CLASS 12TH, CHAPTER 1
Chapter at a Glance
Electricity is not just about current flowing in wires—it begins with electric charges at rest and the fields they create. This chapter builds the foundation for electrostatics and introduces powerful ideas like Coulomb’s law, electric field, flux, and Gauss’s law.
Key Learning Outcomes
After studying this chapter, a learner should be able to:
Explain the nature and properties of electric charge
Apply Coulomb’s law in different media
Analyze electric field and field line patterns
Use Gauss’s law for symmetric charge distributions
Solve numericals using vector concepts.
. Electric Charge
✔ Definition
Electric charge is a fundamental property of matter responsible for electrical interactions.
✔ Types of Charges
Positive charge
Negative charge
Like charges repel, unlike charges attract.
✔ SI Unit
Coulomb (C)
Charge on one electron = −1.6 × 10⁻¹⁹ C
Properties of Electric Charge
(i) Additivity
Total charge = Algebraic sum of individual charges
(ii) Conservation of Charge
Charge can neither be created nor destroyed, only transferred.
(iii) Quantisation of Charges.
Coulomb’s Law
✔ Statement
The force between two stationary point charges is:
Directly proportional to the product of charges
Inversely proportional to the square of the distance between them
✔ Mathematical Form
✔ Nature of Force
Repulsive for like charges
Attractive for unlike charges.
Electric Field
✔ Definition
Electric field at a point is the force experienced per unit positive test charge placed at that point.
✔ SI Unit
N C⁻¹ or V m⁻¹.
Electric Field Lines
Important Characteristics
Start from positive charges and end on negative charges
Never intersect
Density indicates strength of the field
Always perpendicular to the surface of conductors.
Electric Flux (Φₑ)
✔ Definition
Electric flux measures the number of electric field lines passing through a surface.
✔ SI Unit
N m² C⁻¹.
Gauss’s Law
✔ Statement
The net electric flux through a closed surface equals 1/ε₀ times the total charge enclosed.
🔹 Applications of Gauss’s Law
Infinite line charge
Infinite plane sheet of charge
Spherical shell (conducting & non-conducting).
Competency-Based Questions (CBQs) with Solutions
🔶 CBQ 1: Conceptual Reasoning
Why is Coulomb’s law applicable only to point charges or spherical charges?
✅ Solution:
Coulomb’s law assumes that the entire charge acts from a single point. This condition is satisfied only when:
Charges are point-like, or
Spherical charges with uniform distribution are considered, where charge behaves as if concentrated at the center.
🔶 CBQ 2: Real-Life Application
Why does a charged balloon stick to a wall?
✅ Solution:
The charged balloon induces opposite charges on the wall’s surface. Attraction occurs due to electrostatic induction, even though the wall is electrically neutral.
🔶 CBQ 3: Numerical + Reasoning
Two charges of +2 μC and −2 μC are separated by 1 m. Calculate the force between them.
✅ Solution:
F = 36 \text{ N (attractive)}
🔶 CBQ 4: Graphical Interpretation
What does the electric field vs distance graph look like for a point charge?
✅ Solution:
Electric field varies as:
🔶 CBQ 5: Assertion–Reason (Competency Focused)
Assertion: Electric field inside a charged spherical conductor is zero.
Reason: Charges reside only on the outer surface of a conductor.
✅ Answer:
Both Assertion and Reason are true, and Reason correctly explains the Assertion.
🔶 CBQ 6: Multi-Concept Question
A Gaussian surface encloses no charge. Is the electric field everywhere zero?
✅ Solution:
No.
Gauss’s law ensures net flux is zero, not that electric field must be zero at all points. Fields due to external charges may exist.
🔶 CBQ 7: Higher-Order Thinking
Why is a test charge always taken very small?
✅ Solution:
A large test charge would disturb the original electric field configuration, leading to incorrect field measurement.
🔶 CBQ 8: Case-Based Question
A metallic sphere is given excess charge.
(a) Where does the charge reside?
(b) What is the electric field inside the sphere?
✅ Solution:
(a) Entire charge resides on the outer surface
(b) Electric field inside = Zero
🔶 CBQ 9: Analytical
How does electric force differ from gravitational force? Give two points.
✅ Solution:
Electric Force
Gravitational Force
Can be attractive or repulsive
Always attractive
Much stronger
Comparatively weak
🔶 CBQ 10: HOTS Numerical
Find the electric field at the center of an electric dipole.
✅ Solution:
At the center of a dipole, electric fields due to +q and −q are equal and opposite.
🔶 Quick Practice with Answers
SI unit of ε₀ → C² N⁻¹ m⁻²
Field lines inside conductor → Zero
Flux through closed surface with no charge → Zero
Nature of force between +q and −q → Attractive
Electric field is a → Vector quantity
Dimension of electric field → MLT⁻³A⁻¹
Work done moving charge perpendicular to field → Zero
Field due to infinite plane sheet → Constant
Gauss’s law applicable when symmetry is → High
Field at midpoint of equal like charges → Zero
✨ Exam-Focused Tips
Draw clear field line diagrams
Always mention direction of vectors
Use Gauss’s law only with symmetry
Learn standard results (line, plane, sphere)
📚 Conclusion
This chapter is the gateway to electrostatics. Mastery here ensures confidence in:
Electric potential
Capacitors
Current electricity.
