Basic Concepts of Electricity
Electricity is the basis of countless technologies in everyday life, from lights, fans, mobile chargers, computers and televisions to industrial machines. But before learning about electric current, potential difference, resistance or electric power, we need to understand what electric charge is and how charge behaves in matter.
This lesson explains the fundamental ideas needed for the Electricity chapter step by step.
👉 Atoms and Electric Charge
The three main subatomic particles of an atom are electrons, protons and neutrons.
| Particle | Location | Nature of Charge | Charge |
|---|---|---|---|
| Electron | Outside the nucleus | Negative | |
| Proton | Inside the nucleus | Positive | |
| Neutron | Inside the nucleus | Neutral | 0 |
When an atom contains equal numbers of protons and electrons, the positive and negative charges balance each other. Its total charge is zero, so the atom is electrically neutral.
Important Idea
In ordinary electrical phenomena, protons in the nucleus do not move from one atom to another. An atom or object generally becomes charged because it gains or loses electrons.
⭐ If an atom gains one or more electrons, it has more electrons than protons and becomes a negative ion.
⭐ If an atom loses one or more electrons, it has more protons than electrons and becomes a positive ion.
Example
If a neutral atom loses 2 electrons, its net charge becomes . If it gains 3 extra electrons, its net charge becomes .
👉 What is Electricity?
Electricity is a broad term for physical phenomena associated with the presence, interaction and motion of electric charge.
So it is not sufficient to describe electricity simply as “a form of energy”. Electric charge, forces between charges, accumulation of charge and flow of charge are all part of electricity.
🔸 Electric charge is a fundamental physical property of matter. An object with a net charge can electrically attract or repel another charged object.
Two Types of Electric Charge
Positive Charge (+)
Negative Charge (−)
Interaction Between Charges
- Like charges repel each other, for example two positive charges or two negative charges.
- Unlike charges attract each other, for example one positive and one negative charge.
Remember: Attraction and repulsion are effects of the interaction between charges and are important properties of electric charge.
👉 SI Unit of Electric Charge and Elementary Charge
The SI unit of electric charge is the coulomb, symbol C.
The charge of a proton is positive and the charge of an electron has the same magnitude but is negative. This smallest basic magnitude is called the elementary charge.
Therefore,
👉 Quantisation of Charge
The total charge on an object occurs in integral multiples of elementary charge:
where,
- = total charge on the object,
- = an integer related to the number of electrons transferred,
- = magnitude of elementary charge.
This property is called quantisation of charge.
🔸 Solved Example 1
An object gains 5 extra electrons. Find its total charge.
So the object is negatively charged.
🔸 Solved Example 2
How many elementary charges correspond to ?
Therefore, the amount of charge is equal to 20 elementary charges.
👉 Conservation of Charge
An important property of electric charge is conservation of charge. The total electric charge of an isolated system remains constant.
In other words, charge is neither created nor destroyed; it can only be transferred from one object to another.
For example, when two neutral objects are rubbed together, one may lose some electrons and the other may gain them. One object then becomes positively charged and the other acquires an equal amount of negative charge, while the total charge remains unchanged.
👉 Conductors and Insulators
Charge cannot move equally easily through all materials. On this basis, materials can initially be classified as conductors and insulators.
| Property | Conductor | Insulator |
|---|---|---|
| Movement of charge carriers | Comparatively easy | Very limited |
| Common examples | Copper, aluminium, iron | Rubber, glass, dry plastic |
| Use | Wires that carry electric current | Protective covering around wires |
Everyday example: Electrical wires usually contain conducting metals such as copper or aluminium inside, with an insulating plastic or rubber covering outside.
👉 What are Free Electrons in Metals?
Some outer electrons in a metal are not tightly bound to one particular atom. They can move comparatively freely through the metallic conductor and are called free electrons.
Without an external electrical influence, these free electrons move randomly. Therefore, there is no net flow of charge in one particular direction.
When a potential difference is created across the conductor, an average motion or drift in a definite direction is superimposed on the random motion of the electrons. This organised movement of charge produces electric current.
Important: During current flow in a metal wire, all electrons do not rush rapidly from one end of the wire to the other. Their drift speed is comparatively small, although the electrical effect becomes established through the circuit very quickly.
👉 Static Electricity and Current Electricity
Electricity can initially be discussed in two forms depending on whether charge is accumulated on an object or flows continuously.
