## Thursday, July 14, 2016

### Electric Charge and Electric Force

Electrostatics is a branch of physics that deals with charges in the state of rest and its applications. We know from very long days that a glass rod rubbed with silk cloth can attract is able to attract small pieces of paper.  Basing on this kind of understanding, charges are divided into two types called positive and negative charges. It is also concluded that the similar charges repel each other and opposite charges attract each other. All bodies by default are neutral and they have equal number of positive and negative charges.

According to modern concepts of physics, negative charge is due to electrons and positive charge is due to protons and they are inside the nuclei. The actual charge that is mobile is due to electrons and protons won’t move and transfer. All charge and current is due to electrons only. The body who gain electrons is said to be negatively charged and the body that lost electrons is said to be positively charges. Thus charge is neither created nor destroyed and it just transfers from one body to other. This is called the concept of conservation of energy and it is always true and it is one of the basic concepts of conservation.

Charge is also available as integral multiples of electrons. It is always available as electron transfer and always it is the complete electron that transfers from one body to other. It won’t happen as integral electrons but not as part of electron.

The body gain electrons will get negative charge and it also gains a little mass of that electrons itself. The body that loose electrons, will lose negative charge and hence it will have more positive charges and its mass also decreases slightly. Charges are conserved and quantized. This charge is treated as scalar and it has only magnitude but not any direction.

The force of attraction or repulsion between two charges depends on the magnitude of charges and is inversely proportional to the square of the distance between the bodies. This is called Coulomb’s inverse square rule. This force also depends on the medium between the two charges. Here we are going to understand the impact of the medium on force between two charges. If the medium is good, it conducts charge is better and vice versa. The impact is studied with parameter called permittivity of medium or free space.

The force of attraction between two charges is independent of presence of third charge. It means even after the third charge, the force between two charges remains same. But on each charge, now there is more force due to extra charge. To find the effective force on any charge, we need to add all the existing forces using the vector laws of addition.

Between two charges, we can find the effective force on any charged particle placed somewhere in between them. We can also find a null point where the effective electric force on it is zero. This can be found when the two charges are same and even when the two charges are different.

Neutral Point

Let us consider two similar charges separated by certain distance. Each charge repels the other one with a certain force. Now let us assume that we have kept a third charge between them. On the third charge there will be force of repulsion if it is similar to the two existing charges. We need to keep the third charge at a certain place so that the two forces are acting on it in opposite directions and they are equal in magnitude so that they can cancel each other.

Thus the resultant force on the third charge will be zero and hence at that particular location, the new charge is not going to experience any force. That point is called neutral point. If the third charge is opposite to the two existing charges, it will experience attractive force and they can cancel each other.

If two charges are opposite in nature and they are separated by a certain distance, then we cannot find a neutral point in between them as the two force acting on a third charge are in the same direction and hence there will be some resultant force and they cannot cancel each other. We can get null point only beyond them but not in between them.

We can find the neutral point not in between the two charges and outside of the system as  shown.

If suppose two similar charges of equal magnitude are separated by some distance, we can keep either a positive or negative charge between them and it can be in neutral mode. It means when we place it properly, it won’t experience any resultant force. But to keep the entire system in equilibrium mode, we shall keep negative charge only when the initial two charges are positive and vice versa.

Between two positive charges, if we keep a positive charge that third charge can be in equilibrium position, but these two charges do experience a resultant force and hence we cannot get equilibrium. If we keep a negative charge, that charge as well as the other two charges also experience two forces and these two forces are in opposite direction. Hence they can cancel each other and can get a equilibrium.

Problem

Let us consider two charges separated by a certain distance and we need to know the location of third charge so that the system is in equilibrium.

Solution

We need to identify either positive or negative charge which can keep the system in equilibrium.

Only negative charge can create the required equilibrium.

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