 APEX INSTITUTE : Class - XII Physics Sample Paper-2

## Tuesday, September 16, 2014

### Class - XII Physics Sample Paper-2

Instructions:
1. Section-A  Q. no. 1-8 carry 1 mark each.
2. Section-B  Q. no. 9-18 carry 2 marks each.
3. Section-C  Q. no. 19-27 carry 3 marks each.
4. Section-D  Q. no. 28-30 carry 5 marks each.
Section-A
1. The P.d. across a given copper wire is doubled .What happens to the drift velocity of charge carriers?
Ans. Doubled as current increases.
2. You are asked to measure of emf of a cell. Which instruments will you use a high resistance voltmeter or a potentiometer?
Ans. Potentiometer as it does not draw current.
3. In which direction would a compass needle align if taken to geographic (i) North and (ii) south pole?
Ans. Parallel and anti parallel to .
4. Steel is preferred for making permanent magnets why?
Ans. Steel has high coercivety and permeability.
5. State the law that gives the polarity of induced emf.
Ans. According the law, induced current will be such that it opposes the cause by which it is induced.
6. What will be average value of a.c. signal I = 10sin314t for time interval t = 0 to t = π/w?
Ans. 7. Write the following radiations in ascending order in respect of their frequencies X-ray microwaves radio waves.
Ans. Radiowave, microwave ,  X-ray.
8. A converging lens of refractive index 1.5 is kept in a liquid having same refractive index .What would be power of lens in this medium.
Ans. SECTION-B
9. The image of a candle is formed by a convex lens on a screen. The lower half of the lens is painted black to make it completely opaque .Draw the ray diagram to show the image formation. How will this image be different from one obtained when lens is not painted.
Ans. 10. Explain the effect of increase of intensity and potential difference on photoelectrons kinetic energy.
Ans. Due  to increase in Intensity , No effect on kinetic energy of Photo electrons as well as on Potential Difference. As due  to increase in Intensity , there  is only an increase in the number of Photons per unit area , and not the energy incident.
11. An alternating voltage of frequency f is applied across LCR circuit. Let fr be the resonance frequency of the circuit . Will the current in the circuit lag, lead or remain phase with the applied voltage when (i)  f > fr  (ii) f <  fr ?
Ans. (i) f > fr  circuit will be inductive. Hence current lag with applied voltage.
(ii) f > fr  capacitive, current leads with applied voltage.
12. Figure shows the variation of intensity of magnetization versus the applied magnet field H for two magnetic materials A & B (i) Identify the materials A and B.
(ii) Predict the variation of susceptibility with temperature for the material A.
Ans. (i) A → para , B → Fero
(ii) 13. With the help of an example, explain how the neutron to proton ratio changes during α –decay of nucleus.
Ans. In general ZXA →   Z-2YA-4  +  2He4 + Q 14. A cell of emf E and internal resistance ‘r’ are connected across a variable resister R. Plot a Graph. Sshowing the variation of terminal potential V with resistance r?
Ans.  15. A cylindrical bar magnet is kept along the axis of a circular coil and near it as shown in the diagram. Will there be any induced emf at terminals of the coil, when the magnet is rotated (a) about its own axis (b) about an axis perpendicular to the length of magnet. Ans. (a) No as flux is not changing.
(b) Yes as flux is changing.
16. Mention four properties of em waves.
Ans. (i) They are produced by accelerating or oscillating charge.
(ii) They do not require material medium for propagation.
(iii) They follow the law of super position.
(iv) They propagate with speed of light in vacuum irrespective of their wavelength.
17. For a photosensitive surface threshold wavelength is λ0 .Does photo emission occur if wavelength (λ) of incident radiation is more than λ0 , less than  λ0. Justify your answer.
