Concept of Charge

Concept of Charge In Physics is a fundamental property of matter that gives rise to electric forces and interactions. It exists in two types, positive and negative, and plays a vital role in understanding electricity and magnetism.

Previous year Questions

Year	Question	Marks
2016	Two identical metallic spheres A and B of exactly equal mass are taken. If A is charged with Q positive charge and B is charged with Q negative charge, then which sphere will be heavier after charging and why ?	2M
2016 Special exam	Distinguish between conductor, semi-conductor and insulator on the basis of band theory solid.	10M

Concept of Charge

Electric charge is the basic unit of electricity and is responsible for electrical interactions in nature.

History:

• The concept of electric charge dates back to 600 BC, when the Greek philosopher Thales of Miletus observed that amber rubbed with wool or silk could attract light objects like straw and paper. This led to the discovery of electrification by friction and the birth of electrostatics.

The Origin of the Term “Electricity”

• The word “electricity” comes from the Greek word “elektron,” meaning amber—the first substance observed to exhibit electrical effects.

Definition: Charge is an intrinsic property of subatomic particles (such as electrons and protons) that determines how they interact with electric and magnetic fields.

• Unit of Charge: Coulomb (C)
• Symbol of Charge: q or Q
• Charge of an Electron: – 1.6 × 10⁻¹⁹ C
• Charge of a Proton: +1.6 × 10⁻¹⁹ C

Types of Charge

• There are two types of electric charge: Through years of careful study, scientists realized that there are only two kinds of charge, which were later named positive and negative by Benjamin Franklin.
  1. Positive Charge (+) – Carried by protons
  2. Negative Charge (-) – Carried by electrons
• Neutrons have no charge (neutral). Atoms are normally neutral because they have an equal number of protons and electrons.
• Objects become charged when they gain or lose electrons, leading to either positive or negative charge.

Properties of Electric Charge

1. Like Charges Repel, Opposite Charges Attract

• Two positive charges (+ +) repel.
• Two negative charges (- -) repel.
• Opposite charges (+ -) attract each other.

2. Additivity of Charges

• Electric charge behaves like a scalar quantity, meaning it can be added algebraically.
• Mathematically, for a system with n charges q₁,q₂,q₃,…,q_n ​, the total charge is:

Q_total=q₁ + q₂ + q₃ +….+ q_n

3. Charge is Conserved

• The total charge in an isolated system remains constant.
• Charge cannot be created or destroyed, only transferred.

4. Charge is Quantized

• Charge always exists in discrete packets and is an integral multiple of a fundamental unit of charge.
• This smallest possible charge is denoted by e, which is the charge of an electron or proton.

Q = ne

where n is an integer (positive or negative), and e=1.602×10^-19 C

• A body can have charges like ±e, ±2e, ±3e, but never 1.5e or 2.7e.

5. Charge is Transferable

• Electrons can move from one body to another, creating charge imbalance.
• Example: Rubbing a balloon on hair transfers electrons, making the balloon negatively charged.

6. Charge is Invariant

• The charge of a particle remains the same regardless of motion or frame of reference.

Measurement of Electric Charge

• In the SI system, electric charge is measured in Coulombs (C).
• One Coulomb (1 C) is defined as:
  • The charge flowing through a wire in 1 second if the current is 1 Ampere (A).
  • That is,1C=1A⋅1s
• Magnitude of Elementary Charge (e):

e=1.602×10^-19 C

• Since 1 Coulomb is a very large charge, we often use smaller units:
  • 1 microcoulomb (µC) = 10^-6C
  • 1 millicoulomb (mC) = 10^-3 C
• Example: 1 Coulomb of charge is approximately equal to the charge of 6.25×10¹⁸ electrons.

Read more…

Static Electricity

Current Electricity

FAQ (Previous year questions)