Physics - 1999 ( I.S.C)
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Answer all questions in Part I and six Questions from Part II, choosing two from each of the Sections A,B and C
All working, including rough work, should be done on the same sheet as, and adjacent to, the rest of the answers.
The intended marks for questions or parts of questions are given in brackets [].
(Material should be supplied : Log tables including trignometric functions).

PART-I(Compulsory)

Q1.(i) State the unit of electric permitivity in terms of the base(fundamentals) SI units.

(ii) Draw a diagram to show the electric lines of force due to an electric dipole.


(iii) The plates of a charged parallel plate capacitor are connected to a voltmeter. What will be the effect of the reading of the voltmeter if the plates of the capacitor are brought closer to each other in an insulated manner(i.e. while conserving charge on the plates.)


(iv)In fig.1,AB is a potentiometer wire of the length 100 cm.The cell of emf 3V has a negligible internal resistance.What is the reading in the voltmeter (V) if BC is equal to 70 cm ?

(v)An electric kettle is rated 3 KW, 240 V. Find out the resistance of its heating element.


(vi)Fig.2 shows two separate wires forming a cycle of radius r with center at O.Both wires carry equal
currents i, Find out the total magnetic field at the center O due to the two currents.

(vii)Why is soft iron is considered more suitable than steel for making the core of transformer ?


(viii)How is galvanometer  converted in to an ammeter ?


(ix)A two slit interference experiment is performed with monochromatic red light and the fringe width is measured.How is this fringe width affected when the source is replaced by monochromatic blue light ?


(x)The wavelength of lightwave is 6400 Å in air.What will be its wavelength when it enters  a medium whose refractive index is 1.6 ?

(xi)Light intensity can be partially reflected and partially transmitted by a medium whose refractive index is Ö3.Find out the angle of polarisation for this medium ?

(xii)A thin convex lens of focal length 20 cm is kept in coaxial contact with another thin lens so that the combination becomes a converging lens of focal length 100 cm. What is the focal length of the second lens ?

(xiii)The focal length of the objective and the eyepiece of an astronomical telescope are 2.5 m and 5 cm respectively.What is the magnifying power of the telescope when the final image is formed at infinity ? 

(xiv)An electron is accelerated from rest through potential difference V volts and it acquires a final velocity v m/s.Obtain an expression for e/m of electron in terms of V and v ?

(xv)State how the wavelength l and momentum p are related to each other in describing wave-particle duality of matter ?

(xvi)Draw a labeled diagram to illustrate Rutherford's a-particle scattering experiment.

(xvii)What is the relation between the maximum frequency of X-rays produced by a coolidge tube and the potential difference V between the cathode and the target(anode) ?

(xviii)Write the radioactive   decay law in the form of an equation stating clearly what the symbols stand for.

(xix)What is the function of the moderator in a nuclear reactor ?

(xx)Draw a labeled diagram to show the circuit symbols of an npn transistor.

PART-II

Q2(a)Define potential energy of a point charge in a uniform electric field E .Obtain the expression U=- P. E for the potential energy of a dipole moment P in a uniform electric field E.

(b)A potentiometer arrangement, used to measure thee emf of a cell E1,is shown in fig-3.AB is the potentiometer wire of length 100 cm and rheostat R is adjusted to 5 ohm.If the null point is obtained at C with Ac equal to 75 cm,what is the emf of E1 ?

Q3(a)A moving coil galvanometer of resistance 25 ohm gives a full scale deflection when 10 mA current is passed through it.Giving necessary calculations.explain with the help of a diagram how this galvanometer may be converted into a voltmeter of range 20 V.

(b)State 'Tangent law 'for superposition of two magnetic fields. Prove this law with the help of a labeled diagram.

Q4(a)Give the defining vector equation of magnetic field B  in terms of the force F acting on a charge q moving in the field with a velocity V .Using this equation express the SI unit of B in terms of the fundamental (base)SI unit.

(b)Derive the expression for the force per unit length acting on either of two very long parallel conductors carrying currents I₁ and I₁ respectively and kept at a distance d apart.State how the directions of the magnetic fields and the forces produced May be indicated.

