# It’s a breeze if your concepts are clear

## The first in a two-part series on cracking the physics exam in Class 12

education Updated: Feb 15, 2012 15:36 ISTNone

Here are some interesting unit-wise options to crack the Class 12 CBSE Board physics paper. Remember, there will be no HOTS this year as well, so just 15% of questions are going to be difficult, giving you the chance to prove your smartness in understanding the concept of the subject. The remaining 85% of questions will be easy and average, testing your planned average study. So, do keep worries, stress and fear at bay.

As you know your physics (theory) paper is divided in 10 units with different weightages. Though questions can be asked from any section of the syllabus, keeping time availability in mind it would be wise to concentrate on important theories, concepts, formulae and derivations. It is equally important to draw the relevant graphs and diagrams (schematic, circuit and Ray) to give the final touch-up to your preparations. So, here it goes:**Unit 1: Electrostatics **

*(08 marks)*

**Revise:**SI units and dimensions of electric charge, field, dipole moment, flux and charge densities, potential, capacitance and polarisation. Drawing field lines and EPS for dipole, two charge and single charge system.

Vector form of Coulomb’s Law. Gauss Theorem, electric dipole, electric field lines and equipotential surface. capacitor, Van de Graff Generator

**Remember:**Charge is scalar but the electric field created by it is a vector, whereas the potential is again a scalar. Electric flux is a scalar. A dipole experiences no force but pure torque in uniform electric field whereas it does experience a force and torque both in non-uniform field. Gauss’s Law is valid only for closed surfaces. Three types of charge densities viz linear, surface and volume are different physical quantities having different unit and dimensions. Along a field line, potential decreases at the fastest rate. The dipole moment per unit volume is called polarisation and is a vector. Whether it’s a solid or a hollow conducting sphere, all free charges reside on its surface. Dielectric constant is also called relative permittivity and is dimensionless, unitless.

**Unit 2:Current electricity**

*(07 marks)*

**Revise :**SI units and dimensions of mobility, resistance, resistivity,conductivity, current density and emf. Ohm’s Law , drift velocity, colour coding. Parallel/ series combination of cells. Potentiometer. Numericals on finding equivalent resistance/current using Kirchoff’s laws

**Remember:**Current is scalar as it does not follow laws of vector addition but current density is vector. Kirchoff’s junction/ loop law is charge/ energy conservation laws. If the Galvanometer and cell are interchanged in balanced Wheatstone bridge, the balance does not get affected. For a steady current along a tapering conductor, current remains constant but current density, drift speed and electric field varies inversely as area of cross-section. Ohm’s law is not universally applicable such as vacuum diode, semiconductor diode.

**Unit 3: Magnetic effects of current and magnetism:**

*(08 marks)*

**Revise :**SI units and dimensions of permeability, relative permeability, magnetic moment, field, flux, intensity, susceptibility, torsional constant and their nature as vector or scalars. Magnetic field lines. Biot-Savart and Ampere’s law, solenoid, toroid, MCG, Cyclotron, para, dia and ferro magnetism, permanent and electromagnets. Numericals on ammeter and voltmeters

**Remember:**Parallel currents attract and anti parallel currents repel. Ampere’s law can be derived from Biot- Savart’s law. MCG has two sensitivities voltage and current as deflection per unit voltage/ current, respectively. Angle of dip is also called inclination, its value at poles and at equator are 90 degrees and 0 degree, respectively.

Superconductors are perfect diamagnets. T(tesla) is SI unit for magnetic field, the other being G(gauss,non-SI),1 T is equal to 10,000 gauss.

Diamagnetism is universal - it is present in all materials.

**Unit 4: EMI and AC**

*(08 marks )*

**Revise :**SI unit and dimensions of self and mutual inductance, capacitive and inductive reactance, impedance, Q-factor, power factor. Faraday’s/ Lenz’s law, eddy current, motional emf, self/ mutual inductance, AC generator, transformer

**Remember:**Lenz’s law is consequence of energy conservation. Eddy current has merits and demerits. AC is scalar but follows phasor treatment as it is periodically varying. At resonance power factor is 1, hence maximum power is dissipated. A transformer works in AC but not in DC. The power consumed in an AC circuit is never negative. Rated values of ac devices for current and voltages are rms whereas for power it is average. Higher the Q-factor sharper the resonance, smaller the bandwidth and better the selectivity

**Unit 5: Electromagnetic waves**

*(03 marks)*

**Revise :**Properties and frequencies, Ampere-Maxwell law, displacement current, drawing of EMW. Numericals on finding frequency, speed etc from given equation.

**Remember :**An oscillating charge produces EMW of the frequency of oscillation. IR waves are also called heat waves as they produce heating. The AM (amplitude modulated) band is from 530 kHz to 1710 kHz. TV waves range from 54 MHz to 890 MHz. The FM (frequency modulated) radio band extends from 88 MHz to 108 MHz. TV remote uses IR waves. LASIK and water purification uses UV rays.

**Unit 6: Optics**

*(14 marks)*

**Revise :**Lens and Lens maker’s formula, magnifying and resolving power, limit of resolution, Hygen’s principle and polarisation, YDSE. Numericals on image location and its nature for lens-mirror combinations

**Remember:**Resolving power is inverse of limit of resolution. Unpolarised light after passing through a polaroid gets linearly polarised with half the intensity for any orientation of the polaroid. Diffraction, interference and polarisation prove the wave nature of light. Polarisation proves the transverse nature of light. Compound microscope has eyepiece of larger aperture and objective smaller vice versa in a telescope. Reflecting telescope removes chromatic and spherical aberration fairly. If the source of light is white in YDSE the central fringe is white and others are coloured in sequence from nearest red to the farthest blue.

**Unit 7: Dual nature of matter and radiation**

*(04 marks)*

**Revise :**Einstein’s photoelectric equation and all the graphs in the NCERT book. Davisson-Germer experiment. Numericals based on de Broglie’s and photoelectric equations.

**Remember :**de Broglie equation relates particle to wave. Wave nature of electrons are used in electron microscope. Photoelectric effect was explained using photon picture of light.

This helpline comes to you from CBSE. For queries, send an email to hteducation@ hindustantimes.com, marked 'CBSE queries

The author is physics expert and can be contacted at krishnaheart@rediffmail.com

First Published: Feb 14, 2012 12:46 IST