Physics - 1998 ( I.S.C)
You are on Questions-1 to 5

(Three hours)

Answer all questions in Part I and six question from part II, choosing two questions 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 answer.
The intended marks for questions or parts of questions are given in brackets [ ] .
(Material to be supplied ; Log tables including Trigonometric functions).

PART I (Compulsory)

Q1, Answer all questions briefly and to the point:
(i) Define 'electric potential' at a point in an electric field.

(ii) A capacitor is charged through a potential difference of 200v when 0.1C charge is stored in it. How much energy will it release when it is discharged?

(iii) Find out the resistivity of a conductor in which a current density 2.5 A/m² is found to exist when an electric field 15 V/m is applied on it.

(iv) Find out the magnitude of resistance X in the given circuit diagram (fig.1) when no current flows through the 5W resistor.

(v) Define magnetic 'permeability'.

(vi) Write an expression for the magnetic field produced by an infinitely long straight wire carrying a current i, at a short perpendicular distance 'a' from itself.

(vii) A bar magnet placed in a uniform magnetic field of strength 0.3 T, with its axis at 30° to the field, experiences a torque of 0.06 Nm. What is the magneitc moment of the bar magnet?

(viii) With reference to alternating currents and voltages, state any one fundamental difference between 'resistance' and 'reactance'.

(ix) Define the term 'coherence' for light waves.

(x) A plane diffraction grating is used with a monochromatic source of wavelenght 5000 Å under normal incidence. The second order maximum is obtained at a diffraction angle of 30°. Find the grating element.

(xi) A monochromatic ray of light is made to fall on a normal 60° prism under minimum deviation condition.  What is the relation between the angle of incidence and the angle of emergence?

(xii) Find the radius of curvature of the convex surface of planocovex lens whose focal length is 0.3 m and the refractive index of the material of the lens is 1.5.

(xiii)  What happens to the wavelength of a photon after it collides with an electron?

(xiv) What should be done to increase the intensity of   X-rays, produced by a Coolidge Tube?

(xv) The radius of the first electron orbit of hydrogen atom is 5.3 x 10^-11 m. What is the radius of the second orbit?

(xvi) What is meant by the term 'isotope'.

(xvii) Give the relation between 'half life' and 'disintegration constant' of a radio-active material.

(xviii) When ₃Li⁷ is bombarded with a certain particle, two alpha particles are produced. Identify the bombarding particle.

(xix) Fig.2 shows two p-n junction diodes along with a resistance R and a dc battery E. Indicate the path and direction of flow of appreciabe current in the circuit.

(xx)What kinds of baising are required to the collector and base of a transistor in a common emmiter amplifier?

PART II
Answer six questions from Part II, choosing two questions from each of the sections A,B and C.
SECTION A

Q2. (a) Fig.3 Shows four point charges at the corners of a square of side 2 cm. Find the magnitude and direction of the electric field at the centre (X) of the square if Q = 0.02m C.

[Use 1/4pÎ₀ = 9x10⁹ Nm² C^-2].

(b) What is meant by 'capacitance'?Give its S.I unit.

(c) In Fig.4C is a large value capacitor connected ion series with a centre zero galvanometer G, a dec battery E and key K.
What will be observed in G when K is suddenly closed? Give reasons for your answer

Q 3 (a)When the key K is closed then the capacitor starts getting charged. The upper plate is positively charged and the lower plate is negatively charged. 3.(a) The circuit diagram in fig.5 has two cells E1 and E2 with emf 4V and 2V respectively, each one having an internal resistance 2W. The external resistance R is of 8W. Find the magnitude and direction of the currents flowing through the two cells.

(b) Fig.6 shows a cell with emf E and internal resistance r, connected to a voltmeter V and a variable resistance R. Deduce the relatiuonship between V, E, R and r. How will the value of V vary when R is reduced? 

Q 4 (a)I  = E/R +r
IR = E - Ir ---------------(1)
V =IR  where V is potential drop across R
When R is reduced, I increases .So V is decreased. length of the dipole is 2l and its magnetic moment is M.

(b) Why is it possible to make permanent magnets out of steel?


(c) The radius of the first electron orbit of hydrogen atom is 0.5 A. The electron moves in this orbit with a uniform speed of 2.2x10⁶ m/s. What is the magnetic field produced at the cnetre of the nucleus due to the motion of this electron? (Use m₀/4p = 1x10^-7 N/A² and electronic charge = 1.6x10^-19C).

Q 5.(a) Define 'coefficient of mutual inductance' for a pair of coils. 
A solenoidal coil has 50 turns per cm along its length and a cross-sectional area of 4 cm². 200 turns of another wire is wound round the first solenoid coaxially. The two coils are elctrically insulated from each other.  Calculate the mutual inductance between the two coils. (Use m₀ = 4p x 10^-7 N/A²).

(b). A 2mF capacitor, 100W resistor and 8 H inductor are conected in series with an A.c source. What should be the frequency of this A.c source for which the current drawn in the circuit is maximum? If the peak value of emf of the source is 200V, find for maximum current:
(i) the inductive and capacitative reactances of the circuit,
(ii)total impedence of the circuit of the circuit,
(iii)peak value of current in the circuit,
(iv)the phase relation between voltages acroos inductor and resistor.
(v)the phase difference between voltages across inductor and capacitor.

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