MCQ Bank, Case-Study Questions & Worksheet with Answer Key
This resource is specially designed for CBSE Class 12 students, keeping in mind the latest competency-based education (CBE) approach, case-study pattern, and board exam expectations.
PART A: MCQ BANK
(Conceptual + Application + HOTS)
MCQs on Electric Charge & Properties
- Electric charge is said to be quantised because:
A. It can be created
B. It exists in integral multiples of e
C. It is conserved
D. It is continuous
✅ Answer: B - Which of the following is not a property of electric charge?
A. Conservation
B. Quantisation
C. Additivity
D. Amplification
✅ Answer: D - If 6.4 × 10⁻¹⁹ C charge is present on a body, number of excess electrons is:
A. 2
B. 3
C. 4
D. 6
✅ Answer: B
MCQs on Coulomb’s Law - Coulomb’s law is valid for:
A. Moving charges
B. Any size of charges
C. Point charges at rest
D. Current carrying conductors
✅ Answer: C - If distance between two charges is doubled, electrostatic force becomes:
A. Double
B. Half
C. One-fourth
D. Four times
✅ Answer: C - The value of Coulomb’s constant depends on:
A. Charge
B. Distance
C. Medium
D. Shape of conductor
✅ Answer: C
MCQs on Electric Field - Electric field is defined as:
A. Force × charge
B. Force / charge
C. Charge / force
D. Energy / charge
✅ Answer: B - SI unit of electric field is:
A. N m² C⁻¹
B. N C⁻¹
C. J C⁻¹
D. V C⁻¹
✅ Answer: B - Electric field inside a charged conductor is:
A. Maximum
B. Minimum
C. Zero
D. Infinite
✅ Answer: C
MCQs on Electric Field Lines - Electric field lines:
A. Intersect each other
B. Are closed loops
C. Start from negative charges
D. Never intersect
✅ Answer: D
MCQs on Electric Flux & Gauss’s Law - Electric flux through a closed surface depends on:
A. Shape of surface
B. Area of surface
C. Charge enclosed
D. Electric field only
✅ Answer: C - If no charge is enclosed inside a Gaussian surface, electric flux is:
A. Maximum
B. Zero
C. Infinite
D. Depends on shape
✅ Answer: B
MCQs (Assertion–Reason Based) - Assertion: Electric field at the center of a dipole is zero.
Reason: Fields due to both charges cancel each other.
A. Both true, reason explains assertion
B. Both true, but reason does not explain
C. Assertion true, reason false
D. Both false
✅ Answer: A
Quick MCQ Practice (Answers at End)
Nature of electric force → Attractive / Repulsive / Both
Electric field is a scalar or vector?
Unit of electric flux
Gauss’s law is based on which property of charge?
Field due to infinite plane sheet is __
MCQ Answers (14–18)
Both
Vector
N m² C⁻¹
Conservation
Constant
PART B: CASE-STUDY QUESTIONS (CBSE BOARD PATTERN)
Case Study 1: Charged Sphere
A metallic sphere is given excess charge and allowed to reach electrostatic equilibrium.
(a) Where does the charge reside?
(b) What is the electric field inside the sphere?
(c) Name the law used to explain this behaviour.
✅ Answers:
(a) On the outer surface
(b) Zero
(c) Gauss’s Law
Case Study 2: Gaussian Surface
A spherical Gaussian surface encloses a point charge q.
(a) Write the expression for electric flux.
(b) Does flux depend on radius of sphere?
(c) What happens to flux if charge is doubled?
✅ Answers:
(a) Φ = q / ε₀
(b) No
(c) Flux doubles
Case Study 3: Electric Field Lines
A student draws electric field lines around two equal positive charges.
(a) Do field lines intersect?
(b) What does spacing between lines indicate?
(c) What happens at the midpoint?
✅ Answers:
(a) No
(b) Strength of electric field
(c) Electric field is zero
PART C: WORKSHEET (WITH ANSWER KEY)
Section A: Very Short Answer (1 Mark)
Define electric field.
SI unit of charge.
Nature of force between like charges.
Is electric flux a scalar or vector?
Answers:
Force per unit charge
Coulomb
Repulsive
Scalar
Section B: Short Answer (2 Marks)
State any two properties of electric charge.
Why is Coulomb’s law called inverse square law?
Answers: - Conservation, Quantisation
- Force ∝ 1/r²
Section C: Numerical (3 Marks)
Two charges of +2 μC are placed 1 m apart. Find force.
Answer:
F = 36 N (repulsive)
Section D: Conceptual (3 Marks)
Why is Gauss’s law not useful for irregular charge distributions?
Answer:
Because symmetry is required to calculate electric field easily.