Electrostatics - Coulomb's Law of Electrostatic

Introduction:
Electrostatic is a branch of physics which works with the static electric charges. Electrostatics class 12 states that Electrostatic meaning is to deal with the phenomena and properties of stationary electric changes or slow moving electric charges. In this article we will study about that what is electrostatic, coulomb’s law of electrostatics and electrostatic definition.

Introduction to electrostatics:

Electrostatics is the study of the static electric charges. Here we will study about the forces, fields and potentials connected with static charges. To study in detail about the electrostatics concepts let us look at the applications of electrostatics.

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Applications of electrostatics.
The force of attraction and force of repulsion amid charged bodies have many industrialised applications. Some of these are as follows:
1. In electrostatic loudspeaker.
2. In electrostatic sprinkling of paints and powder coating.
3. In flyash collection in chimneys.
4. In a Xerox copying machine.
5. In the designing of a cathode-ray tube used in television and radar.

Two kinds of electric charges:
Around 100 years ago, Charles Du Fay of France presented that electric charges on numerous objects are of only two categories. The subsequent simple experiments prove this information.

Experiment no. 1:

(1) Brush a glass rod with silk and append it from a rigid backing by means of a silk thread. Bring alternative alike charged rod near it. The two rods repel each other.
(ii)Bring a plastic rod brushed with wool near the charged glass rod. The two rods attract each other.
(iii) Now brush a plastic rod with wool and append it from a rigid backing. Bring alternative alike charged plastic rod near it. There will be a repulsion amongst the two rods.

Experiment 2:

If a glass rod, brushed with silk, is made to touch two small pith balls (or polystyrene balls) which are suspended by silk threads, then two balls repel each other. Correspondingly, two pith balls touched with a plastic rod brushed with fur are set up to repel each other. But it is seen that a pith ball touched with glass rod attracts another pith ball touched with a plastic rod.
From the overhead experiments, we note that the charge formed on a glass rod is dissimilar from the charge created on a plastic rod. Also the charge formed on a pith ball touched with a glass rod is changed from the charge created on pith ball touched with a plastic rod. We can conclude that:
1. There are simply two categories of electric charges - positive and negative.
2. Alike charges repels each other whereas unlike charges attracts each other.
3. The statement 2 is known as the fundamental law of electrostatics.

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The above experimental electrostatic examples also exhibit that the charges are transported from the rods to the pith balls in contact. We say that the pith balls have been electrified or charged by interaction. This property which segregates the two categories of charges is entitled as the polarity of charge.

Conservation of charge implies that electric charges can be created or destroyed always in the form of equal and opposite pairs but never in isolation. For example, in the beta decay of a neutron (zero charge), a proton (charge + e) and an electron (charge - e) are produced. Total charge remains 0 previously and afterwards the decay.

The law of conservation of charge is a precise law of nature. It is valid in all areas of nature. Even in the domains of high energy physics, where mass changes into energy and vice-versa, the law of conservation of charge strictly holds good.

Let us define electrostatics in physics. In physics electrostatics is the phenomena and properties of static electric changes or slowly moving electric charges. The electricity created by friction is termed as frictional electricity. If the changes in a body do not move, then the frictional electricity is also known as Static electricity.

Coulomb's law of electrostatic:

In 1785, the French physicist Charles Augustin Coulomb (1736-1806) used a torsion balance to experimentally measure the electric forces between charged small spheres. He expressed his observations in the form of Coulomb's law, which is similar in electricity to the law of universal gravitation in Newtonian mechanics.

Coulomb meaning in Tamil is கூலம்.
Coulomb's law states that the attractive or repulsive forces amongst two fixed-point charges (i) is proportionate to the product of the magnitudes (size) of the 2 charges, and (ii) is in reverse proportionate to the square of the distance amongst them. This force acts along the line relating the two charges.

If two point charges q₁ and q₂ are parted by a distance r, then the attractive or repulsive force F amongst them is such that

F∝q₁q₂ and F∝ 1 / r²
∴F∝ q₁q₂ / r² and F∝K q₁q_{2 /} r².
Where k is a proportional constant, called the electrostatic force constant. The value of k depends on the nature of the medium between the two charges and the unit system chosen to measure F and r.
For two charges in free space and SI units, we have
K= 1 / 4πε₀= 9 x 10⁹ N m² C^-2

The constant ε₀ is termed as the permittivity of free space. So we can define Coulomb's law in SI units as
F= 1 / 4πε₀. q₁q₂ / r²

Charge units.

(i) The unit of charge in the International System of Units is the coulomb. In the above equation, if q₁=q₂=1 C and r = 1m, then
F= 1 / 4πε₀= 9 x 10⁹ N
Therefore, Coulomb is an equal and similar charge with a force of 9 x 10⁹ N when placed at a distance of one meter of it.
(ii) In the electrostatic CGS system, the charge unit is called the electrostatic charge unit (e.s.u. of charge) or statcoulomb (stat C). Chapter
The one e.s.u. or one statcoulomb is the repulsion of the same charge with a force of 1 dyne in a vacuum at a distance of 1 cm.
1coulomb = 3 x 10⁹ statcoulomb
= 3 x 10⁹ e.s.u. charge

A torsion balance is a delicate device to measure the extent of force. When the undeviating sizes of charged bodies are much lesser than the distance amongst them, their sizes may be unnoticed and the charged bodies are entitled as point charges. Coulomb's law is effective only for point charges. In SI units, the exact value of the combination 4πε₀ is

4πε₀ = 10⁷ / c² C² N^-1 m^-2

Where c is the speed of light in vacuum having the exact value 2.99792458 x 10⁸ meter per second.

Electrostatic force constant, k=8.98755 x 10⁹ N m² C^-2 = 9 x 10⁹ N m² C^-2

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Range of coulombian forces:

Coulombian forces act over an enormous range of separation (r), from nuclear dimensions (r=10^-15 m) to macroscopic distances as large as 10¹⁸ m. Inverse square is effective over this range of parting to a high degree of accuracy.

Electrostatic force vs. gravitational force.

Electrostatic force is the force of attraction or repulsion between two charges at rest while the gravitational force is the force of attraction between two bodies by virtue of their masses.

Similarities:

1. Both forces comply with inverse square law.

2. Both forces are proportionate to the product of masses or charges.

3. Both are central forces i.e., they act along the line connecting the centres of the two bodies.

4. Both forces are conservative that is the work done in contrast to these forces does not be governed by the path followed.

5. Both forces can operate in vacuum.

Dissimilarities:

1. Gravitational force is attractive whereas electrostatic force may be attractive or repulsive.

2. Gravitational force does not be determined by the nature of the medium whereas electrostatic force be governed by the nature of the medium between the two charges.

3. Electrostatic forces are much tougher than gravitational forces.

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