Wednesday 28 December 2011

The animation will  discuss the relation between resistance length,resistivity and area.In that animation You can change the length area and resistivity value using movable switch given there .Try to play with that.And get a feel on how exactly it changes.


Animation of Variation of resistance

Question of the day


.       A horizontal force of 10 N is necessary to just hold a block stationary against a wall. The coefficient of friction between the block and the wall is 0.2. The weight of the block is
            (A) 20 N                                                          (B) 50 N
            (C) 100 N                                                        (D) 2 N

Microscopic View of Electric current Animation

Atom
Animation
                                                                                                        microscopic view of atom
microscopic view of flow of current inside the conductor











Tuesday 27 December 2011

Question of the Day


The capacitance of two oppositely charged metal spheres of same radius when far apart will be equal to
        [a]2C                       [b]C/2
        [c]C/4                      [d]4C

Monday 26 December 2011

Alternating Current Fundamentals

Basics of Alternating Current

Friday 23 December 2011

CET Questions

Question1:
A charge Q is divided in two parts q and Q - q separated by a distance R. If force between the two charges is maximum, find the relationship between q & Q.

Solution: F=K q(Q-q)/r2
for max. &min.dF/dq=0 , q=Q/2

Question2:A circular disc X of radius R is made from an iron pole of thickness t, and another disc Y of radius 4R is made from an iron plate of thickness t/4. then the relation between the moment of inertia Ix and Iy is 
(A) Iy=32Ix
(B) Iy=16Ix
(C) Iy=64Ix
(D) None of these

Solution2:
If t is the thickness and R is the radius of the disc, then mass = πR2tρ 
ρ = density of the material of the disc. 
Moment of inertia of disc X, 

Ix=(1/2)πR4tρ 
…(i)
Moment of inertia of disc Y, 
Iy=32πR4tρ 
…(ii)
From equation (i) and (ii)
Iy=64Ix 



Question3:
On a planet a freely falling body takes 2 sec when it is dropped from a height of 8 m, the time period of simple pendulum of length 1 m on that planet is

Solution3:

On a planet, if a body dropped initial velocity (u = 0) from a height h and takes time t to reach the ground
then h=(1/2)gpt2
or gp=4m/s2
Using
T=2π(L/g)1/2
T=3.14 sec

Question4:
A car is moving with uniform velocity on a rough horizontal road. Therefore, according to Newton's first law of motion
(a) No force is being applied by its engine
(b) An acceleration is being produced in the car
(c) The kinetic energy of the car is increasing
(d) force is surely being applied by its engine


Solution4: d Since, force needed to overcome frictional force

Question 5:
A particle is projected upwards. The times corresponding to height h while ascending and while descending are a and b respectively.Find the velocity of projection

Solution:
If a and b are time of ascent and descent respectively then time of flight
T=a+b

Now T=2u/g

So u=g(a+b)/2

CBSE Important questions chapterwise

Physics XII

Thursday 22 December 2011

Semiconductor Physics Notes (With good 3D pictures and energy level diagrams)

Semiconductor physics

Wednesday 21 December 2011

Semiconducor Questions

Semiconductor Physics Questions and answers (CBSE)

Tuesday 20 December 2011

Light Emitting Doide


Almost all LEDs are made of gallium with arsenic, nitrogen and phosphorus additives.
A light emitting diode is a standard diode made of semiconductors of the Direct Band-Gap type of material with its junction physically arranged so it freely emits its photons in the desired direction. The diode is encased in transparent coloured or clear plastic often rounded so as to act as a lens. ( ref )
In direct band-gap materials, electrons do not change momentum when moving from conduction band to valence band so no momentum is transferred to the surrounding structure. Heat is not generated but light is.

As a DBG diode conducts in the forward direction, electrons fall from the Conduction Band to the lower energy Valence Band. From basic quantum considerations of atoms, electrons falling inwards under the influence of the Electric Field of the nucleus must emit photons with an energy equal to the difference in energy of the two bands.
ΔECV= hf 
Because red light is of lower photonic energy, these were the first to be created as the technology was simplest. Blue LEDs are relatively recent devices ( ref ) using Gallium Nitride as the basic semiconductor as it has the appropriate gap between bands ( band gap ).
From the above equation, red has ΔECV~ 2.0 eV  whereas for blue light ΔECV~ 4.0 eV.
Ironically, white LEDs currently are blue/UV LEDs with fluorescent impurities in the casing plastic similar to those used in domestic fluorescent lights. The deep blue/UV components are absorbed by the fluorescents and reemitted across the whole spectrum. This is because the current generation of green LEDs are very inefficient and produce too much heat compared to light.

