# CBSE Physics Class 12th Board Question Paper 2019 | SET -3

SECTION — A

1. Distinguished between unpolarized and linearly polarised light. (1)

2. Define the term “threshold frequency”, in the context of photoelectric emission. (1)
OR
Define the term “intensity” in the photon picture of electromagnetic radiation.

3. How is the drift velocity in a conductor affected with the rise in temperature? (1)

4. In sky wave mode of propagation, why is the frequency range of transmitting signals restricted to less than 30 MHz? (1)
OR
On what factors does the range of coverage in ground wave propagation depend?

5. Draw the pattern of electric field lines, when a point charge +q is kept near an uncharged conducting plate. (1)

SECTION — B

6. (a) Define the terms, (i) threshold frequency and (ii) stopping potential in photoelectric effect.
(b) Plot a graph of photocurrent versus anode potential for a radiation of frequency ν and intensities I1 and I2 (I1<I2). (2)

7. Why a signal Transmitted from a TV tower cannot be received beyond a certain distance? Write the expression for the optimum separation between the receiving and the transmitting antenna. (2)

8. State Bohr’s quantization condition of angular momentum. Calculate the shortest wavelength of the Bracket Series and state to which part of the electromagnetic spectrum does it belong. (2)
OR
Calculate the orbital period of the electron in the first excited state of hydrogen atom.

9. Why is wave theory of electromagnetoic radiation not able to explain photoelectric effect? How does photon picture resolve this problem? (2)

10. Two bulbs rated (P1,V) and (P2,V). If they are connected (i) in series and (ii) in parallel across a supply V, find the power dissipated in the two combinations in terms of P1 and P2. (2)

11. Obtain the expression for the ratio of the de-Broglie wavelengths associated with the electron orbiting in the second and third excited states of hydrogen atom. (2)

12.A charged particle q is moving in the presence of a magnetic field B which is inclined at an angle of 30° with the direction of the motion of the particle. Draw the trajectory followed by the particle in the presence of the field and explain how the particles described this path. (2)

SECTION — C

13. (a) Explain briefly how Rutherford scattering of α-particle by a target nucleus can provide information on the size of the nucleus.
(b) Show that the density of nucleus is independent of its mass number A. (3)

14. State the underlying principle of a cyclotron. Explain its working with the help of a schematic diagram. Obtain the expression for cyclotron frequency. (3)

15. Two infinitely long straight wire A1 and A2 carrying currents I and 2I flowing in the same direction are kept ‘d‘ distance apart. Where should a third straight wire A3 carrying current 1.5I be placed between A1 and A2 so that it experiences no net force due to A1 and A2? Does the net force acting on A3 depend on the current flowing through it? (3)

16. (a) Draw the equipotential surfaces due to an electric dipole.
(b) Derive an expression for the electric field due to a dipole of dipole moment p at a point on its perpendicular bisector. (3)

17. Using Kirchhoff’s rules, calculate the current through the 40 Ω and 20 Ω resistors in the following circuit : (3)

OR

What is the end error in a metre bridge? How is it overcome? The resistance in the two arms of the metre bridge are R = 5 Ω and S respectively. When the resistance S is shunted with an equal resistance, the new balance length found to be 1.5 l1, where l1 is the initial balancing length. Calculate the value of S.

18. (a) Describe briefly the functions of the three segments of n-p-n transistor.
(b) Draw the circuit arrangement for studying the output characteristics of n-p-n transistor in CE configuration. Explain how the output characteristics is obtained. (3)
OR
Draw the circuit diagram of a full wave rectifier and explain its working. Also give the input and output waveforms.

19. Define the term wavefront. Using Huygen’s wave theory, verify the law of reflection. (3)
OR
Define the term “refractive index” of a medium. Verify Snell’s law of refraction when a plane wavefront is propagating from a denser to a rarer medium.

