ANAND CLASSES Study Material and Notes to explore the concept of electric charge, its units, types, and properties with solved examples, MCQs, and worksheets tailored for JEE, NEET, and Class 10 students. Strengthen your understanding with exam-focused content.
Definition of Electric Charge
Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric or magnetic field. It is the source of all electric and electromagnetic interactions.
🔷 What is Electric Charge?
Electric charge is a basic property of subatomic particles (like electrons and protons) that determines how they interact through electric and magnetic forces. Just like mass causes gravitational force, electric charge causes electromagnetic force.
⚡ Types of Electric Charges
Experiments show that there are two kinds of electric charges:
- Positive Charge (Carried by Protons)
- Negative Charge (Carried by Electrons)
🧪 How are these charges identified?
- When a glass rod is rubbed with silk, the glass rod acquires a positive charge.
- When an ebonite rod is rubbed with wool, it acquires a negative charge.
🔁 Interaction Between Charges
- Like charges repel:
- Positive ↔ Positive → Repulsion
- Negative ↔ Negative → Repulsion
- Unlike charges attract:
- Positive ↔ Negative → Attraction
🧲 Properties of Electric Charges
- Two types : Positive and Negative
- Like charges repel, unlike charges attract
- Charge is conserved : Cannot be created or destroyed
- Charge is quantized : Always in multiples of elementary charge (e)
- Charge is transferable : Can be transferred from one object to another by friction, conduction, or induction
📏 4. SI Unit of Electric Charge
- The SI unit is coulomb (C)
- 1 coulomb = charge that exerts 9 × 10⁹ N force on an equal charge at 1m in vacuum
🧮 Relation with Electrons and Protons
- Charge on electron: –1.6 × 10⁻¹⁹ C
- Charge on proton: +1.6 × 10⁻¹⁹ C
🔋 Understanding the Equation: q = ne
This is a fundamental formula in electricity that connects total electric charge (q) in a conductor with the number of electrons (n) and the charge of one electron (e).
📘 The Equation
q = ne
Where:
- q = Total electric charge (in coulombs, C)
- n = Number of electrons or elementary charges
- e = Charge of one electron = $1.6 \times 10^{-19} \, \text{C}$
🔍 Explanation of the Formula
This formula tells us that total charge is equal to the number of electrons multiplied by the charge of a single electron.
✨ Quantization of Charge
Electric charge is quantized, which means it exists in discrete packets rather than being continuous. The smallest unit of charge is the elementary charge (e), which is the charge of one proton (+e) or one electron (–e).
So any charge q must be an integral multiple of e, i.e., $$q = n \times e$$
where n is an integer (positive or negative depending on the nature of the charge).
📘 Sample Problem
❓ Calculate the number of electrons constituting one coulomb of charge. (NCERT Book Question)
✅ Solution:
Given, charge on one electron, e = 1.6 × 10⁻¹⁹ C
Total Charge, q = 1C :
q = ne
1 = n e
1 = n x 1.6 × 10⁻¹⁹
To find number of electrons (n) for 1 C:
$$n = \frac{1}{1.6 \times 10^{-19}} $$
$$n = 6.25 \times 10^{18}$$
Answer:
6.25 × 10¹⁸ electrons make up 1 coulomb of charge.
The above example tells us that the SI unit of electric charge ‘coulomb’ (C) is equivalent to the charge contained in 6.25 × 1018 electrons. Thus, coulomb is a very big unit of electric charge.
❓ Frequently Asked Questions (FAQs)
Q1. Why are there only two types of charges?
Because experiments have consistently shown only two behaviors: attraction and repulsion. These behaviors are explained by the existence of two types of charges—positive and negative.
Q2. What is the origin of electric charge?
Electric charge arises from subatomic particles: electrons (–ve) and protons (+ve). Electrons can be transferred, creating charge imbalance.
Q3. Can a body have a fractional charge?
Not practically. In everyday observations, charge is always in multiples of the elementary charge (1.6 × 10⁻¹⁹ C).
💡 Conceptual Questions with Answers
Q1. Why is the charge on an electron negative and on a proton positive?
Answer:
These are conventions agreed upon in physics. The signs are assigned to differentiate the nature of interactions, not due to any inherent physical direction.
Q2. What happens if two charged objects are brought near each other?
Answer:
- If charges are the same, they repel.
- If charges are opposite, they attract.
🎯 Multiple Choice Questions (MCQs)
Q1. What is the SI unit of electric charge?
A. Newton
B. Joule
C. Coulomb
D. Volt
✅ Correct Answer: C. Coulomb
Explanation: SI unit of electric charge is defined as coulomb (C).
Q2. If an object has 3.2 × 10⁻¹⁹ C of charge, how many electrons does it carry?
A. 1
B. 2
C. 3
D. 4
✅ Correct Answer: B. 2
Explanation: $$\text{No. of electrons} = \frac{3.2 \times 10^{-19}}{1.6 \times 10^{-19}} = 2$$
📚 Do You Know?
- Coulomb is a very large unit; 1 C = charge of 6.25 × 10¹⁸ electrons!
- Benjamin Franklin named positive and negative charges.
- A body is electrically neutral if the number of electrons = number of protons.
📝 Worksheet: Practice Questions
Fill in the Blanks
- The SI unit of electric charge is ______.
- Like charges ______ each other.
- 1 C = ______ × 10¹⁸ electrons.
- The charge of a proton is ______.
True or False
- A glass rod rubbed with silk becomes negatively charged.
- Opposite charges attract.
- Electrons carry a positive charge.
Short Answer Questions
- Define electric charge.
- Name the two types of electric charges.
- What is meant by quantization of charge?
🧪 Test Paper (Marks: 20 | Time: 30 mins)
| Section | Questions | Marks |
|---|---|---|
| A (1 mark each) | 1. Define electric charge. 2. Write the SI unit of electric charge. 3. Name two particles that carry charge. | 3 × 1 = 3 |
| B (2 marks each) | 4. What happens when a glass rod is rubbed with silk? 5. State two properties of electric charge. | 2 × 2 = 4 |
| C (3 marks each) | 6. What are like and unlike charges? Explain with examples. 7. Why is coulomb considered a large unit of charge? | 2 × 3 = 6 |
| D (4 marks) | 8. Calculate the number of electrons in 2 coulombs of charge. | 4 |
| Total | 20 Marks |
⚡ Important Points for Quick Revision
- Two charges: Positive (+) and Negative (–)
- Like charges repel, unlike charges attract
- SI unit: Coulomb (C)
- Charge on electron: –1.6 × 10⁻¹⁹ C
- 1 C = 6.25 × 10¹⁸ electrons
- Charge is conserved and quantized
