Physics Foreign Set 2 2014-2015 Science (English Medium) Class 12 Question Paper Solution

Why does the Sun look reddish at sunset or sunrise ?

The power factor of an a.c. circuit is 0.5. What is the phase difference between voltage and current in this circuit ?

Is it necessary for a transmitting antenna to be at the same height as that of receiving antenna for LOS communication ?

Plot a graph showing the variation of resistivity of a conductor with temperature.

Write the expression for the torque \[\vec{\tau}\] acting on a dipole of dipole moment  \[\vec{p}\] placed in an electric field \[\vec{E}\].

Write the β-decay of tritium in symbolic form.

Why is it experimentally found difficult to detect neutrinos in this process ?

Given the resistances of 1 Ω, 2 Ω, 3 Ω, how will be combine them to get an equivalent resistance of (11/3) Ω?

Given the resistances of 1 Ω, 2 Ω, 3 Ω, how will be combine them to get an equivalent resistance of  (11/5) Ω?

The V − I characteristic of a silicon diode is as shown in the figure. Calculate the resistance of the diode at (i) I = 15 mA and (ii) V= −10 V.

How does the refractive index of a transparent medium depend on the wavelength of incident light used ?

Velocity of light in glass is 2 × 10⁸ m/s and in air is 3 × 10⁸ m/s. If the ray of light passes from glass to air, calculate the value of critical angle.

An equiconvex lens of focal length 'f' is cut into two identical plane convex lenses. How will the power of each part be related to the focal length of the original lens ?

A double convex lens of + 5 D is made of glass of refractive index 1.55 with both faces of equal radii of curvature. Find the value of its radius of curvature.

The kinetic energy of the electron orbiting in the first excited state of hydrogen atom is 3.4 eV. Determine the de Broglie wavelength associated with it.

Write briefly the important processes that occur during the formation of p−n junction. With the help of necessary diagrams, explain the term 'barrier potential'.

A beam of monochromatic radiation is incident on a photosensitive surface. Answer the following question giving reason :

Do the emitted photoelectrons have the same kinetic energy?

A beam of monochromatic radiation is incident on a photosensitive surface. Answer the following question giving reason :

Does the kinetic energy of the emitted electrons depend on the intensity of incident radiation?

A beam of monochromatic radiation is incident on a photosensitive surface. Answer the following question giving reason :

On what factors does the number of emitted photoelectrons depend?

What is an 'integrated  circuit (I.C.) ?

Distinguish between (i) linear I.C. and (ii) digital I.C.

 Identify the equivalent gate for the following circuit and write its truth table.

Why does a galvanometer when connected in series with a capacitor show a momentary deflection, when it is being charged or discharged?

How does this observation lead to modifying the Ampere's circuital law?

Hence write the generalised expression of Ampere's law.

In the given circuit in the steady state, obtain the expressions for (a) the potential drop (b) the charge and (c) the energy stored in the capacitor, C.

A circular coil of radius 10 cm, 500 turns and resistance 200 Ω is placed with its plane perpendicular to the horizontal component of the Earth's magnetic field. It is rotated about its vertical diameter through 180° in 0.25 s. Estimate the magnitude of the emf and current induced in the coil. (Horizontal component of the Earth's magnetic field at the place is 3.0 ✕ 10⁻⁵ T).

Using Rydberg formula, calculate the longest wavelength belonging to Lyman and Balmer series. In which region of hydrogen spectrum do these transitions lie?
[Given R = 1.1 ✕ 10⁷ m⁻¹]

Why cannot two independent monochromatic sources produce sustained interference pattern?
Deduce, with the help of Young's arrangement to produce interference pattern, an expression for the fringe width.

Answer the following question :
Define 'bandwidth' and describe briefly its importance in communicating signals ?

Answer the following question :

Distinguish between digital and analogue signals.

Answer the following question :

Write the functions of transducer and repeater ?

Four charges +q, −q, +q and −q are to be arranged respectively at the four corners of a square ABCD of side 'a'.
(a) Find the work required to put together this arrangement.
(b) A charge q₀ is brought to the centre of the square, the four charges being held fixed. How much extra work is needed to do this ?

Three point charges +q each are kept at the vertices of an equilateral triangle of side 'l'. Determine the magnitude and sign of the charge to be kept at its centroid so that the charges at the vertices remain in equilibrium.

An electron moving horizontally with a velocity of 4 ✕ 10⁴ m/s enters a region of uniform magnetic field of 10⁻⁵ T acting vertically upward as shown in the figure. Draw its trajectory and find out the time it takes to come out of the region of magnetic 

field.

A straight wire of mass 200 g and length 1.5 m carries a current of 2 A. It is suspended in mid air by a uniform magnetic field B. What is the magnitude of the magnetic field?

A plane wavefront propagating in a medium of refractive index 'μ₁' is incident on a plane surface making the angle of incidence 'i' as shown in the figure. It enters into a medium of refractive index 'μ₂' (μ₂ > μ₁). Use Huygens' construction of secondary wavelets to trace the propagation of the refracted wavefront. Hence verify Snell's law of refraction.

Sushil is in the habit of charging his mobile and then leaving the charger connected through the mains with the switch on. When his sister Asha pointed it out to him, he replied there was no harm as the mobile had been disconnected. Asha then explained to him and convinced him, how the energy was still being wasted as the charger was continuously consuming energy.

Answer the following questions :

(a) What values did Asha display in convincing her brother?

(b) What measures, in your view, should be adopted to minimise the wastage of electric energy in your households?

(c) Imagine an electric appliance of 2 W, left connected to the mains for 20 hours. Estimate the amount of electrical energy wasted.

A small conducting sphere of radius 'r' carrying a charge +q is surrounded by a large concentric conducting shell of radius Ron which a charge +Q is placed. Using Gauss's law, derive the expressions for the electric field at a point 'x'
(i) between the sphere and the shell (r < x < R),
(ii) outside the spherical shell.

Show that if we connect the smaller and the outer sphere by a wire, the charge q on the former will always flow to the latter, independent of how large the charge Q is.

Consider a system of n charges q₁, q₂, ... q_n with position vectors `vecr_1,vecr_2,vecr_3,...... vecr_n`relative to some origin 'O'. Deduce the expression for the net electric field`vec E` at a point P with position vector `vecr_p,`due to this system of charges.

Find the resultant electric field due to an electric dipole of dipole moment, 2aq, (2a being the separation between the charges ±± q) at a point distant 'x' on its equator.

Draw a ray diagram showing the image formation of a distant object by a refracting telescope ?

Define the magnifying power?

Write the two important factors considered to increase the magnifying power?

Describe briefly the two main limitations and explain how far these can be minimized in a reflecting telescope ?

Draw a ray diagram showing image formation in a compound microscope ?

 Define the term 'limit of resolution'?

name the factors on which  'limit of resolution' depends ?

How is 'limit of resolution' related to resolving power of a microscope ?

Suggest two ways by which the resolving power of a microscope can be increased?

"A telescope resolves whereas a microscope magnifies." Justify this statement ?

Write any two important points of similarities and differences each between Coulomb's law for the electrostatic field and Biot-Savart's law of the magnetic field ?

Use Biot-Savart's law to find the expression for the magnetic field due to a circular loop of radius 'r' carrying current 'I', at its centre ?

What are eddy currents?

 How are eddy currents produced ?

Describe briefly three main useful application of eddy currents ?