Conservation of Charge
Physics is that one field of education where we can see what is happening, meaning the results, and the examples are macro, and we can see them in our daily lives. One of the major concepts of physics revolves around the charge, which is stored in a given body. When you come across a charge in your physics book, you know things are going to get serious from this point onwards. Don't worry, and we will help you learn everything about the conservation of charge and its real-world example so that you can get a better understanding of the concept.
Charge in physics is what atoms, protons, and electrons are to chemistry, it is the base of electronic physics. Everything you see, from your computer, TV, to your washing machine runs on the conservation of charge, and today we are going to break down this concept.
Law of Conservation of Charge
Now let's define the conservation of charge. It states that a positive charge present in the given body will always have the same amount of negative charge to keep the body in a neutral state. These types of bodies are called the neutral body, and you can't say they don't charge them, as they have both negative and positive charges in equal portions to cancel them out. As a result, we concluded that a charge in a given body could not be created, nor could it be destroyed. We can only transfer it from one system to another, and the material that provides transfer of charge is called conductors. From conductors, a charge can be displaced in the form of heat or the displacement of electrons. This is what the law of conservation of charge is in physics.
What is Conservation of Charge?
There are two ways in which a body can leave its neutral state of charge.
Object Getting a Negative Charge
A given object will get a negative charge if the electrons get transferred to it from another source.
Let's take an example here, and we take a negatively charged rod with a net charge of -4e. When this rod touches the surface of a neutral body, which is a conductor, displacement of the electron takes place from the rod to the neutral body. This is because electrons are repulsive to each other, and they want to spread in a wider area to get away from each other, after the transfer of charge we have -2e and -2e on the rod and the sphere respectively.
The total charge in the system was -4e, and after the transfer, it remains -4e, but now it is divided into two bodies. That's how a body gets a negative charge.
The Object is Positively Charged
An object present in the neutral charge state will get a positive charge when the electrons present inside of it, get transferred to another body.
We take a rod that is positively charged, and we touched the surface of the neutral body with the rod. During this process, electrons present in a neutral body are attracted to the charge present in the rod, and thus, they are displaced from the neutral body to the rod. As a result, what we have left is a body that has less negative charge than its positive charge making it a positively charged body.
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Here we have a +4e charge present in the neutral body when the transfer of the electrons took place, and we are left with a +2e charge on the rod and a +2e charge on the body. So the object becomes positively charged, and the protons that were present in the body remain the same. Here once again, we have seen the conservation of charge.
State The Law of Conservation of Charge
The net charge present in the isolated system will always remain constant; thus, the given system will not be doing any exchange of mass and energy with its surrounding atmosphere and will never charge that is different from its initial state. This is what the law of conservation of charge is according to physics.
Conservation of Charge Examples
Let's take one example here, and you might have seen an old trick of comb and hair, where your hair rises and sticks to the comb. It might look like magic, but it comes in the simple conservation of charge examples. Your hair is here in a neutral state, having both positive and negative charges in an equal amount. A combination has a positive charge, and when you use it, it takes away the positive charge from your hair and leaves it with a negative charge.
Thus, the negative charge starts to repel each other, and you will have yourself floating hair in the air.
FAQs on Law of Conservation of Charge
1. What is the Law of Conservation of Charge as per the CBSE Class 12 syllabus for 2025-26?
The Law of Conservation of Charge is a fundamental principle in physics which states that the total electric charge in an isolated system remains constant over time. This means that charge can neither be created nor destroyed; it can only be transferred from one body to another. For any process, the net algebraic sum of all charges in a system before the process is equal to the net algebraic sum of all charges after the process.
2. How can you demonstrate the conservation of charge with a real-world example?
A classic example is rubbing a neutral glass rod with a neutral silk cloth. Before rubbing, both objects have a net charge of zero. During the process of rubbing:
- Electrons are transferred from the glass rod to the silk cloth.
- The glass rod loses electrons and becomes positively charged.
- The silk cloth gains an equal number of electrons and becomes negatively charged.
3. Does the law of conservation of charge apply at the microscopic and subatomic levels?
Yes, the law of conservation of charge is a universal law that holds true at all scales, including microscopic and subatomic levels. In nuclear and particle physics reactions, the net charge is always conserved. For instance, in the beta decay of a neutron (charge 0), it decays into a proton (+e), an electron (-e), and an antineutrino (0). The initial charge is zero, and the final total charge is (+e) + (-e) + 0 = 0, thus conserving the charge.
4. How is the law of conservation of charge related to Kirchhoff’s Junction Rule in electric circuits?
Kirchhoff’s Junction Rule (or First Law) is a direct application of the law of conservation of charge to electrical circuits. The rule states that the algebraic sum of currents entering any junction in a circuit is equal to the sum of currents leaving that junction. Since electric current is the rate of flow of charge, this rule essentially means that charge does not accumulate at a junction. The amount of charge flowing into the junction per unit time must equal the amount of charge flowing out, upholding the principle of charge conservation.
5. If charge cannot be created, how do phenomena like pair production and annihilation align with this law?
These phenomena seem to contradict the law, but they actually uphold it by conserving the net charge.
- Pair Production: A high-energy gamma-ray photon (charge = 0) converts into an electron (-e) and a positron (+e) pair near a heavy nucleus. The initial charge was zero, and the final net charge is (-e) + (+e) = 0.
- Annihilation: An electron (-e) and a positron (+e) collide and annihilate each other, producing two gamma-ray photons (charge = 0). The initial net charge was (-e) + (+e) = 0, and the final charge is zero.
6. Why does any excess electric charge given to a conductor reside only on its outer surface?
This happens due to two main reasons based on the properties of conductors:
- Free Movement of Charges: Conductors have a large number of free electrons that can move easily throughout the material.
- Electrostatic Repulsion: When excess charge is added, the like charges repel each other with a strong electrostatic force. To minimize this repulsion, they move as far apart as possible. The most stable configuration they can achieve is by distributing themselves uniformly over the outer surface of the conductor.
7. What is the key difference between the conservation of charge and the quantisation of charge?
While both are fundamental properties of electric charge, they describe different aspects:
- Conservation of Charge refers to the total quantity of charge. It states that the net charge of an isolated system is always constant. It deals with the 'before and after' of any process within the system.
- Quantisation of Charge refers to the discrete nature of charge. It states that any observable charge (q) on a body is always an integral multiple of the elementary charge (e), which is the charge of a single electron or proton (q = ne, where n is an integer). It means you cannot have a fraction of the elementary charge.
