomb’s Law class 12.
What is a Coulomb: SI Unit of Electric Charge
Coulomb is named after Charles-Augustin de Coulomb, a French scientist of 1700s in the definitive later half. This is a derived unit that draws from current and time in how they relate with each other. We know that the base unit of electric current is Ampere, while for time, we have the base unit in seconds. We can then look at these three quantities in how they relate like this:
One Coulomb tells us how much charge flows through a conductor in one second and when electric current is one ampere.
In maths, we show that as the formula for electric charge (which you can also check in our blog on Electric Charges and Fields formula sheet)
q = I x T
Or better put,
1C = 1 A x 1 s
While this is the practical, human-scale unit for measuring charge, we should also understand the subatomic behaviour of charge. After all, it’s the flow of electrons that conducts electricity. So how many of those electrons are actually flowing with one ampere of charge?
Now, that comes from digging into the maths of how many electrons actually move when we say one ampere of charge is flowing.
SI Unit of Charge: How Many Coulombs Exist in a Single Electron?
A confusion some may have is how many electrons will be there in 1 C or the SI unit of charge.
Know that this one electron charge is a constant in the International System of Units. It’s called the elementary charge. The value? It’s 1.602176634×10-19C - a really microscopic value.
So to find the number of electrons in one coulomb, we have to satisfy the quantisation of electric charge property with the formula that shows charge (q) is equal to an integer (n) times elementary charge (e).
q = ne
In 1 coulomb, the electron number would be 1/ 1.602×10-19 when we take integer n as 1. This becomes close to 6.242×1018 electrons.
This relationship shows us how big of a number of single electron charges have to be there to show one Coulomb.