All About Limitations Of Coulomb’s Law

Coulomb’s law, Coulomb’s inverse-square law or electrostatic force, was discovered by Charles-Augustin. It calculates the amount of energy that exists between two particles that are electrically charged and stationary. The law is applicable only for the point charges at rest.

The law states that the force existing between two charged particles is inversely proportional to the square of the distance between those particles but directly proportional to the product of charge on each body. Coulomb’s Law can be only applied in those cases where the inverse square law is obeyed.

History of Coulomb’s law  

The first observation of Coulomb’s law was done back in 600 BC by Thales of Miletus, a Greek philosopher. He observed that two bodies were statically charged; they either attracted or repelled each other. He made the observation, but he did not establish any mathematical equation on the same.

Later in 1785, Charles-Augustin de Coulomb, a French physicist, derived a mathematical relation to calculate the force of attraction or repulsion existing between those two particles. The law hence came to be known as Coulomb’s law.

The formula for Coulomb’s law

Let us consider two bodies kept at a distance s from each other, having charges q1 and q2, respectively. The charges kept in a medium have permittivity 𝞮0𝞮r.

Now the force between these particles can be expressed as:

F=q1q2/4𝞹𝞮0𝞮rs2

Coulomb’s First Law

The first Coulomb’s law states the law of attraction. According to the law, like charges repel each other whereas unlike charges attract each other.

Coulomb’s Second law

The second law states that the force existing between the electrically charged particles is inversely proportional to the distance between them and directly proportional to the product of charges present on those particles.

This proportionality is removed by a constant K which depends on the medium in which the particles are placed.

This constant K=1/4𝞹𝞮0𝞮r

Limitations of Coulomb’s law

1. Coulomb’s law cannot be applied if the point charges are not at rest. The reason for this is that when the two charged particles are brought together, the distribution of charges on them changes.

2. Coulomb’s law cannot be applied if the shape of the charges is arbitrary because, in irregular shapes, it becomes difficult to determine the distance between the particles.

3. Coulomb’s law can only be applied if large solvent particles exist between two charged particles.

4. Coulomb’s law cannot be used directly to calculate the charge on big planets. This is because of the arbitrary shapes of planets which makes determining the exact distance between them difficult.

Application of Coulomb’s law

Various applications of Coulomb’s law can be witnessed in our day to day life. Some of them are:

1. Mixing salt or oil in water: While mixing substances, their polarity needs to be observed. Salt is polar in nature and attracts water, while oil is nonpolar and repels waters. This is the reason salt gets mixed in water whereas oil doesn’t.

2. A charged scale attracts bits of paper towards itself. 

3. A comb gets charged and is capable of attracting bits of paper towards itself.

4. Xerox machine.

5. When You rub a balloon with a cloth and bring it close to a wall that is either neutral or positively charged, it attracts the balloon, and the balloon gets stuck on the wall. This showcases the process of Coulomb’s law.

Conclusion

Coulomb’s law is an essential concept in physics. Despite its limitations, the law is used in various places and can be observed in our day to day life. Applicable to only point size charges, Coulomb’s law works on the principle of attraction. If there are two particles having charges q1 and q2 are put together and then separated, each will have a charge q1+q2/2. If one body has a negative charge, then the charge acquired by each particle will be equivalent to q1-q2/2. The coulomb’s constant K has a value of 1 for air, whereas the value of K for metals is infinity.