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Here’s part-II of cracking physics

education Updated: Feb 22, 2012 18:26 IST
Krishna Deo Pandey
Krishna Deo Pandey
Hindustan Times

Last week we carried a review of units 1 to 7 in the physics syllabus. We now conclude the series by looking at all that you need to revise from unit 8 onwards.

Unit 08: Atoms and nuclei (06 marks)

Revise: Bohr model, scattering experiment. Radioactive decay laws, BE and BE per nucleon, isotopes, isobars, fusion and fission.

Remember: Rutherford’s model of atom is called planetary/nuclear model. The n value for first excited state in Bohr’s equation is 2 and not 1. Bohr model is only applicable to hydrogen/hydrogen-like atoms. All radioactive decay follows first order kinetics. Beta decay corresponds to both plus/minus beta decay releasing neutrino/antineutrino, respectively. Alpha decay increases n/p ratio. Nuclear reactors use fission reaction for producing powers. Source of energy in sun/stars is nuclear fusion.

Unit 9: Electronic devices (07 marks)

Revise: Diode as full and half wave rectifier. Zener diode as voltage regulator. Transistor as switch, amplifier, oscillator, Logic gates.

Remember: The order of thickness of depletion layer is one-tenth of a micrometre. Zener diode cannot regulate AC. Si and GaAs are used in solar cells. Full wave rectification doubles the frequency of the input whereas half wave does not change. An oscillator uses positive feedback. NAND and NOR gates are called universal gates.

Unit 10: Communication system (05 marks)

Revise: Various frequncy range tables, need for modulation, AM wave production and detection and derivation of relation between radio horizon and antenna height.

Remember: Mobile telephones operate typically in the UHF range of frequencies (about 800-950MHz). In modulation, some characteristics of the carrier signal like amplitude, frequency, or phase varies. Space waves are used for line-of-sight (LOS) communication as well as satellite communication.

Write smart
Write the answers neatly, to the point, in your own words, properly numbered, labelled schematic diagram (if required), with correct units for numericals. Follow these tips:

* Avoid silly errors by paying attention to confusing words such as conductance, reactance, inductance, impedance, resistivity, conductivity, retentivity, coercivity, permittivity, permeability, AC/DC, Q-value, and Q-factor.

* While describing any device, write whether it is a machine, instrument, arrangement or device for a particular purpose. Note that Van de Graff generator, cyclotron and AC Generator are machines, gold leaf electroscope, galvanometer, voltmeter, potentiometer are electrical instruments. Telescope, microscope (reflecting/refracting), human eyes are optical instruments and capacitor, metre, bridge, transformer, rectifier, amplifier, oscillator are devices.

* Whenever required, give ‘schematic line diagram’ only with proper labelling. Avoid 3D and unnecessary shading of diagrams.

* Put the correct arrows while drawing electric/magnetic lines of force, ray diagrams in optics, vectors, phasors and electrical circuits.

* Be clear with symbols and signs like A: mass number/ampere, N: Number of neutrons, Z: number /impedance, BE:binding energy, L:self inductance M: mutual inductance, D: least distance of distinct vision/dioptre etc

* Write the relevant formula clearly as it can carry more weightage than calculation.

The author is a physics expert and can be contacted on

First Published: Feb 22, 2012 09:48 IST