Electric Charge – Types, Properties with Solved Examples

What is Electric Charge? 

An Electric charge is the basic physical property of matter. It is the charge on a particle that allows it to experience a force in a magnetic or electric field.

Electrostatic interactions are a common observation in everyday life. The activity of rubbing a plastic comb on dry hair and placing it above paper bits wherein the paper bits get attracted by the comb is a popular example of an electrostatic charge. But what is an electrostatic charge? Learn about charge definition, charge formula, types of electric charges, and their properties. 

Charge Definition: A characteristic of a unit of matter that explains the extent to which a matter has fewer or more electrons than protons. In other words, it is the charge on a particle that allows it to experience a force in a magnetic or electric field.

What is Charge? 

A charge is also called an electric, electrostatic, or electrical charge. It is denoted by the symbol q. Electrostatic charge is the electric charge at rest on an insulation body’s surface.  

Types of Electrical Charge

It is vital to understand the structure of an atom to understand the two types of charge: positive and negative. 

An atom has a nucleus with positively charged protons and neutral neutrons. Electrons or negatively charged particles revolve around the nucleus in specific shells. An atom as a whole is electrically neutral as the number of protons is equal to the number of electrons in the ground state.

The outermost shell, or the valence shell, has valence electrons. In some materials, this shell has loosely bound electrons that flow from one atom to another. These freely moving electrons decide whether a material is an insulator or a conductor. 

A conductor allows free electrons to freely move throughout the solid. Insulators are materials that limit the movement of electrons as they hold them tightly. Some common examples of good conductors are metals such as copper, aluminium, gold, silver, iron, bronze, and more. Insulators include plastic, glass, rubber, wood, fibreglass, oil, porcelain, and more. 

Properties of Electric Charge

The properties of charge are as follows:

• Additivity of Electric Charge: If a system of charges contains n point charges with magnitude q1, q2, q3…qn, the system’s total charge can be calculated by algebraically adding all the charges. Since the charge can either be negative or positive, the operation must consider the charge sign.
• Conservation of Electric Charge: Charges are neither created nor destroyed. They are only transferred from one object to another.  
• The quantisation of Electric Charge: All the free charges are integral multiples of a predefined unit, e. So, the charge possessed by a system is given as follows:

q=ne

In the above equation, n is an integer. It can be zero, positive or negative. The basic unit of charge is symbolised by e. It is the charge carried by an electron or a proton and has a value of 1.6 ×10­­^-19 C.

The Law of Conservation of Electric Charge 

According to the law of conservation, you can’t create a net electric charge. It can only move from one place to another. 

The following example explains the law of conservation:

When you rub your feet across a carpet, the charge gets transferred to your feet. This charge is called a static charge. Now a charge imbalance is created wherein you are a conductor and the carpet an insulator. So, when you touch a doorknob, the charge tends to leave you and go to the doorknob, restoring the charge imbalance. Thus, you get a shock on touching a surface as the electrons leave you. 

Nature maintains a neutral charge equilibrium, and the process doesn’t create new charges. Thus, the law of conservation of electric charge is obeyed. 

Unit of Electric Charge 

The SI unit of electrical charge is Coulomb. One Coulomb can be defined as the quantity of charge transferred in a second. The charge formula for calculating charge is as follows:

Q = I.t

Where Q is the electrical charge, t is the time, and I is for electric current.

The other units of charge are Faraday and Ampere-Hour. 1 Faraday = 96,500 Coulomb.

Coulomb’s Law

Like charges attract each other while unlike ones repel each other. Coulomb’s law allows you to calculate the strength of this attractive or repulsive force between two points.

The law states that the magnitude of force (attractive or repulsive) between any two point charges is in direct proportion with the product of the magnitude of the charges and inverse proportion with the square of the distance between the two charges.  

When q1 and q2 are electric charges, Fe is the electric force, k is the Coulomb’s constant, and r is the distance between the charges, the Coulomb’s law can be given as

F = (K q1 q2) / r²

or 

F = (q1 q2) / 4πε0 r²​ (where ε0  is the permittivity of free space)

Example 1: Calculate the force between two charged spheres containing charges of 2 × 10-7 C and 4 × 10-7 C when they are kept at a distance of 30 cm. Given: 1/ 4πε0 =9 × 109Nm2  C-2Solution: q1 = 2 × 10-7 C q2 = 4 × 10-7 Cr= 30 cm = 0.3 mUsing the formula F = (q1 q2) / 4πε0 r2, you can calculate the force between the two particles.F = (9 × 10 9 × 2 × 10 −7 × 4×10 −7  C) / (0.3) 2  F = 72 x 10 −7 / (0.3)2  Hence, the force between the two spheres is 8 ×10 −3 N. Since the charges are of the same nature, the force will be repulsive.

Example 2: The repulsive force between two equal positively charged ions is 3.7 × 10 −9 N when placed at a distance of 5 A∘. Calculate the number of electrons missing from each ion.Solution: Given: r = 5 A∘ = 5 × 10 −10 mq1 = q2 So, let q1 = q2   = qF= 3.7 × 10−9 NUsing the formula F = (q1 q2) / 4πε0 r2, you can calculate the charge q. q2 = F ×4πε0 r2​  q2 = 3.7 × 10 −9 N × (5 × 10−10)2× (1/ 9×109) q2 = 10.28 × 10−38 q= 3.2×10 −19 C To calculate the number of electrons, you will use the following formula:  q = ne n= q/e n= 3.2×10 −19 C / 1.6 ×10−19C n= 2 Answer: Two electrons are missing from each ion.

How to Charge an Object?

The process of supplying the electrical charge to an object or vice versa is called charging. You can charge an uncharged object by the following three methods:  

Charging by Friction (Triboelectric Method)

The simplest way to transfer charge is by rubbing two objects against each other. In this process, an object loses electrons while the other gains electrons. The object that loses electrons develops a positive charge and is said to be positively charged, and the one that gains electrons is negatively charged. This method of charging objects by friction is also called electrification by friction or the Triboelectric method.

Charging by Conduction

Charging by conduction involves uncharged object charging by bringing it near a charged object. The charged conducting object has electrons and protons in an unequal number disturbing the electrically neutral state; as a result, when an uncharged conductor comes close to it, it discharges electrons and consequently stabilises itself.

Charging by Induction

When an uncharged conductor is brought near a charged conductor, but there is no physical contact between the two, it is called charging by induction. Lack of physical contact differentiates charging by induction from charging by conduction. 

Conclusion

An electrical charge is an important property in Physics. It explains the force experienced by a particle in an electric or magnetic field. The process of charging and discharging is about the electric charge’s movement.

However, it is noteworthy that the charge is neither created nor destroyed; it is only transferred from one body to another.

Frequently Asked Questions

Q1. Is electric charge a vector quantity?

A. Electric charge is not a vector quantity because to be a vector; a quantity must obey the laws of vector addition while having magnitude and direction. However, when two currents meet at a point, the resultant electric current is an algebraic sum and not the vector sum. So, despite having a direction and magnitude, the electric charge or electric current is a scalar quantity. 

Q2. What is an elementary charge?

A. The charge possessed by a single electron or proton is called an elementary charge. It is denoted by e and is equal to 1.6 × 10⁻¹⁹C.

Q3. What is one Coulomb?

A. One Coulomb can be defined as the quantity of charge transferred in a second. 1 C equals approximately 6.24 x 10¹⁸ elementary charges.