Coulomb's Law

It was first observed in 600 BC by Greek philosopher Thales of Miletus, if two bodies are charged with static electricity, they will either repulse or attract each other depending upon the nature of their charge. This was just an observation but he did not establish any mathematical relation for measuring the attraction or repulsion force between charged bodies. After many centuries, in 1785, Charles Augustin de Coulomb who is a French physicist, published the actual mathematical relation between two electrically charged bodies and derived an equation for repulsion or attraction force between them. This fundamental relation is most popularly known as Coulomb's law.

Statement of Coulomb’s Law

First Law

Like charge particles repel each other and unlike charge particles attract each other.

Second Law

The force of attraction or repulsion between two electrically charged particles is directly proportional to the magnitude of their charges and inversely proportional to the square of the distance between them.

Formulas of Coulomb’s Law

According to the Coulomb’s second law,

Where,

1. ‘F’ is the repulsion or attraction force between two charged bodies.
2. ‘Q1’ and ‘Q2’ are the electrical charged of the bodies.
3. ‘d’ is distance between the two charged particles.

1. ‘k’ is a constant that depends on the medium in which charged bodies are presented. In S.I. system, as well as M.K.S.A. system k=1/4πε. 

The value of ε0 = 8.854 X 10-12C2/Nm2.


Principle of Coulomb’s Law

Suppose if we have two charged bodies one is positively charged and one is negatively charged, then they will attract each other if they are kept at a certain distance from each other. Now if we increase the charge of one body keeping other unchanged, the attraction force is obviously increased. Similarly if we increase the charge of second body keeping first one unchanged, the attraction force between them is again increased. Hence, force between the charge bodies is proportional to the charge of either bodies or both.

Now, by keeping their charge fixed at Q1 and Q2 if you bring them nearer to each other the force between them increases and if you take them away from each other the force acting between them decreases. If the distance between the two charge bodies is d, it can be proved that the force acting on them is inversely proportional to d2.

This development of force is not same for all mediums. As we discussed in the above formulas, εr would change for various medium. So, depends on the medium, creation of force can be varied.

Limitation of Coulomb’s Law

1. Coulomb's Law  is valid, if the average number of solvent molecules between the two interesting charge particles should be large.

1. Coulomb's Law  is valid, if the point charges are at rest.
2. It is difficult to apply the coulomb's law when the charges are in arbitrary shape. Hence, we cannot determine the value of distance‘d’ between the charges when they are in arbitrary shape

OHM'S LAW

One the most important & fundamental laws of electronics is the ohms law. This law defines the relationships between current, voltage and resistance. A good way to understand ohms laws is an analogy with a domestic water system.

Thinking how? Let’s see:

Let’s start with the terms present in the definition:

• V = voltage: The easiest way to think of voltage is to call it potential. We know the potential is something which is very useful to do work. Imagine two tanks of water connected with a pipe. If one tank of water is placed higher than the other then there is potential for water to flow from the high level tank to the low level tank. This water pressure is similar to that used to drive electricity around a circuit, called the potential difference, this is measured in volts. This potential difference is provided by a battery or in the case of huge electricity a generator at the generating station.

• I = current: As the water flows through the pipe in a water system due to potential difference ,in the same way an electric current flows through a copper wire. So, current is simply the transfer of something from one place to another. The standard unit of electric current is one ampere that is the current produced by a one volt source in a circuit having a resistance of one ohm.

• R = resistance: Resistance meaning anything that opposes the flow of current. In this case of two tanks of water connected by a pipe, imagine resistance is formed by the pipe. As the pipe gets wider, more water flows & as the pipe gets narrower, less water flows. If there were no pipe between the two bodies of water, we can say there is infinite resistance. The unit by which electrical resistance is measured is Ohm & one ohm is equal to the current of one ampere which will flow when a voltage of one volt is applied.

Now that you got the concept ohm, volt and ampere, so now it’s the time to introduce you to the relationship in between them that is ohms law. The statement is, the electric current passing through a conductor is directly proportional to the potential difference across it, provided that the temperature remains constant. The constant of proportionality is the resistance of the conductor.

The definition above simply states that the current passing through a conductor increases if you increase the voltage.

So, we can say: V proportional to I

Thus, V = IR, Where , V = potential difference in volts (V)

I = current in amps (A) and

R = the constant of proportionality that is the resistance.