(c)The coil of a simple A.C. generator is rotating at the rate of 50 revolution per second in a uniform magnetic field of 1 Tesla.What is the peak value of the ac voltage generated ?The number of turns in the coil is 100 and its face area is 100 cm^2.\

Q.5(a) A capacitor C is connected to an ac voltage source

V = V₀ sinw t.

Draw a circuit diagram for the above arrangement. obtained and expression for the current I flowing in the circuit. Express it as a sine function. What is the phase difference between current I and voltage V ? What is the reactance of the capacitor ?

(b) Fig. 4 shows that an alternating emf e applied across a series LRC circuit. The peak values of the alternating voltages developed across the three components are marked in the figure. Determine the peak value of the applied emf and the phase difference between the applied emf and the current in the circuit.

Q6.(a) Describe briefly, with the help of a diagram, Michelson's method for the determination of the speed of light.

(b) What happens to the fringe width of the fringe pattern of a Fresnel's biprism arrangement if the distance between the slit source and the biprism is reduced ? Explain your answer stating (not deriving) the relation between fringe width, distance between the two apparent sources and the distance of the biprism and the slit. Draw a ray diagram to show the arrangement.


Q7(a) A regular 600 glass prism has its refractive index Ö2 for a given wavelength. Calculate the angle of minimum deviation d_m. At what angle of incidence should a monochromatic light of this wavelength fall on a face of the prism so as to get a minimum deviation ?
(Use sin 30⁰ = 1/2 and sin 45⁰ = 1/Ö2).

(b) Draw a graph showing variation of intensity on the screen of a single slit Fraunhofer diffraction experiment against different angles of diffraction, when the slit is illuminated by a monochromatic light of wavelength l. If the slit width is a, at what diffraction angle is the first minima obtained ? (No derivation is required)


Q8.(a) A double convex lens of glass has a focal length 20 cm in air and the absolute refractive index of the material of lens is 1.5, What will be the focal length of this lens if it is immersed in a liquid of absolute refractive index 1.65 ? State the nature of the lens when immersed in the liquid.

(b) Describe with the help of a diagram, showing approximate wave length ranges, the electromagnetic spectrum from gamma rays to radiowaves.

Q9(a) Derive the condition for two thin lenses placed coaxially in contact so as to form an achromatic combination.

(b) Define 'angular magnification' of a telescope. Draw a neat ray diagram with proper arrows to show the image formation in an astronomical refracting telescope in normal adjustment for relaxed eye. Obtain the expression for angular magnification of the telescope in this adjustment. Q10.(a) Draw a labeled diagram of a cathode ray oscilloscope. Name three important sections of this tube.

Q10.(a) Draw a labeled diagram of a cathode ray oscilloscope. Name three important sections of this tube.

(b) Work function of copper metal is 4eV. Calculate the energy of the incident photon in joule. Also, express the work function in joule. Find out the Kinetic energy of the fastest electron ejected from it when a monochromatic light of wavelength 4000 Å falls on its surface. (Use h = 6.6 x 10^-34 Js, c = 3 x 10⁸ m/s).

Q11.(a) State Bohr's postulates for the model of atom. Use these postulates to derive an expression for the energy of an electron in the n^th orbit of the hydrogen atom.

(b) State the differences between emission and absorption spectra with respect to their nature and production.


Q12.(a) The nucleus of an atom with Z = 54 and A = 115 emits a negative b-particle. How many protons and  neutrons will be present in the resulting nucleus after this  b-decay ? What change takes place inside the nucleus ? Mention two properties of b-particles.

(b) Complete the following nuclear reaction :

(c) What is nuclear fusion ? Describe how the Sun is emitting huge amounts of energy (in the form of heat and light) continuously for billions of years. (No reaction equation are required.)

Q13.(a) Distinguish between conductors, insulator and semiconductors in terms of energy band diagrams. Draw the diagrams.

(b) Why do we get a small current (in mA) through a p-n junction diode under reverse bias condition ? In which direction does this current flow through the junction ?

(c) Draw a labeled circuit diagram of common emitter type transistor (npn or pnp) amplifier, showing the biasing correctly.