Photo Electric Effect

How the emission of electrons depend upon the intensity of light and frequency is well explained in this animation Please go through it once,Its really awesome.
For animation click here

Semiconductor diode animation link

How exactly charge carries flows inside the diode with different bias voltages are wonderfully explained in this animation . Please refer the below animation for Crystal clear concept in diode.


Semiconductor Physics Doide Biasing


Diode Biasing
REVERSE BIAS
If the crystal has a negative voltage applied to the p-type and a positive voltage to the n-type crystals, then we REVERSE bias the diode. ( We could use a AA battery to do this! ) The positive holes are attracted AWAY from the junction. Similarly the negative electrons are also attracted AWAY from this region.
We have reinforced the internal E Field with the imposed external field. A large energy difference for electrons in the Conduction Band will now appear across the junction and electrons will fail to cross and merge with holes - it will fail to conduct. ( There is a breakdown point however, diodes deliberately designed to use this are called "Zener diodes" and are used to lock voltages in circuits! )
Similarly a large energy difference in the Valence Band will appear for holes so they will not cross the junction.
FORWARD BIAS
If the p-type is connected to positive and n-type to negative, then the different types of charge will merge at the junction and a full current will flow around the circuit.
The External Electric Field now cancels the internal field.
The Bands will be brought to the same energies and electrons and holes will be able to cross from one type to another. Electrons will then fall into holes and a current is established.
Energy is released at the junction in the form of light /and or heat depending on the types of diode material. Silicon, an Indirect Band-Gap semiconductor, tends to have heat created through change of momentum of the electrons as they drop from Conduction Band to Valence Band. However in Direct Band-Gap semiconductors, each fall of an electron releases a photon whose frequency is given by
ΔECV= hf 


Monday 19 December 2011

Solution For Objective Papers

Solution will be published as soon as possible.One important note to my students.Please become a member of this website.Because only the members can see the solution.And if you have any doubt You can post there.I will try to answer by the end of the day.Please make this website  as more interactive as possible.

For membership you can directly sign in your Google accounts.Inform your friends also about this.


Physics Objective Type Questions with Answers for the preparation of Engineering Entrance Test : AIEEE, (5)



  1. The equation applicable in the adiabatic process of n mole ideal gas is...

    1. ΔU = nCpΔT
    2. ΔU = nCvΔT
    3. ΔQ = nCvΔT
    4. ΔQ = nCpΔT
  2. In a Carnot's engine, the working substance is...

    1. ideal gas
    2. steam
    3. petrol
    4. CO2
  3. Two particles A and b initially at rest, move towards each other under a mutual force of attraction. At the instant when the speed of A is v and the speed of B is 2v, the speed of centre of mass of the system is

    1. zero
    2. v
    3. 1.5v
    4. 3v
  4. Imagine a light planet revolving around a very massive star in a circular orbit of radius R with a period of revolution T. The gravitational force of attraction between the planet and the star is proportional to R(-5/2), then T2 is proportional to...

    1. R3
    2. R(7/2)
    3. R(3/2)
    4. R3.75
  5. Electrical conductivity increases with the increase in temperature of...

    1. P-type semiconductor
    2. N-type semiconductor
    3. intrinsic semiconductor
    4. none
  6. Amount of charge in coulomb required to deposit one gram equivalent of substance by electrolysis is...

    1. 96500
    2. 48 x 10-10
    3. 6 x 1024
    4. 9600
  7. A camera lens is adjusted to f/4 and has an effective diameter of 2.0 cm. Its focal length is...

    1. 0.2 cm
    2. 1.6 cm
    3. 2.0 cm
    4. 8.0 cm
  8. The self inductance of a long solenoid is directly affected by changes in all but...

    1. the number of turns
    2. the current
    3. the area of cross section
    4. the relative permeability of the core
  9. Which of the following is not the unit of energy ?

    1. joule
    2. erg
    3. newton.metre
    4. dyne
  10. All collisions conserve...

    1. kinetic energy
    2. momentum
    3. both kinetic energy and momentum
    4. neither of kinetic energy and momentum
  11. If a rocket of initial mass m is to rise from its launching pad, its initial thrust must exceed...

