Physics is the one subject where students either score very high or very low in Class 12 boards. The difference is almost always derivations and numericals. Students who have a clear method for both consistently score 60+ out of 70.
Why Derivations Are Actually Easy Marks
CBSE Physics derivations are fully predictable. The same 15–20 derivations have appeared in board papers for over a decade. They do not change. This means every mark in a derivation is available before the exam — you just have to practice writing each one until it's automatic.
The 12 Most Important Derivations (Learn These First)
- Derivation of mirror formula and lens maker's equation
- Electric field due to an infinite plane sheet of charge (Gauss's Law application)
- Expression for capacitance of a parallel plate capacitor
- Derivation of drift velocity and relation with current
- Biot-Savart Law and magnetic field due to a straight wire
- Force between two parallel current-carrying conductors
- EMF induced by a rotating coil in a magnetic field
- Expression for energy stored in an inductor
- Relation between refractive index and critical angle (total internal reflection)
- De Broglie wavelength and its derivation
- Expression for radius of nth Bohr orbit
- Binding energy per nucleon and mass defect
Tip
Write each derivation three times — once with the book open, once with only the starting formula as a hint, once completely from memory. Only the third repetition actually sticks for the exam.
The 4-Step Method for Every Numerical
- 1.Write 'Given' — list every quantity from the question with symbols and units.
- 2.Write 'To find' — clearly state what is being asked.
- 3.Write 'Formula' — the exact formula you will use. CBSE awards marks for writing the correct formula even if the calculation is wrong.
- 4.Substitute, calculate, and write the final answer with units and direction (if vector).
Following this method consistently means you earn partial credit even on numericals you cannot fully solve. A student who writes the correct formula and substitutes correctly — but makes an arithmetic error — typically earns 2 out of 3 marks. A student who leaves the question blank earns zero.
High-Frequency Numerical Topics
| Chapter | Numerical Type | Marks |
|---|---|---|
| Electrostatics | Electric field, potential, capacitance calculations | 5–8 |
| Current Electricity | Kirchhoff's laws, Wheatstone bridge | 3–5 |
| Magnetism | Force on charge/wire, cyclotron frequency | 3 |
| Optics | Lens/mirror formula, magnification, power of lens | 5–8 |
| Modern Physics | Photoelectric effect, de Broglie wavelength, nuclear binding energy | 5 |
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percent of Physics marks come from derivations and numericals — master these two categories