Electrostatics - Definition, Example & Explanation

What is Electrostatics?

Electrostatics is a branch of physics that deals with the study of electromagnetic phenomena where electric charges are at rest, i.e., where no moving charges exist after a static equilibrium has been established. In physics, the electrostatic phenomenon pertains to the properties of slow-moving or stationary electric charges. Moreover, as this phenomenon arises from the forces exerted by the electric charges on each other, it is defined by Coulomb's Law. Hence, to understand the concept of electrostatic, there is a need to be aware of electric charge and Coulomb’s Law.

 

What is Electric Charge?

Electric charge, also known as charge or electrostatic charge, is defined as the basic property of subatomic particles that causes them to experience a force when placed in an electromagnetic field. In general, electric charges are of two types – positive carried by the charge carriers named protons and negative by charge carriers termed as electrons. If the net charge of an object is equal to zero, i.e. neither positive nor negative, then it is said to be neutral. Electric charge is symbolized as Q and measured using coulomb. 

 

S.I Unit of Charge is Coulomb. 

 

Positively Charged Particles:

In this type of particle, numbers of positive ions (protons) are larger than the numbers of negative ions (electrons). To neutralize positively charged particles, electrons from the surroundings come to this particle until the number of protons and electrons become equal.

 

Negatively Charged Particles:

Similarly, electrons are larger in number than that of protons. To neutralize negatively charged particles, electrons move to the ground or any other particle around until the number of protons and electrons become equal. 

 

Coulomb's Law:

It is an experimental law that measures the amount of force amid two stationary charges. According to Coulomb's law, opposite charges always attract whereas like charges always repel each other with force directly proportional to the product of the charges and inversely proportional to the square of the distance amid them.

 

Explanation:

To understand this law, let's take an example where two electrically charged particles are positioned near to each other. Depending on the nature of the charge (positive or negative) carried by the particles, either an attraction or a repulsion force will act on them. This force between the two electrically charged bodies can be calculated by using a formula first developed by Charles-Augustin de Coulomb under Coulomb's Law. 

 

Coulomb's Law Formula

Let Q1 and Q2 are the charges of the two electrically charged bodies and ‘d’ be the distance amid the centre of the bodies. Now, if the electrically charged bodies are placed in a medium of permittivity, then force can be expressed as:

F = k ((Q1/Q2)/d2)

Where, 

Q1, Q2 = charges

F = electric force

k = Coulomb constant

r = distance of separation

 

Coulomb’s First Law:

Bodies with like charges repel each other, and bodies with unlike (opposite) charges, attract each other.

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Coulomb’s Second Law:

The force, whether of attraction or repulsion, between two charged bodies is directly proportional to the product of their charges and inversely to the square of the distance amid them. 

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According to the second law, 

F ∝ Q1 Q2 and F ∝ 1/d2

Hence, F ∝ ((Q1 Q2) / d2)

F = k ((Q1/Q2)/d2)

Where,

Q1 and Q2 = Charges of the charged bodies

d = distance amid the centre of the two charged bodies

k = constant based on the medium in which the bodies are positioned

F = Force of attraction or repulsion between the charged bodies

Note that in both the S.I and M.K.S system, k = 1/4 εoεr and the value of εo =8.854 x 10⁻¹² C²/Nm². The value of εr that changes with change in medium is 1 in vacuum and air.  

 

Principle of Coulomb's Law

To understand the principle of Coulomb's Law, suppose you have two bodies, out of which one is positively charged, and the other is negatively charged. In this case, the two bodies will attract each other as they have opposite charges. Now, if you increase the charge of one body, leaving the other one as it is, then the force of attraction will increase. Hence, we can conclude that the force amid the charged bodies is directly proportional to the charge of the bodies. Now, keeping the charge Q1 and Q2 of the two bodies constant, if you bring them closer, the force amid them will increase. However, if you place them far from each other, then the force will decrease. So, we can say that the force between two charged bodies is inversely proportional to the distance between them. 

Remember that the force developed between two charged bodies isn't the same in all the mediums, and vary with the mediums. 

 

Electric Field

An electric field refers to a field developed around an electrically charged body and exerts force on other charged objects within that region. 

Electric Field is abbreviated as E-field. It is calculated by the term called electric field density. For instance, if you place a positive (+) unit charge close to a positively charged body, a repulsive force will occur. The resulted force will make the unit charge to move away from the body. The imaginary line over which this unit charge will move is termed as the line of force.

Similarly, if you place a positive (+) unit charge in the field around a negatively charged object, it will experience a force of attraction. The resulted force will compel the unit positive charge to come closer to the negatively charged object. In this case, the line through which the unit charge moves is the line of force.

You can place the positive unit charge anywhere in the field around the positively charged object. The line of force that the charge takes to move will be different for each position where you place it and get radiated from or come out of the charged object.

On the other hand, the lines of force in the case of the negatively charged object will come into the negatively charged object.  

Hence, Coulomb's law states that the force with which like charges repel each other and opposite charges attract each other is proportional to the product of the charges and inversely proportional to the square of the distance between them.

We show charge with ‘q’ or ‘Q’ and, the smallest unit charges are 1.6021x10⁻¹⁹ Coulomb(C). One-electron and a proton have the same amount of charge.