    1. mg/2
    2. mg
    3. 2mg
    4. mg2/2
  12. 9 gram of water contains approximately...

    1. 6 x 1023 molecules of water
    2. 6 x 1023 atoms of oxygen
    3. 6 x 1023 atoms of hydrogen
    4. 3 x 1023 atoms of hydrogen
  13. A cable can support a maximum load of W without exceeding the elastic limit. If it is replaced by a cable of same material but half as long and half the diameter, the elastic limit will not be exceeded by the new cable up to a load of...

    1. W/4
    2. W/2
    3. W
    4. 2W
  14. Surface tension does not depend on...

    1. surface area
    2. the type of liquid
    3. temperature
    4. medium in contact with the liquid
  15. -40° F is equal to...

    1. -40° C
    2. -8° C
    3. -72° C
    4. -104° C
  16. The magnitude of the field within a conductor...

    1. depends on the shape of the conductor
    2. is always zero
    3. is always positive
    4. may be zero or positive
  17. A permanent magnet does not exert force on...

    1. an unmagnetized iron bar
    2. a magnetized iron bar
    3. a stationary electric charge
    4. a moving electric charge
  18. A primary rainbow cannot be seen by an observer looking up if the altitude of the Sun exceeds...

    1. 10°
    2. 20°
    3. 22.5°
    4. 40°
  19. According to the descending order of temperature the stars are classified in to the orders O,B,A,F,G,K and M. Our Sun comes in the order...

    1. A
    2. B
    3. G
    4. K
  20. Position of the centre of mass of a rigid body depends on...

    1. its shape only
    2. distribution of its matter only
    3. both its shape and distribution of matter
    4. its position relative to the earth.
  21. Force is to linear motion as _____ is to rotational motion.

    1. acceleration
    2. torque
    3. moment of inertia
    4. angular momentum
  22. A hole is drilled to the centre of the earth and a stone is dropped in to it. When the stone is at the earth's centre, compared to the values at the earth's surface...

    1. its mass and weight both change
    2. its mass and weight both are zero
    3. its mass is unchanged and its weight is zero
    4. its mass is zero and its weight is unchanged
  23. For an IC engine, the thermal efficiency depends on...

    1. the compression ratio
    2. the speed of the piston
    3. the temperature of working medium
    4. the diameter of the cylinder
  24. A 12-V potential difference is applied across a series combination of four 6-ohm resistors. The current in each resistor is...

    1. 0.5 A
    2. 2 A
    3. 8 A
    4. 18 A
  25. The temperature at which the thermo emf becomes zero is known as the...

    1. neutral temperature
    2. absolute temperature
    3. inverse temperature
    4. thermoelectric temperature
  26. A transformer uses the principle of...

    1. self-induction
    2. mutual induction
    3. variable resistance
    4. electrostatic attraction
  27. If V and Vm are the instantaneous and maximum values, respectively , of voltage in an a.c. circuit, then the mean value of the voltage is...

    1. Vm sinωt
    2. √ 2 Vm
    3. Vm/√ 2
    4. none of the above
  28. Electromagnetic waves having wavelengths greater than 200 m are called...

    1. sky waves
    2. ground waves
    3. micro waves
    4. X-rays
  29. If θm is the angle of diffracted wave with the direction of incidence and d is the width of slit, then mth order minima is obtained for sinθm equal to...

    1. mλ/d
    2. dλ/m
    3. md/λ
    4. m/λd
  30. As a sample of a radioactive element decays, its half-life...

    1. decreases
    2. increases
    3. remains the same
    4. changes exponentially.

Physics Objective Type Questions with Answers for the preparation of Engineering Entrance Test : AIEEE, (4)



  1. A current of 5 A passes along a wire of length 1.0 m. the wire is at right angles to a magnetic field of flux density of 0.15 T. The force acting on the wire is...

    1. 0
    2. 0.03 N
    3. 0.75 N
    4. 33 N
  2. The end product of most radioactive decay series is usually some form of

    1. carbon
    2. helium
    3. hydrogen
    4. lead
  3. The cause of surface tension is...

    1. adhesive forces
    2. cohesive forces
    3. external forces
    4. gravitational force
  4. The unit 'poise' is...

    1. dyne.sec/cm2
    2. dyne.cm/sec2
    3. newton.sec/m2
    4. dyne.sec/cm
  5. Emission line spectrum will be obtained from...

