CBSE Physics Class 12: Complete Chapter-Wise Formula Breakdown
Every high-weightage Physics formula for CBSE Class 12 boards, organised by chapter and weighted by how often boards ask them. Stop memorising the wrong formulas.
One of the most common mistakes CBSE Class 12 Physics students make is trying to memorise every formula in the NCERT textbook. There are hundreds. Boards don't ask hundreds. Boards ask the same 40–50 formulas in slightly different contexts, year after year. Here's the chapter-wise breakdown, weighted by actual exam frequency.
Electrostatics & Capacitance (8–9 marks)
This is consistently one of the highest-weighted units in Physics boards. The formulas that appear most often: Coulomb's law, the parallel plate capacitor, and energy stored in a capacitor.
- Coulomb's law: F = kq₁q₂/r² where k = 9×10⁹ N·m²/C²
- Electric field of point charge: E = kq/r² | Potential: V = kq/r
- Parallel plate capacitor: C₀ = ε₀A/d | With dielectric: C = kε₀A/d
- Energy stored: U = ½CV² = Q²/2C = ½QV (know all three forms — boards test each one)
- Gauss's law: Net flux Φ = q_enclosed/ε₀
The most commonly lost marks in Electrostatics: forgetting that capacitors in series and parallel follow the OPPOSITE rule compared to resistors. For capacitors: series → 1/C_eq = Σ(1/Cᵢ). Parallel → C_eq = ΣCᵢ.
Current Electricity (7–8 marks)
- Ohm's law: V = IR | Resistivity: R = ρL/A | Temperature: R_T = R₀(1 + αΔT)
- Kirchhoff's laws: KCL: ΣI = 0 at junction | KVL: ΣV = 0 around any loop
- Power: P = VI = I²R = V²/R (know all three — questions change which variables are given)
- Drift velocity: I = nAev_d → v_d = I/(nAe)
- Terminal voltage: V = E − Ir (E = EMF, r = internal resistance)
Magnetism & EMI / AC Circuits (8 marks each)
- Force on charge: F = qvBsinθ | On wire: F = BILsinθ
- Solenoid field: B = μ₀nI | Straight wire: B = μ₀I/2πr
- Faraday's law: ε = −NdΦ/dt | Flux: Φ = BAcosθ
- Reactance: X_L = ωL | X_C = 1/ωC | Impedance: Z = √(R² + (X_L − X_C)²)
- Resonance: f_r = 1/2π√LC | At resonance: Z = R (minimum), power factor = 1
Optics (10 marks — highest weighted unit)
Optics is consistently the highest-yield single unit in CBSE Physics. It covers Ray Optics and Wave Optics, which together are worth approximately 10 marks.
- Mirror equation: 1/v + 1/u = 1/f | Lens equation: 1/v − 1/u = 1/f
- Lens maker's equation: 1/f = (n − 1)(1/R₁ − 1/R₂)
- Magnification (mirror): m = −v/u | (lens): m = v/u
- Snell's law: n₁sinθ₁ = n₂sinθ₂ | Critical angle: sinC = n₂/n₁
- Young's double slit: fringe width β = λD/d | Condition for bright: nλ | Dark: (2n−1)λ/2
Modern Physics (10 marks)
- Photon energy: E = hν = hc/λ where h = 6.63×10⁻³⁴ J·s
- Photoelectric effect: KE_max = hν − φ | Stopping potential: eV₀ = KE_max
- de Broglie wavelength: λ = h/p = h/mv
- Bohr model: Energy E_n = −13.6/n² eV | Radius r_n = 0.529n² Å
- Radioactive decay: N = N₀e^(−λt) | Half-life: T₁/₂ = 0.693/λ | E = mc²
The 5 Most-Forgotten Physics Facts (Boards Test These Every Year)
- Dimension of ε₀ = C²N⁻¹m⁻² (equivalent to F/m) — asked as a 1-mark every few years
- At resonance in LCR circuit: impedance Z = R (minimum, not zero) — a 1-mark trap
- Sign convention for concave mirror: f is negative in New Cartesian — extremely common mistake
- RMS values: V_rms = V₀/√2 | I_rms = I₀/√2 — used in almost every AC power question
- TIR requires n₁ > n₂ AND the angle must exceed the critical angle — both conditions needed