Class XII_All India_Physics_Set- 17. Draw V I characteristics of a p-n junction diode. Answer the following questions, giving reasons: (i) Why is the current under reverse bias almost independent of the applied potential up to a critical voltage? (ii) Why does the reverse current show a sudden increase at the critical voltage? Name any semiconductor device which operates under the reverse bias in the breakdown region. 3 V-I characteristic of p-n junction diode: (i) Under the reverse bias condition, the holes of p-side are attracted towards the negative terminal of the battery and the electrons of the n-side are attracted towards the positive terminal of the battery. This increases the depletion layer and the potential barrier. However the minority charge carriers are drifted across the junction producing a small current. At any temperature the number of minority carriers is constant so there is the small current at any applied potential. This is the reason for the current under reverse bias to be almost independent of applied potential. At the critical voltage, avalanche breakdown takes place which results in a sudden flow of large current. (ii) At the critical voltage, the holes in the n-side and conduction electrons in the p- side are accelerated due to the reverse bias voltage. These minority carriers acquire sufficient kinetic energy from the electric field and collide with a valence electron. Thus the bond is finally broken and the valence electrons move into the conduction band resulting in enormous flow of electrons and thus formation of hole-electron pairs. Thus there is a sudden increase in the current at the critical voltage. Zener diode is a semiconductor device which operates under the reverse bias in the breakdown region.
Class XII_All India_Physics_Set- 18. Define the current sensitivity of a galvanometer. Write its S.I. unit. Figure shows two circuits each having a galvanometer and a battery of 3V. When the galvanometers in each arrangement do not show any deflection, obtain the ratio R 1 /R. 3 Current sensitivity of a galvanometer is defined as the deflection in galvanometer per unit current. Its SI unit is radians/ampere. For balanced wheat stone bridge, there will be no deflection in the galvanometer. 4 6 4 9 R1 6 R 9 6 1
Class XII_All India_Physics_Set- For the equivalent circuit, when the wheat stone bridge is balanced, there will be no deflection in the galvanometer. 1 6 6 8 R 4 8 R 1 R R 1 6 3 4 19. A wire AB is carrying a steady current of 10 A and is lying on the table. Another wire CD carrying 6 A is held directly above AB at a height of mm. Find the mass per unit length of the wire CD so that it remains suspended at its position when left free. Give the direction of the current flowing in CD with respect to that in AB. [Take the value of g = 10 ms ] 3 Force per unit length between the current carrying wires is given as: o II F 4 r 1, where I 1 = current in wire AB = 10 A and I = current in wire CD = 6 A r = distance between wires = mm = 10-3 m. Let m be the mass per unit length of wire CD. As the force balances the weight of the wire II 10 6 4 r 10 10 6 1 m 10 6 10 kg m 10 10 o 1 7 mg 10 m 10 3 7 4 1 3 0. A rectangular conductor LMNO is placed in a uniform magnetic field of 0.5 T. The field is directed perpendicular to the plane of the conductor. When the arm MN of length of 0 cm is moved towards left with a velocity of 10 ms 1, calculate the emf induced in the arm. Given the resistance of the arm to be 5 (assuming that other arms are of negligible resistance) find the value of the current in the arm. 3 OR
Class XII_All India_Physics_Set- A wheel with 8 metallic spokes each 50 cm long is rotated with a speed of 10 rev/min in a plane normal to the horizontal component of the Earth s magnetic field. The Earth s magnetic field at the place is 0.4 G and the angle of dip is 60. Calculate the emf induced between the axle and the rim of the wheel. How will the value of emf be affected if the number of spokes were increased? Let ON be x at some instant. The emf induced in the loop = e. d d( Blx) dx e Bl Blv 0.5 0. 10 1 V dt dt dt e 1 Current in the arm, I 0. A R 5 OR o 4 H B cos I 0.4cos 60 0. G 0. 10 T This component is parallel to the plane of the wheel. Thus, the emf induced is given as, 1 N E Bl, where. t 1 4 3.14 10 5 E 0. 10 0.5 3.14 10 V 60 The value of emf induced is independent of the number of spokes as the emf s across the spokes are in parallel. So, the emf will be unaffected with the increase in spokes. 1. (a) What is linearly polarized light? Describe briefly using a diagram how sunlight is polarised. (b) Unpolarised light is incident on a polaroid. How would the intensity of transmitted light change when the polaroid is rotated? 3 (a) Natural light is unpolarised i.e. the electric vector takes all possible directions in the transverse plane, rapidly and randomly, during a measurement. A polarizer transmits only one component (parallel to a special axis). This resulting light is called linear or plane polarized.