Two Basic Forms of Electricity
Static Electricity
Current Electricity
Static Electricity
When a net electric charge remains accumulated on an object and does not continuously flow through a closed path, the situation is called static electricity.
Example: A plastic comb rubbed through dry hair may attract small pieces of paper.
Current Electricity
When charge carriers move in an organised manner along a definite path, it is called current electricity. The operation of most electrical appliances in our homes is based mainly on current electricity.
👉 What is Electric Current?
The amount of net electric charge passing through a particular cross-section of a conductor per unit time is called electric current.
If charge flows in time , then
where,
- = electric current,
- = charge that flows,
- = time.
The SI unit of electric current is the ampere, symbol A.
Thus, when 1 coulomb of charge passes through a cross-section of a conductor every second, the current is 1 ampere.
🔸 Solved Example 3
If 12 C of charge flows through a wire in 4 s, find the current.
Therefore, the current is 3 A.
👉 Conventional Current Direction vs Electron Motion
Historically, the direction of electric current was defined as the direction in which positive charge would flow. This is called the conventional current direction.
In metallic conductors, the main moving charge carriers are negative electrons. Therefore,
In a metal wire, electron drift is opposite to the direction of conventional current.
| Quantity | Direction |
|---|---|
| Conventional current | In the external circuit, it is taken from higher potential to lower potential |
| Electron drift | Opposite to the conventional current direction |
👉 Why is Potential Difference Needed for Current?
A conducting wire alone does not produce a steady electric current. A potential difference must exist between its two ends. A cell or battery helps create this potential difference in a circuit.
The potential difference establishes an electric field in the conductor and gives the free electrons an average drift in a particular direction. As a result, electric current is established.
This idea will be especially useful when studying electric potential and electromotive force (EMF).
👉 How Large is 1 Coulomb of Charge?
The magnitude of charge on one electron is extremely small:
Therefore, approximately
extra electrons are required to make −1 C of charge. Thus, 1 C is a very large amount of charge on a macroscopic scale.
👉 Applications in Everyday Life
⭐ Mobile charger: A power source creates a potential difference in a circuit, allowing energy transfer through the movement of charge.
⭐ Electrical wire: Copper or aluminium is used as a conductor because free electrons in these metals respond easily.
⭐ Plastic covering of a wire: Plastic is an insulator, so it helps isolate the user from the conducting part.
⭐ Static electricity: In dry weather, friction between clothes or hair can cause charge accumulation, producing small sparks or attraction.
👉 Try It Yourself (Quick Practice)
🔸 1. An object loses 10 electrons. Find the magnitude and nature of its charge.
🔸 2. How many elementary charges are equal to ?
🔸 3. If 20 C of charge passes through a conductor in 5 s, find the current.
🔸 4. Why are conventional current direction and electron drift direction opposite in a metallic conductor?
🔸 5. Why are electrical wires covered with plastic or rubber?
Exam tip: In numerical questions on charge and current, first write the given quantities with their units. Then use or as required.
👉 Common Mistakes to Avoid
🔸 1. “Electricity simply means energy.” This is not a complete definition. Electric charge, electric forces, accumulation of charge and flow of charge are all part of electricity.
🔸 2. “Protons move through a wire to produce current.” In metallic conductors, the main moving charge carriers are free electrons.
🔸 3. “The direction of electron motion is the direction of current.” In metallic conductors, the two directions are opposite.
🔸 4. “New charge is created when an object becomes charged.” Usually electrons are transferred; total charge is conserved.
🔸 5. “A conductor always carries current because it contains free electrons.” A steady net current requires a potential difference.
👉 Quick Revision
🔹 Electric charge is of two types: positive and negative.
🔹 Electrons are negative, protons are positive and neutrons are electrically neutral.
🔹 An object can become charged by gaining or losing electrons.
🔹 The SI unit of electric charge is coulomb (C).
🔹 Elementary charge: .
🔹 Quantisation of charge: .
🔹 Total charge is conserved in an isolated system.
🔹 Free electrons play an important role in electrical conduction in metals.
🔹 Electric current: .
🔹 The SI unit of current is ampere (A).
🔹 In a metallic conductor, electron drift is opposite to conventional current.
🔹 A potential difference across a conductor is required for steady electric current.
