# Coulomb’s Law

When our synthetic clothing or sweater remove from our bodies, especially in dry weather, a spark or crackling sound appears. With females’ clothing like a polyester saree, this is almost unavoidable. Lightning, in the sky during thunderstorms, is another case of electric discharge. It is an electric shock always felt while opening a car door or grabbing the iron bar of a bus after sliding out of our seats.

The cause of these sensations is the discharge of electric charges that have collected as a result of rubbing insulating surfaces. This is due to static electricity generation. Anything that does not have movement or change with time is referred to as static. The study of forces, fields, and potentials coming from static charges is known as **Electrostatics**.

Let’s now understand one of the basic and important laws of electrostatic called Coulomb’s Law as:

### Coulomb’s Law

Coulomb’s law is a mathematical formula that describes the force between two point charges. When the size of charged bodies is substantially smaller than the separation between them, then the size is not considered or can be ignored. The charged bodies can be considered as point charges.

The force of attraction or repulsion between two charged things is directly proportional to the product of their charges and inversely proportional to the square of the distance between them, according to

Coulomb’s law. It acts along the line that connects the two charges that are regarded to be point charges.

Coulomb studied the force between two point charges and found that it is inversely proportional to the square of the distance between them, directly proportional to the product of their magnitudes, and acting in a line that connects them.

### Coulomb’s Law Formula

The amount of the force (F) between two point charges q

_{1}and q_{2}separated by a distance r in a vacuum is given by,

F ∝ q_{1}q_{2}and

F ∝ 1/r^{2}or

F ∝ q_{1}q_{2}/ r^{2}or

F = k q_{1}q_{2 }/ r^{2}where k is proportionality constant and equals to

1/4πε_{0}.The symbol ε

_{0}is called epsilon not, and it signifiespermittivity of a vacuum.The value of k is

9 × 10when we take the S.I unit of value of ε^{9}Nm^{2}/ C^{2}_{0}is8.854 × 10.^{-12}C^{2}N^{-1}m^{-2}

**Coulomb’s Law in Vector Form**

Coulomb’s law is better written in vector notation because force is a vector quantity. Charges q_{1} and q_{2} have location vectors r_{1} and r_{2}, respectively. F_{12} denotes force on q_{1} owing to q_{2} and F_{21} denotes force on q_{2} owing to q_{1}. For convenience, the two-point charges q_{1} and q_{2} have been numbered 1 and 2, respectively, and the vector leading from 1 to 2 has been designated by r_{21}.

Similarly, the vector leading from 2 to 1 is denoted by r

_{12,}r

_{21}and r_{12}are the magnitudes of the vectors and , respectively and magnitude r_{12}is equal to r_{21}. A unit vector along the vector specifies the vector’s direction. The unit vectors are used to denote the direction from 1 to 2 (or 2 to 1). The unit vectors define as,Similarly,

Coulomb’s force law between two point charges q

_{1}and q_{2}located at vector r_{1}and r_{2}is then expressed as,

**Key Points on Coulomb’s Law:**

- The above expression holds true regardless of whether q
_{1}and q_{2}are positive or negative. F_{21 }is toward , which repulsive force, as it should be for like charges that are if q_{1}and q_{2}are of the same sign (either both positive or both negative). When the signs of q_{1}and q_{2}are opposite or dislike charge, F_{21}is toward , that is toward which shows attraction, as expected for dissimilar charges. As a result, we don’t need to construct separate equations for like and unlike charges. Both instances are handled correctly by the above expression for Coulomb’s force law. - The above expression for Coulomb’s force law can be used to calculate the force F
_{12}on charge q_{1}due to charge q_{2}by simply swapping 1 and 2 as,

Coulomb’s law therefore agrees with Newton’s third law.

- In a vacuum, Coulomb’s law expression determines the force between two charges q
_{1}and q_{2}. If the charges are deposited in matter or there is matter in the intervening area, the situation becomes more complicated due to the presence of charged matter constituents. - Two identical conductors with charges q
_{1}and q_{2}are brought into contact and subsequently separated, resulting in each conductor having a charge equal to (q_{1}+q_{2})/2. Each charge will be equal to (q_{1}-q_{2})/2 if the charges are q_{1}and –q_{2}.

**Force Between Multiple Charges**

Consider a system in a vacuum with n motionless that is stationary charges q_{1}, q_{2}, and q_{3}. It has been proven experimentally that the vector sum of all the forces on a charge due to a number of other charges, taken one at a time, is the vector sum of all the forces on that charge owing to the other charges. Due to the presence of other charges, the separate forces remain unaffected. This is known as the superposition principle.

