Physics: formula sense and short numericals
Physics in PECE checks whether you can apply Class 10-level laws to short numericals and to statements about everyday phenomena. The paper favours direct formula use, unit sense and clear definitions over long derivations.
Core rules to keep ready
- Motion and force: speed = distance/time; acceleration = change in velocity/time; F = ma; weight = mg (g ≈ 10 m/s² in most Indian school papers).
- Electricity: Ohm's law V = IR; series resistance R = R₁ + R₂ + …; parallel 1/R = 1/R₁ + 1/R₂; power P = VI = I²R = V²/R; commercial energy in kWh.
- Light: mirror formula 1/f = 1/v + 1/u, lens formula 1/f = 1/v − 1/u; magnification m = hᵢ/hₒ = −v/u (mirror) or v/u (lens). Follow the sign convention consistently.
- Work, energy and power: W = Fs cos θ; KE = ½mv²; PE = mgh; P = W/t.
Worked example
A 100 W lamp is used 5 hours a day for 30 days. Find the energy consumed in kWh. Reasoning: convert power to kW first: 100 W = 0.1 kW. Daily energy = 0.1 kW × 5 h = 0.5 kWh. For 30 days = 0.5 × 30 = 15 kWh. If the question asks for joules, multiply by 3.6 × 10⁶ J/kWh, giving 5.4 × 10⁷ J.
Traps that cost marks
Mixing watt and kilowatt; using diameter as radius; dropping the sign in mirror/lens problems; treating series and parallel formulas as interchangeable; writing weight in kilograms; and forgetting that resistance in parallel is smaller than the smallest branch.