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Physics
Class- XII
2000(CBSE)
You are on Set no 1 Qno. 21 to
30
Q21) State Huygen's postulates of wave theory. Sketch the wavefront
emerging from a (i) point source of light and (ii) linear source of
light like a slit. (Marks 1+2)
Q22) State the conditions for total internal reflection of light to
take place at an interface separating two transparent media. Hence derive
the expression for the critical angle in terms of the speeds of light in
the two media. (Marks 3)
Q23) State the dependence of work function on the kinetic energy of
electrons emitted in a photocell. If the intensity of incident radiation
is doubled, what changes occur in the stopping potential and the
photoelectric current. (Marks 1+2)
Q24) With the help of a labelled circuit diagram, explain how will
you determine the internal resistance of a primary cell using a
potentiometer. State the formula used. (Marks 3)
Q25) A short bar magnet of magnetic moment 0.9 joule/tesla, is
placed with its axis at 45o to a uniform magnetic field. If it
experiences a torque of 0.063 joule, (i) calculate the magnitude of the
magnetic field and (ii) what orientation of the bar magnet corresponds to
the stable equilibrium in the magnetic field ? (Marks
3)
Q26) A conductor of length 'I' is connected to a d.c source of
potential 'V' . If the length of the conductor is tripled, by stretching
it, keeping 'V' constant, explain how do the following factors vary in the
conductor :
(i) Drift speed of electrons, (ii) Resistance and (iii)
Resistivity. (Marks 3)
Q27) A proton and an alpha particle of the same velocity enter in
turn a region of uniform magnetic field acting in a plane perpendicular to
the magnetic field. Deduce the ratio of the radii of the circular paths
described by the particles. Explain why the kinetic energy of the particle
after emerging from the magnetic field remains unaltered. (Marks
3)
Q28) State the postulates of Bohr's model of hydrogen atom. The
electron, in a given Bohr orbit has a total energy of -1.5 eV. Calculate
its (i) kinetic energy. (ii) potential energy and (iii) wavelength of
light emitted, when the electron makes a transition to the ground state.
(Ground state energy = -13.6 eV) (Marks 5)
Q29) For a given a.c. circuit, distinguish between resistance,
reactance and impedance. An a.c. source of frequency 50 hertz is connected
to a 50 mH inductor and a bulb. The bulb glows with some brightness .
Calculate the capacitance of the capacitor to be connected in series with
the circuit , so that the bulb glows with maximum brightness.
(Marks 3+2)
Q30) Drawing a labelled circuit diagram, explain the working
principle of a common-emitter transistor amplifier. State the phase
relation between input and output signals.
Or
With the help
of a labelled circuit diagram, explain how a transistor oscillator
works. (Marks 5)
Useful constants
(I) Planck's
constant, h = 6.6 x 10-34 Js
(ii) Speed of light in air, c=
3 x 108 m s-1
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