20. (a) Identify the part of the electromagnetic spectrum used in (i) radar and (ii) eye surgery. Write their frequency range.
(b) Prove that the average energy density of the oscillating electric field is equal to that of the oscillating magnetic field. (3)

21. Draw a labelled ray diagram of an astronomical telescope in the near point adjustment position.
A giant refracting telescope at an observatory has an objective lens of focal length 15 m and an eyepiece of focal length 1.0 cm. If this telescope is used to view the moon, find the diameter of the image of the moon formed by the objective lens. The diameter of the moon is 3.48 x 106 m, and the radius of the lunar orbit is 3.8 x 108 m. (3)

22. (a) If A and B represent the maximum and minimum amplitude of an amplitude modulated wave, write the expression for the modulation index in terms of A and B.
(b) A message signal of frequency 20 KHz and peak voltage 10 V is used to modulate a carrier of frequency 2 MHz and peak voltage of 15 V. Calculate the modulation index. Why the modulation index is generally kept less than one? (3)

23. (a) State Gauss’s law for magnetism. Explain its significance.
(b) Write the four important properties of the magnetic field lines due to a bar magnet. (3)
OR
write three points of difference between para-, dia-, and ferro- magnetic materials giving one example for each.

24. (a) Three photodiodes D1, D2 and D3 are made of semiconductors having band gaps of 2.5 eV, 2 eV and 3 eV respectively. Which of them will not be able to detect light of wavelength 600 nm?
(b) Why photodiodes are required to operate in reverse bias? Explain. (3)

SECTION — D

25. (a) Describe briefly the process of transferring the charge between the two plates of a parallel plate capacitor when connected to a battery. Derive an expression for the energy stored in a capacitor.
(b) A parallel plate capacitor is charged by a battery to a potential difference V. It is disconnected from the battery and then connected to another uncharged capacitor of the same capacitance. Calculate the ratio of the energy stored in the combination to the initial energy on the single capacitor. (5)
OR
(a) Derive an expression for the electric field at any point on the equatorial line of an electric dipole.
(b) Two identical point charges, q each , are kept 2m apart in air. A third point charge Q of unknown magnitude and sign is placed on the line joining the charges such that the system remains in equilibrium. Find the position and nature of Q. ⭐

26. (a) In a series LCR circuit connected across an ac source of variable frequency. Obtain the expression for its impedance and draw a plot showing its variation with frequency of the ac source.
(b) What is the phase difference between the voltages across the inductor and the capacitor at resonance in the LCR circuit?
(c) When an inductor is connected to a 200 V dc voltage, a current of 1 A flows through it. When the same inductor is connected to a 200 V, 50 Hz ac source, only 0.5 A current flows. Explain, why? Also, calculate the self-inductance of the inductor. (5)
OR
(a) Draw the diagram of a device which is used to decrease high ac voltage into a low ac voltage and state its working principle. Write four sources of energy loss in the device.
(b) A small town with a demand of 1200 KW of electric power at 220 V is situated 20 Km away from an electric plant generating power at 440 V. The resistance of the two wire line carrying power is 0.5 Ω per Km. The town gets the power from the line through a 4000-220 V step-down transformer at a substation in the town. Estimate the line power loss in the form of heat.

27. (a) Describe any two characteristic features which distinguish between interference and diffraction phenomena. Derive the expression for the intensity at a point of the interference pattern in the Young’s Double Slit experiment.
(b) In the diffraction due to a single slit experiment, the aperture of the slit is 3 mm. If monochromatic light of wavelength 620 nm is incident normally on the slit, calculate the separation between the first order minima and the 3rd order maxima on one side of the screen. The distance between the slit and the screen is 1.5m. (5)
OR
(a) Under what conditions is the phenomenon of total internal reflection of light observed? Obtain the relation between the critical angle of incidence and the refractive index of the medium.
(b) The lenses of focal length +10 cm, -10 cm and +30 cm are arranged coaxially as in the figure given below. Find the position of the final image formed by the combination.