The force on one charge, say q_{1}, due to two other charges, q_{2} and q_{3}, may be determined By conducting a vector addition of the forces due to each of these charges. As a result, if F_{12} denotes the force exerted on q_{1} as a result of q_{2,}

Similarly, F_{13} denotes the force exerted on q_{1} as a result of q_{3, }which again is the Coulomb force on q_{1} due to q_{3} even though other charge q_{2} is present.

Thus, the total force F_{1} on q_{1} due to the two charges q_{2} and q_{3} can be expressed as,

The above force calculation can be applied to a system with more than three charges. The principle of superposition states that in a system of charges q_{1}, q_{2…….}q_{n}, the force on q_{1} owing to q_{2 }is the same as Coulomb’s law, i.e., it is unaffected by the presence of other charges q_{3}, q_{4},…, q_{n}. The vector sum of the forces F_{12}, F_{13},…, F_{1n} on the charge q_{1} owing to all other charges gives the overall force F_{1} can be written as

The vector sum is calculated by using the parallelogram law of vector addition. Coulomb’s law and the superposition principle are the foundations of electrostatics.

**Limitations of Coulomb’s Law**

- Only the point charges at rest are covered by the law.
- Coulomb’s Law is only applicable in situations where the inverse square law is followed.
- When charges are in an arbitrary shape, it is difficult to apply Coulomb’s law since the distance between them cannot be determined.
- The charge on the larger planets cannot be calculated directly using the law.

**Applications of Coulombs Law**

- Calculate the distance between the two charges as well as the force between them.
- The Coulombs law can be used to compute the electric field that is:

E = F / q_{T}

where E is the electric field, F is the force and q_{T }is the test charge.

Its SI unit is **NC ^{-1}**.

### Sample Problems

**Problem 1: Charges of magnitude 100 micro coulombs each are located in vacuum at the corners A, B and C of an equilateral triangle measuring 4 meters on each side. If the charge at A and C are positive and the charge B negative, what is the magnitude and direction of the total force on the charge at C?**

**Solution:**

The Force F

_{CA}is applied toward AC and the expression for the F_{CA}is expressed asSubstitute the values in the above expression,

The Force F

_{CB}is applied toward CB and the expression for the F_{CB}is expressed asSubstitute the values in the above expression,

Therefore, the two forces are equal in magnitude but in different directions. The angle between them is 120º. The resultant force F is given by,

**Problem 2: A positive charge of 6×10 ^{-6 }C is 0.040m from the second positive charge of 4×10^{-6} C. Calculate the force between the charges.**

**Solution:**

Given,

A positive charge q

_{1}is 6×10^{-6}C.The second positive charge q

_{2}is 4×10^{-6}C.The distance between the charges r is 0.040 m.

Substitute the values in the above expression,

**Problem 3: State Coulomb’s law and its expression.**

**Solution:**

Coulomb’s law is a mathematical formula that describes the force between two point charges. When the size of charged bodies is substantially smaller than the separation between them, then the size is not considered or can be ignored. The charged bodies can be considered as point charges. Coulomb studied the force between two point charges and found that it is inversely proportional to the square of the distance between them, directly proportional to the product of their magnitudes, and acting in a line that connects them.

The amount of the force (F) between two point charges q

_{1}and q_{2}separated by a distance r in a vacuum is given byWhere F is the force between two point charges, q

_{1}and q_{2}are the point charge, r is the distance between the point charge and k is proportionality constant.For subsequent simplicity, the constant k in the above expression is commonly written as

Here, is known as the permittivity of free space. The value of in SI units is

**Problem 4: Why does Coulombs’ force act between two charges only in the line joining their centers?**

**Solution:**

Because of the fundamental features of electrical charge, this is the case. Charges that are similar repel each other. Charges that are diametrically opposed attract each other.

The force of attraction or repulsion between two charges will be directed in the direction so that the force does the least amount of work. As a result of this requirement, the action is directed along the straight line connecting the two charges, which is the shortest distance between them.

**Problem 5: Compare the nature of Electrostatic and Gravitational Forces.**

**Solution:**

Between two huge masses, a gravitational force acts. However, an electrostatic force is activated when two charged bodies come into contact.

Similarities:

- These two forces are central forces.
- Follow the law of inverse squares.
- They’re both long-range forces.
- Both forces are naturally conservative.

Dissimilarities:

- In nature, electrostatic force can be both attractive and repellent. In nature, gravitational force can only be attractive.
- The material medium between two charges affects the electric force between them. The material medium between huge bodies has little effect on gravitational force.
- Electric forces are extremely powerful (approximately 10 38 times stronger) than gravitational forces.

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