Electrostatics – Definition, Example & Explanation

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The study of stationary electric charges is known as electrostatics. The charge associated with matter because of which it produces and experiences a magnetic and electric effect is known as electric charge. When a plastic rod is rubbed with fur or a glass rod rubbed with silk will attract pieces of paper which signifies that both the rods are electrically charged.

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 be the charges of the two electrically charged bodies and ‘d’ be the distance amid the center 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 1.6021×10⁻¹⁹ Coulomb(C). One-electron and a proton have the same amount of charge.

Characteristics of Electrically Charged Objects

The important characteristics of electrically charged objects are-

1. Like charges repel which means + repels +, – repels –

2. Unlike charges attract which means + attracts – and vice versa.

3. There is no net charge on a neutral object which means the charge is conserved. If the fur and plastic rod were neutral at first. After the rod becomes charged by the fur, the negative charge of the fur will be transferred to the plastic rod. The net negative charge on both fur and rod are equal.

Conductors and Insulators

The materials through which electric charges can flow easily as they allow the flow of charge are known as conductors. The most common conductors are all metals.

The materials through which electric charges cannot flow easily due to the presence of electrons that are tightly bound and are not ready to move are known as insulators. The most common examples of insulators are paper and rubber.

The device which is used to indicate the presence of charges is known as an electroscope. The electroscope only indicates the presence of a charge, it does not indicate the nature of charges whether they are positive or negative.

Coulomb’s Law

Coulomb’s law states that in between two charges the magnitude of the electrostatic force Fis given by

\[F = \frac{kq_zq_2}{r^2}\]

Where q1& q2are charges and ris the distance between the two charges

k is the proportionality constant

The SI unit for charge is “Coulomb”. 

If the charge is in coulomb and distance is in meters the approximate value of k will give the unit of force in Newton.

\[k\] = 9.0 \[\times 10^{9}  N. m^{2}/C^{2}\] 

Limitations of Coulomb’s Law

1. Coulomb’s law can be applied only for the point charges which are at rest.

2. Coulomb’s law is applicable only in the cases where the inverse square law is obeyed.

3. Where the charges are in an arbitrary shape, it is difficult to imply Coulomb’s law as it is not possible to determine the location between two objects.

4. Coulomb’s law cannot be directly used to calculate charge on big planets.

Electric Field

When a small positive test charge and a large negative charge are brought together, the small positive test charge experiences a force from the large negative test charge which forces it away. When the test charge is far away from the large charge, the electrostatic force that it experiences is smaller. Therefore an electrostatic field is constituted by this data of direction and the magnitude of the electrostatic force, due to one fixed charge or a set of fixed charges.

The electric field can be defined as the force per unit charge that is exerted on a small positive test charge (q0) which is placed at that point. This can mathematically be given by-

\[E\] =\[\frac{f}{q}\]

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Categories: Physics