CBSE Class 12 Physics Key Expected Questions (Chapters 1-7)

Electric Charges and Fields Essentials

Physics students often struggle with conceptual clarity in electromagnetism. After analyzing this video lecture, I've identified core questions that frequently appear in CBSE half-yearly exams. Mastering these builds foundational understanding for higher studies.

Gauss's law remains crucial: The electric flux through a closed surface equals the net charge enclosed divided by ε₀. For example, when a Gaussian sphere's radius triples while enclosing the same point charge, flux remains unchanged. This demonstrates inverse-square law behavior.

Key SI units to memorize:

Electric flux: volt-meter (V·m)
Permittivity of free space: 8.85 × 10⁻¹² C²/N·m²

Field Properties and Calculations

Electric field lines radiate outward from positive charges. Two fundamental properties: they never intersect, and their density indicates field strength.

Millikan's oil-drop experiment calculations require understanding charge quantization. Given a droplet charge of -6.4 × 10⁻¹⁹ C:

Number of electrons = Total charge / Charge per electron  
= (-6.4 × 10⁻¹⁹) / (-1.6 × 10⁻¹⁹)  
= 4 electrons

For infinite line charges producing 4.5 × 10⁻⁴ N/C field at 4 cm:
E = λ/(2πε₀r) → λ = E × 2πε₀r
Substitute values to find linear charge density λ.

Electrostatic Potential and Capacitance

Potential inside uniformly charged spherical shells remains constant, a critical distinction from electric fields. This demonstrates electrostatic shielding principles.

Capacitors store energy as electric field energy between plates. When air-filled capacitors (2μF) show increased capacitance (12μF) in diequectrics:
k = C_medium / C_air = 12/2 = 6
The dielectric constant is 6.

Practical Problem Solving

Calculate potential at 1m from 10⁻⁹ C charge:
V = (1/(4πε₀)) × q/r = 9 × 10⁹ × 10⁻⁹ / 1 = 9V

For parallel plate capacitors:

Capacitance doubles when plate distance halves
Adding dielectric (k=5) multiplies capacitance by k

Current Electricity Fundamentals

Drift velocity defines as average electron velocity in electric fields. Its magnitude per unit field strength is mobility, crucial for semiconductor physics.

Ohm's law limitations include:

Non-ohmic materials like diodes
Temperature-dependent resistance in conductors

Kirchhoff's junction rule: Current entering equals current leaving. Wheatstone bridge derivations rely on this for null deflection conditions.

Moving Charges and Magnetism

Moving charges produce both electric and magnetic fields. Key principles:

Force on charges moving parallel to B-field: zero
Perpendicular motion: F = qvB (maximum)

Biot-Savart law calculates magnetic fields from current elements. Ampere's circuital law: ∮B·dl = μ₀I_enc, essential for solenoid field derivations.

Magnetism in Matter

Magnetization is magnetic moment per unit volume. Material behaviors:

Type	Susceptibility	Example
Diamagnetic	Negative	Bismuth
Paramagnetic	Positive	Aluminum

Permanent magnets require:

High coercivity
High retentivity
Examples: Alnico, Neodymium magnets

Electromagnetic Induction

Faraday's law: Induced EMF equals rate of flux change. Lenz's law conserves energy by opposing flux changes.

For current dropping from 5A to 0 in 0.15s with 100V average EMF:
ε = -L(di/dt) → 100 = L×(5/0.15) → L = 3H

Alternating Current Concepts

AC circuit relationships:

Peak current I₀ = I_rms × √2
Average current over cycle: zero

At resonance in LCR circuits:

Power factor = 1
Purely capacitive circuits: power factor 0

Actionable Revision Checklist

Derive three key formulas from Gauss's law applications
Solve 2 numericals from each chapter
Practice labeled diagrams for solenoids and AC generators
Compare diamagnetic/paramagnetic materials

Recommended resources:

Concepts of Physics by HC Verma (explains derivations intuitively)
PhET Interactive Simulations (visualize electromagnetic concepts)

Mastering these questions builds conceptual depth. Which derivation do you find most challenging? Share your progress in comments!

Part 2 covering chapters 8-14 coming soon