Properties of Electric Charge: Quantisation, Conservation, and Additivity Explained

The table below will be ideal when revising NCERT class 12 physics for the first chapter on the topic of charge properties. 

Property	What It Means	Basic Example
Quantisation of Charge	Charge will only or always have discrete packets, appearing as integer multiples or real numbers of the elementary charge, when using the charge formula: q = n×e. There can never be any fractions of charge.	An object can have 1, 2, or 1000 electrons, but never 1.5 electrons.  Nature deals only in whole numbers.
Additivity of Charge	The total charge in a system is the sum of all individual charges in a system. Summation is algebraic here.	Q_total = q_1 + q_2 + q_3 + ... + q_n You just add up like rupees in your wallet.
Conservation of Charge	The total charge in an isolated system is always constant. We cannot ever create a charge or even destroy it. But we can transfer it from one point to another.	When two bodies exchange charge, the sum before will be equal to the sum after.

 

Section 1.4.3 in the NCERT Class 12 Physics book highlights the “fact that electric charge is always an integral multiple of e is termed as quantisation of charge.”

That means that the total electric charge (q) on any object will only be able to exist as whole numbers, or more precisely, in integer multiples of the elementary charge (e). 

Mathematically, we can express it as

q = ne

n is an integer (0,1,2, and so on)

e is 1.602×10^-19 C, and it’s also the charge of one single electron. That’s the base unit of charge, which you might refer to in Units and measurements class 11 notes or chapter.  

That means, all possible values of electric charge would be 0e, 1e, 2e, and so on. They could also have negative counterparts, depending on the polarity of charges. Like -1e or - 2e. 

If you ask why this quantisation of charge is there in discrete units, it’s because all matter with electrons, protons, and neutrons will have one elementary charge. There can never actually be half an electron. So the charge will only have discrete values.  It’s not physically quantifiable to be 1.5e or 7.4e at the microscopic level. 

So the main idea for quantisation of charge at the microscopic level is to remember.

• e is too tiny (1.602×10^-19 C)
• n is a whole number 

But there is another aspect when quantisation is not used strictly. That is, when calculating large scale charges on everyday objects, ie, at the macroscopic level. 

Quantisation of Charge: When Can You Ignore It?

At the macroscopic level, the elementary charge unit is way too tiny when you compare it with everyday charges.
Or the other way around, the total charge (q) becomes extremely large when compared to a single electron charge. 

Just think about this comparison. 1μC=10^−6 C. That itself is 6.25×10^12 electrons. 

It’s much like looking at a sandy beach from a distance. You know it’s made up of grains but from afar, it has a smooth, continuous surface. 

So, in practical calculations for your NCERT Exemplar Class 12 Physics Chapter 1 exercises, you will come across some tricky problems. You have to figure when to or not to consider quantisation of charge.

There is a law of conservation of charge that applies to isolated systems. The total or net electric charge (q) is constant with respect to time. It tells that it is impossible to create a charge and even destroy it. Electrical charge can only be transferred because it never changes. 

Major Misconception on the Law of Conservation of Charge 

A common mistake for some students while learning this conservation property of charge is thinking that the total charge sum is zero. 

If you’re having this same doubt, too, you must overcome this by considering that the charge can be transferred from one body to another. 

Take the example of an object A that gains electrons from another object B upon rubbing or contact. Object A is negatively charged. Object B becomes positively charged. What that means is the total charge (q) remains unchanged even after rubbing. Not necessarily zero. 

So when you have to calculate charge transfer, you must look at the net charge before and after the transfer. They must be equal. 

Q_before = Q_after

or, better put, 

$\sum q_{\text{initial}}=\sum q_{\text{final}}$

As a scalar quantity, electric charge just has magnitude and sign. To calculate the total charge of the system that has multiple point charges (q_1, q_2, q_3, and so on), you just have to sum them using algebra.
Representatively, 

Q_total = q_1 + q_2 + q_3…+q_n

Do Not Neglect the Sign

A major error some students overlook is the sign of the charge while calculating the total charge. 

For instance, a system has charges: 5 nC, -6 nC, +3 nC, +4 nC, and +1 nC. 

To calculate the total charge (Q_total), we have 

Q_total = (+5) + (-6) + (+3) + (+4) + (+1) = +7nC

Now, this rule of additivity of charge is one of the most important basic properties of charge that you will be applying while learning Coulomb’s Law Class 12.  There, you must add up all the point charges, including their signs, algebraically. 

For electric charge properties Class 12 CBSE exams, expect a mix of conceptual and numerical-based questions. 

Sample Questions on Properties of Charge Class 12	Electric Charge Property Tested
What is the meaning of 'charge is quantised'?	Quantisation, scalar quantity
Why can we ignore quantisation for macroscopic charges?	Macroscopic approximation
Will any charge exist without mass?	Charge existence
The net charge of the system is 0, so does that mean there are no charges present?	Net charge, algebraic sum
Calculate electrons in $Q = 3 x 10^-7C	Number of electrons, charge transfer
Find the charge if n electrons are added or removed.	Scalar quantity, algebraic sum
Time to get 1C if $10^9 electrons or second move?	Macroscopic approximation
Find net charge: +5nC, -6nC, +3nC, +4nC, +1nC	Additivity, net charge, algebraic sum

 

You can check the complete study material for Class 12 CBSE exams based on NCERT Textbooks. Check below. 

Complete CBSE Class 12 Study Material
NCERT Class 12 Maths Solutions	NCERT Physics Class 12 Solutions	Class 12 Chemistry NCERT Solutions
Class 12 Maths NCERT Notes	Physics Class 12 NCERT Notes	Class 12 Chemistry NCERT Notes
CBSE Sample papers for class 12 Maths	CBSE Class 12 Physics Sample Papers	CBSE Class 12 Sample Paper for Chemistry
CBSE Class 12 Maths Previous Year Question Papers	CBSE Class 12 Physics Previous Year Question Papers	CBSE Chemistry Class 12 Question Papers