CBSE Class 12 Physics Chapter 1 u2014 Electric Charges and Fields is the foundation of the entire electrostatics unit, which carries approximately 8 marks in the CBSE board exam and is tested in NEET and JEE as well. These complete notes cover all NCERT concepts, important formulas, and CBSE-pattern questions.
Chapter Overview u2014 CBSE Marking Scheme
| Unit | Chapters | Marks (Board 2027) |
|---|---|---|
| Unit I: Electrostatics | Ch 1 (Electric Charges) + Ch 2 (Potential) | 8 |
| Unit II: Current Electricity | Ch 3 | 7 |
| Unit III: Magnetic Effects | Ch 4, 5 | 8 |
| Unit IV: EMI + AC | Ch 6, 7 | 8 |
| Unit V: Electromagnetic Waves | Ch 8 | 3 |
| Unit VI: Optics | Ch 9, 10 | 10 |
| Unit VII: Dual Nature + Atoms + Nuclei | Ch 11, 12, 13 | 12 |
| Unit VIII: Electronic Devices | Ch 14 | 7 |
Electric Charges u2014 Key Concepts
Properties of Electric Charge
- Additive nature: Total charge = algebraic sum of all charges (q = qu2081 + qu2082 + …)
- Conservation: Charge is neither created nor destroyed; total charge of universe is constant
- Quantisation: Charge exists in integral multiples of e (electron charge = 1.6 u00d7 10u207bu00b9u2079 C): q = ne
Coulomb’s Law
F = kqu2081qu2082/ru00b2 where k = 1/(4u03c0u03b5u2080) = 9 u00d7 10u2079 Nmu00b2Cu207bu00b2
- Force is along the line joining the charges
- Like charges: repulsive force; Unlike charges: attractive force
- Valid for point charges in vacuum
- In medium: F_medium = F_vacuum / u03b5_r (u03b5_r = relative permittivity / dielectric constant)
Superposition Principle
The net force on a charge due to multiple charges = vector sum of individual forces.
F_net = Fu2081u2082 + Fu2081u2083 + Fu2081u2084 + … (vector addition)
Electric Field
Definition and Formula
E = F/qu2080 = kQ/ru00b2 (for a point charge Q at distance r)
Electric field is a vector u2014 direction is the direction of force on a positive test charge.
Electric Field Lines u2014 Properties (CBSE Favourite)
- Field lines originate from positive charges and terminate on negative charges
- Field lines never intersect (if they did, field would have two directions at that point)
- Number of field lines represents magnitude; tangent at any point gives direction
- Field lines are always perpendicular to conducting surfaces
- No field lines exist inside a conductor (E = 0 inside)
Gauss’s Law u2014 CBSE Board Favourite (5-mark question)
Statement: The total electric flux through any closed surface is equal to 1/u03b5u2080 times the total charge enclosed.
Formula: u03a6 = u222eEu00b7dA = Q_enc/u03b5u2080
Applications of Gauss’s Law (Must Know for Boards)
| Configuration | Electric Field |
|---|---|
| Infinite line charge (u03bb) | E = u03bb/(2u03c0u03b5u2080r) |
| Infinite plane sheet (u03c3) | E = u03c3/(2u03b5u2080) on each side |
| Two parallel plates (capacitor) | E = u03c3/u03b5u2080 between plates; 0 outside |
| Solid conducting sphere (Q, radius R) | Outside: E = kQ/ru00b2; Inside: E = 0; Surface: E = u03c3/u03b5u2080 |
| Solid insulating sphere (uniform charge) | Outside: E = kQ/ru00b2; Inside: E = kQr/Ru00b3 |
CBSE Important Questions u2014 3-Mark and 5-Mark
Common 3-mark questions:
- State and explain Gauss’s Law. Derive E for an infinite plane sheet.
- Define electric field lines. State any 4 properties.
- Two charges +4u03bcC and -4u03bcC are placed 20cm apart. Find the field at a point on the axis 10cm from each charge.
Common 5-mark questions:
- Derive Coulomb’s law from Gauss’s Law.
- Using Gauss’s Law, derive the electric field due to an infinitely long straight uniformly charged wire.
- Explain the concept of a Faraday cage (electrostatic shielding) with diagram.
Practice Quiz u2014 CBSE Physics Chapter 1
Quiz data missing.
FAQ
How many marks does Chapter 1 carry in CBSE Class 12 Physics board exam?
Chapter 1 (Electric Charges and Fields) is part of Unit I (Electrostatics), which together with Chapter 2 carries 8 marks in the CBSE Class 12 Physics board exam. Gauss’s Law derivation, properties of electric field lines, and application problems are the most frequently asked topics.
What is the best way to score full marks in Electrostatics in Class 12?
Read NCERT once for concepts. Memorise Coulomb’s Law and Gauss’s Law formulas precisely. Practice drawing electric field line diagrams u2014 they appear in most board papers. For 5-mark derivations, practice writing Gauss’s Law proof for each configuration (wire, sheet, sphere) at least 3 times.
Get AI-powered answer checking for board exam practice at Ready For Boards. Practice more chapters in our Board Exam Course. See CBSE Board Exam Preparation guide.