Class XII_All India_Physics_Set- (b) The incident sunlight is unpolarised. The dot and double arrows show the polarization in the perpendicular and in the plane of the figure. Under the influence of the electric field of the incident wave, the electrons in the molecules of the atmosphere acquire components of motion in both these directions. An observer looking at 90 to the direction of the sun, the charges accelerating parallel to the double arrows do not radiate energy towards this observer since their acceleration has no transverse component. The radiation scattered by the molecule is therefore represented by dots. It is linearly polarized perpendicular to the plane of the figure. If the unpolarised light is incident on a Polaroid the intensity is reduced by half. Even if the Polaroid is rotated by angle θ the average over cos θ = ½. Thus from Malus law: I = I 0 cos θ Or, <I> = < I 0 cos θ > = I 0 < cos θ > = I 0 / Thus, the intensity of the transmitted light remains unchanged when the Polaroid is rotated.. Write Einstein s photoelectric equation and point out any two characteristic properties of photons on which this equation is based. Briefly explain the three observed features which can be explained by this equation. 3 Einstein s photoelectric effect equation: (NOTE: Use any one of three given equations) hv K. E o where, h Planck's constant, v frequency of the incoming photons o work function of the material and K.E kinetic energy of the emitted electrons OR
Class XII_All India_Physics_Set- 1 hv o mv max where, h Planck's constant, v frequency of the incoming photons 1 o work function of the material, mv max kinetic energy of the emitted electrons vmax maximum velocity of emitted electron OR 1 mv max h ( v vo ) where h Planck's constant, v frequency of the incoming photons 1 vo threshold frequency, mv max kinetic energy of the emitted electrons vmax maximum velocity of emitted electron The two characteristic properties of photons on which this equation is based are as follows: (i) Photons have particle characteristic. It is emitted or absorbed in units called quanta of light. (ii) Photons have wave characteristic. It travels in space with particular frequency, a characteristic of waves. Three observed features which can be explained by this equation are: (i) Solar cells: Also called photo-voltaic cells. It converts solar radiations to electrical emf. (ii) Television telecast: The dark and bright light part of images are interpreted as high and low electrical charges as given by photoelectric emission principle. These are further processed and transmitted. (iii) Burglar alarm: The moment the ultraviolet radiation is cut due to thief, it stops the supply of photons and thus works as off mode and the ring bells automatically. 3. Name the type of waves which are used for line of sight (LOS) communication. What is the range of their frequencies? A transmitting antenna at the top of a tower has a height of 45 m and the height of the receiving antenna is 80 m. Calculate the maximum distance between them for satisfactory communication in LOS mode. (Radius of the Earth = 6.4 10 6 m) 3 Space wave are used for the line of sight (LOS) communication. The range of their frequencies is 40 MHz and above. We have, height of transmitting antenna, h T = 45 m and height of receiving antenna, h R = 80 m
Class XII_All India_Physics_Set- Then, maximum distance between the two antennas, d m Rh T+ Rh R d m 6 6 3 3 6.4 10 45 6.4 10 80 = 4 10 3 10 56 km Thus, the maximum distance between the antennas is 56 km. 4. (a) For a given a.c., i = i m sin t, show that the average power dissipated in a (b) resistor R over a complete cycle is 1 im A light bulb is rated at 10 W for a 0 V a.c. supply. Calculate the resistance of the bulb. 3 R. (a) The average power dissipated p i R i Rsin t i R sin t m m 1 sin t (1 cos t) 1 1 1 p imr sin t 1 cos t ( cos t 0) (b) Power of the bulb, P = 10 W Voltage, V = 40 V The resistance of the bulb is given as V 40 R 480 P 10
Class XII_All India_Physics_Set- 5. Draw a labelled ray diagram of a refracting telescope. Define its magnifying power and write the expression for it. Write two important limitations of a refracting telescope over a reflecting type telescope. 3 Refracting telescope: Magnifying Power: The magnifying power m is the ratio of the angle α subtended at the eye by the final image to the angle β which the object subtends at the lens or the eye. h fo fo m. fe h fe Limitations of refracting telescope over reflecting type telescope: (NOTE: Write any two) (i) Refracting telescope suffers from chromatic aberration as it uses large sized lenses. (ii) The image formed by refracting telescope is less brighter than the image formed by the reflecting type telescope due to some loss of light by reflection at the lens and by absorption. (iii) The resolving power of refracting telescope is less than the resolving power of reflecting type telescope as the mirror of reflecting type telescope has large diameter. (iv) The requirements of big lenses tend to be very heavy and therefore difficult to make and support by their edges. (v) It is also difficult and expensive to make such large sized lenses.