# ISC GUESS PAPER IN PHYSICS FOR 2012.

**ISC Guess Paper in Physics for the year – 2012. **

**Physics **

**Time – 3 Hours. ** **Full Marks – 70. **

** Part – I **

** (Answer all the questions)**

Q.1.

(i) For measuring the emf of a cell, why potentiometer is preferred over a voltmeter.

(ii) Find the work done in turning an electric dipole through 360º.

(iii) State the source of energy of stars.

(iv) How are p-type semiconductor produced?

(v) Can 8.6×10^{–19} coulumb of charge be given to a conductor?

(vi) Find the value of disintegration constant of a radioactive substance if its half life is 13.86 days.

(vii) Calculate the work function of an alkali metal whose photo electric threshold frequency is 5.0×10^{14} Hz.

(viii) State the angle of refraction made by a ray of light inside a regular glass prism in the case of minimum deviation.

(ix) State the main reason for axial chromatic aberration in the formation of image by a lens.

(x) The intensity of illumination of 5 lux is required at a distance of 19 m from an electric lamp. What must be the intensity of the lamp.

(xi) In Young’s double slit experiment, state the effect , if any, on the fringe width if red light is used in place of violet light.

(xii) State the type of wavefront generated from a line source of light.

(xiii) How much kinetic energy in Joule is gained by an electron, initially at rest, when it moves through a potential difference of 1000 V?

(xiv) State the nature of the electric potential at a place where the electric field, E = 0?

(xv) Show graphically how electrical resistance of a piece of silicon or germanium varies with variation of temperature.

(xvi) A circular coil having 60 turns, each with an area of 0,01 m^{2}, carries a current of 2A. Find the magnetic dipole moment.

(xvii) How are the electric vector E, the magnetic vector B and velocity vector C oriented in an electro magnetic wave?

(xviii) A straight conductor of length 40 cm is moved with a speed of 7 ms^{–1} along the direction of a uniform magnetic field. Calculate the emf induced, if any, across its ends.

(xix) An alternating current of frequency f is flowing through an ideal choke coil of self inductance L. If V_{0} and I_{0} be the peak values of the voltage and current, find the average power consumed if any by the choke coil.

(xx) Identify the logic gate from the truth table given below. Draw its logic symbol also.

**Part – II **

**Answer six questions from this part, choosing two questions from each of the ****Section A, B and C.**

**Section – A**

**(Answer ANY TWO questions) **

Q.2.

(a) Find the expression for the electric potential at a point at a distance of r from the point charge Q. [3]

(b)Find the equivalent capacitance of the following circuit between the junction A and B, given that C_{1} = C_{2} = C_{3} = C_{4} = 10 μF and C_{5} = 5 μF. [3]

(c) If the resistance of a wire is 10 Ω, what must be the resistance of another, which when joined in parallel with the first, makes the joint resistance 8 Ω. [3]

Q.3.

(a) The emf E of a thermocouple varies with the temperature θ (in degree C) of the hot junction (cold junction at 0º C) as E = 15θ – 0.05θ^{2}. Find the neutral temperature.[3]

(b) A 100 W, 220 V bulb is connected to a 110 V DC source. Determine the power consumed by the bulb. [3]

(c) A helium nucleus completes one round of a circle of radius 80 cm in 2 seconds. Calculate the magnetic field at the centre of the circle. [3]

Q.4.

(a) When Irms is the root mean square value of alternating current and I_{0} is its peak value, show that Irms = I_{0}/√2. [3]

(b) A 30 μF capacitor, 0.2 H inductor and a 40 Ω resister are connected in series to an AC source with emf given by E = 310 sin (314 t) where E is in volt and t in second. Calculate :

(i) the impedance of the circuit and

(ii) peak value of the current in the circuit. [3]

(c) When the primary of a transformer is connected to a 120 V AC mains, the current in the primary is 18.5 mA. Find the voltage across the secondary when it delivers 1.5 mA current through it, assuming the transformer to be an ideal one. State any one type of energy loss in a transformer. [3]

**Section – B **

**(Answer ANY TWO questions)**

Q.5.

(a) Draw a simple ray diagram to show the formation of a primary rainbow. [2]

(b) Using Huygen’s principle, prove the laws of refraction of light. [4]

(c)

The figure above shows the positions of the first bright fringes B_{1} and B_{2} on either side of the central bright fringe O in Young’s double slit experiment. Find the wavelength of monochromatic light used. (M is the mid point of S_{1}S_{2}) [2]

Q.6.

(a) Why images observed in a plane mirror are reversed left to right but are not reversed top to bottom? [2]

(b) State the essential difference between polarized light and un-polarized light. Find Brewste’s angle for water having refractive index μ = 1.33. [3]

(c) Two lenses of focal lengths f_{1} and f_{2} are kept in contact, prove that their combined focal length f is given by

1/f = 1/f_{1} + 1/f_{2}. [3]

Q.7.

(a)(i) Draw a ray diagram to show the formation of the image of a distant object by a refracting astronomical telescope in normal use.

(ii) State one advantage of a reflecting astronomical telescope over a refractive astronomical telescope. [3]

(b) The refractive indices of a substance for red, violet and yellow light are 1.52, 1.62 and 1.59 respectively. Calculate the dispersive power of the substance. If the mean deviation is 40º, find the angular dispersion produced by a prism of this substance. [3]

(c) State the cause of spherical aberration in lenses. Write a method to minimize this defect. [2]

**Section – C **

**(Answer ANY TWO questions)**

Q.8.

(a) In a cathode ray tube an electron passes through a field where the electric field E and the magnetic field B are uniform and mutually perpendicular. Derive an expression for the velocity at which the electron reaches the screen un-deflected. [3]

(b) (i) What is meant by the term activity of a radioactive substance?

(ii) A radioactive substance has a half life of 1.4×10^{16} seconds and a mass number of 238. Calculate the number of disintegrations of 1 g of this sample. [3]

(c) Which of the radiations α rays, β rays or γ rays are

(i) easily absorbed by the matter?

(ii) similar to X-rays? [2]

Q.9.

(a) From Bohr’s energy quantization formula, find the largest wavelength in the Balmer series of hydrogen atom spectrum. [3]

(b) Draw a labeled diagram of a nuclear reactor and state the functions of each parts. [3]

(c) Discuss how the intensity and the penetrating power of X-rays are controlled in a Coolidge X-ray tube. [2]

Q.10.

(a) If 200 Mev energy is released per fission of _{92}U^{235}, how many fissions must occur per second to produce 1 MW power? [3]

(b) What is the photoelectric effect? How is this effect explained in terms of the quantum theory? Why is this effect cannot be explained in terms of the wave theory of light? [5]

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