    1. candle flame
    2. sodium lamp
    3. Sun
    4. white hot filament of an electric bulb
  6. A hollow insulated brass sphere is positively charged. The electric potential inside the sphere is...

    1. zero
    2. greater than the potential at the surface
    3. smaller than the potential at the surface
    4. the same as that on the surface
  7. The linear momentum of a body is P = a + (bt2/2). The force acting on the body is...

    1. a + (bt/2)
    2. a + bt
    3. bt/2
    4. bt
  8. The weight of a body on the surface of earth is 100 N. Its weight at a depth half way to the centre of earth will be...

    1. 50 N
    2. 100 N
    3. 25 N
    4. 125 N
  9. Two wires C and D of the length in the ratio 2 : 3, diameter in the ratio 2 : 3 and of the same material, are subjected to same force. Then the ratio of their extentions lc : ld is...

    1. 3 : 2
    2. 2 : 3
    3. 4 : 9
    4. 8 : 27
  10. The potential energy of a harmonic oscillator is maximum when the displacement is equal to...

    1. zero
    2. 1/2 amplitude
    3. amplitude
    4. 1/4 amplitude
  11. When the forces acting on a body are in equilibrium, its...

    1. velocity is zero
    2. displacement is zero
    3. acceleration is zero
    4. momentum is zero
  12. An object at rest may possess...

    1. velocity
    2. momentum
    3. kinetic energy
    4. potential energy
  13. A nonconservative force does not give rise to...

    1. thermal energy
    2. kinetic energy
    3. light energy
    4. potential energy
  14. Atoms forming a molecule attain the arrangement in which they obtain...

    1. zero potential energy
    2. minimum potential energy
    3. maximum potential energy
    4. none of the above
  15. Bulk modulus of a liquid is...

    1. higher than that for a solid
    2. higher than that for a gas
    3. lower than that for a gas
    4. zero
  16. The CGS unit of coefficient of viscosity is...

    1. dyne.second/cm2
    2. newton.second/cm2
    3. dyne.(second)2/cm2
    4. newton.second/cm
  17. The volume of a gas sample is directly proportional to its...

    1. Fahrenheit temperature
    2. Celsius temperature
    3. Kelvin temperature
    4. pressure
  18. The approximate relationship between the coefficient of linear expansion(α) and the coefficient of cubical expansion(γ) is...

    1. γ = α2
    2. γ = α3
    3. α = 3 γ
    4. γ = 3 α
  19. When capacitors are in parallel...

    1. they are all at the same potential
    2. they are at different potentials
    3. their capacitances are equal
    4. the equivalent capacitance is zero
  20. The earth's magnetic field at sea level is...

    1. 3 x 10-9 T
    2. 3 x 109 T
    3. 3 x 10-5 T
    4. 3 x 105 T
  21. Monoatomic gases give...

    1. line spectra
    2. band spectra
    3. continuous spectra
    4. line and band spectra
  22. The centre of mass of a rigid body...

    1. lies inside the body
    2. lies outside the body
    3. can be inside or outside of the body
    4. may not exist
  23. If 'l' is the length of a simple pendulum and 'T' its period, then 'g' is given by...

    1. (4π2l)/T2
    2. 2π/(√ l/t)
    3. 2πl/t
    4. (4πl2)/t2
  24. The rate at which an object radiates electromagnetic energy does not depend on its...

    1. surface area
    2. mass
    3. temperature
    4. ability to absorb radiation
  25. Which of the following is neither a basic physical law nor derivable from one?

    1. Coulomb's law
    2. Ohm's law
    3. Kirchhoff's first law
    4. Kirchhoff's second law
  26. The power dissipated as heat in an a.c. circuit depends on its...

    1. resistance
    2. inductive reactance
    3. capacitive reactance
    4. impedance
  27. Impedance is a maximum at resonance in...

    1. a series RLC circuit
    2. a parallel RLC circuit
    3. all RLC circuits
    4. no RLC circuit
  28. Red shift indicates...

    1. recession of star
    2. approaching movement of star
    3. that red light changes to violet
    4. that violet light changes to red
  29. The penetration power of beta particles is 1. Then the relative penetration power of gamma rays is...

    1. 1
    2. 100
    3. 1/1000
    4. 1000
  30. In a common-base circuit the current gain is...

    1. 0
    2. 1
    3. greater than 1
    4. smaller than 1

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