Electric Charges and Fields

Introduction

Electric charge is one of the important physical properties of any matter. You might have seen often a crackle while taking off synthetic clothes or a sweater. Or, you would have noticed how a comb or a plastic ruler attract bits of paper after you comb your hair or rub the ruler on your hair. 

These are all examples of electric charges that everybody discharges. Lightning in the sky during thunderstorms or rain is another common example of electric discharge. All these electric discharges by matter occur as a result of the discharge of accumulated charges or by the generation of static electricity in some cases.

Let’s find out more about electric charges and fields in this article.


What is an Electric Charge?

The term ‘electricity’ means amber and it derived from a Greek word called ‘Elektron’. Magnetic and electric forces present in the matter including atoms and molecules determine the properties of the matter. 

Scientists conducted some experiments to conclude that two kinds of electrification are found to be true i.e. i) like charges repel each other, and ii) unlike charges attract each other.

The polarity of charge is also the property that differentiates these two kinds of charges in nature.


Conductors and Insulators

Upon conducting an experiment to demonstrate electric charges due to frictional electricity, it was found that conductors assist the movement of electric charges, however, insulators do not behave in a similar manner. The common examples of conductors include metal, Earth, human body, and so on. On the other hand, porcelain, nylon, wood all offer high resistance to the passage of electricity through them as they are insulators.


What are the properties of Electric charge?

An electric charge has three fundamental properties:

  • Quantization- The quantization property tells us that the total charge of a body represents the integral multiple of a basic quantum of charge.

  • Additive- The total charge of a body as the algebraic sum all the singular charges acting on the system is given by the additive property.

  • Conservation- According to the conservation property, the total charge of a system remains unaffected with time. In other words, a transfer of charge from one object to another occurs when objects are charged due to friction. Charges can neither be created nor destroyed.


Coulomb’s Law

The Coulomb's law states that when two-point charges A and B having mutual electrostatic force acting between them is proportional to their product of the two. That is AB and it is inversely proportional to the square of the distance between them (rAB). 

It can be expressed as-

FBA = Force on B due to A = k(AB)÷AB

Constant terms appear in this law which is also known as a constant of proportionality and is represented by ‘k’.

k can be expressed as-  k=9∗109 Nm2C-2 Nm2C−2

Forces Between Multiple Charges

Even if Coulomb's law gives the electric force between two charges, a force on a charge where there is not one but several charges around cannot be calculated by it. 


Superposition Principle

According to this principle, the property of two charges to repel and attract each other remains unaffected even though there is a presence of a third additional charge.


Properties of Electric Fields and Lines

  • In a charge-free region, Field lines show a continuous curve without having any breakage.

  • Two-line never cross each other on electric fields.

  • These electric field lines always begin on the positive charge and end in the negative charge.

  • No closed loops are formed by electrostatic field lines.

FAQs (Frequently Asked Questions)

1. Explain the concept of Electric Flux.

Ans: The total number of electric field lines passing a given area in a unit time is defined as the electric flux. However, it is noticeable that there is no flow of a physically observable quantity that is seen in the case of liquid flow. To define this mathematically,  Electric flux  Δθ through an area element ΔS is can be defined as-

Δθ = E.ΔS = E ΔS  cosθ

Also the number of field lines cutting the area element remains proportional to this. The angle θ here is the angle between E and ∆S. In a closed surface, where the convention is already stated, θ is the angle between E and the outward normal to the area element. For calculating total flux, the simplest method is to divide the surface into small area elements and find the flux at each element and add them up. Therefore, the total flux θ through a surface S can be given as- θ ~ Σ E. ∆S. Here the approximation symbol is to indicate that the electric field E is taken to be constant over the small area element.

2. Explain the meaning of Electric Dipole.

Ans: Electric dipole is a pair of equal or opposite charges A and -B which are separated by distance 2x. The dipole moment vector (let’s assume it as p) has a magnitude 2Ax and is in the direction of the dipole axis from -B to A