Ans. (i) No, as work function Energy if incident radiation Decreases.
(ii) Yes, Energy of incident radiation increases.
18. The ground state energy of H-atom is -13.6ev. If an electron makes a transition from energy level -0.85ev to -3.4ev. Calculate the wavelength of spectral line emitted to which series of H-atom spectrum does this belong ?
Ans. ΔE = -0.85 – (-3.4)
= 2.55ev =  SECTION-C
19. Deduce an expression for energy of an electron in H-atom in nth orbit.
Ans.   20. Define the term resistivity. Derive the expression for the resistivity of a conductor in terms of number density?
Ans. Resistance offered by a conductor of unit length and unit area of cross section is known as resistivity.
Derivation -
I = neAvd  ...(1)  ...(2)
Comparing (1) and (2) 21. Show mathematically that in A.C. circuit containing only inductance , the current lags behind the voltage by a phase of π/2.
Ans. As E0sinωt – L   22. Explain the term side band frequency, Band width of signal. What will be the Band width of signal used for music and T.V. broadcast?
Ans. Band width :- Δ = max - minimum
e.g. for Audio signal ranges 20 Hz. To 20,000 Hz.
Bandwidth Δ = 20000 – 20 ≈ 20 KHz ,
For : –  Music →  Δ = 20KHz .
For : –  T.V   → Δ = 6MHz .
23. Sketch a graph b/w angle of incident radiation and stopping potential for a given photosensitive material. What information can be drawn from the value of intercept on the potential axis.
Ans. E = h = Φo + eVo  24. A convex lens made up of glass  of refractive index 1.5 is dipped in turn in :
(i) Medium A of refractive  index 1.65
(ii) Medium B of refractive index 1.33
Explain giving reasons ; whether it will behave as a converging or diverging lens.
Ans. (i) If  μm > μg   =>  fm =-ve => Nature will change it will be diverging.
(ii) μm < μ=> fm =increased, Nature will be same.
25. The magnitude of magnetic field in a plane e.m. wave is given as BX = 0, BY = 2 x 10-7 sin(0.5 x 103 x + 1.5 x 1011t)T.
(a) Determine the wavelength and frequency of wave.
(b) Write an expression for the electric field.
Ans. BY = BO  sin (1.5 x 1011t + 0.5 x 103x)T
BO = 2 x 10-7 T,                          ω = 2πν = 1.5 x 1011
ω = 1.5 x 1011                       ⇒ K = 0.5 x 103 EX = 0 , EY = 0, EZ = CBO sin (1.5 x 1011t + 0.5 x 103x)
EZ = 60 sin (1.5x 1011t + 0.5 x 103x)
26. (a) Show that the decay rate r of a sample of a radionuclide is given as R = λN.
(b) The half life of 92U238 against α-decay is 1.5 x 1017s. what is the activity of a sample of 92U238 having 25 x 1020atoms.
Ans. (a)  N = Noe-λt  27. Draw a diagrams to show the behavior of magnetic field lines near a ‘bar’ of (i) copper (ii) Aluminum and (iii) mercury cooled to a very low temperature.
Ans.   SECTION-D
28. Using Kirchhoff’s law calculate the value of current I1, I2 and I3 . Ans. In Loop ABEFA,
2I3 + 3I1 = 6
2I1 + 2I2 + 3I1 =6
5I1 + 2I2 = 6 -------------(1)
In Looop BEDCB
2I1 + 6I2 = 12
I1 + 3I2 = 6      ---------(2)
(II) x5  - I  ⇒ 15I2 – 2I2 = 30 – 6
13I2 = 24 Using equation (2) 29. An a.c. generator consist of a coilof 100 turns and crossectional area of 3m2, rotating at a constant angular speed at 60 rad/s in a uniform magnetic field of 0.04T the resistance of coil is 500? . Calculate the maximum current drawn from the generator and maximum power dissipation in the coil.
Ans. N = 100 , A = 3m2, W= 60rad/s
B = 0.4T          R = 500Ω
As induced emf e = NBAW sinwt
Max emf = e0 = NBAW = 100 x 0.4 x 3 x 60
= 7200 volt
Induced current (max) I0 = 7200/500 = 18A
Power = e0I0   7200 x 18 = 1.296 x 103 watt
30. Draw a graph to show the variation of angle of Deviation ‘D’ with that of the angle of incidence i  for a monochromatic ray of light passing through a glass prism of refracting angle A. Hence deduce relation. Ans.  As δ = (i r1) + (e- r2)
δ = (i +e) - (r1 + r2)        ­­­­­­­­­    - (1)
In Δ QNR              r1 + r2  +  