PHYSICS (Theory) Time Allowed: 3 hours Maximum Marks: 70

Transcription

1 Sample Paper (CBSE) Series PHY/SP/1B Code No. SP/1-B PHYSICS (Theory) Time Allowed: hours Maximum Marks: 70 General Instructions: (i) (ii) (iii) (iv) (v) All questions are compulsory. This question paper has five sections: Section A, Section B, Section C, Section D and Section E. Section A contains five questions of one mark each, Section B contains five questions of two marks each, Section C contains twelve questions of three marks each, Section D contains one value based question of four marks and Section E contains three questions of five marks each. There is no overall choice. However, an internal choice has been provided in one question of two marks, one question of three marks and all three questions of five marks each weightage. You have to attempt only one of the choices in such questions. You may use the following values of physical constants wherever necessary: c = 10 8 m/s h = Js e = C μo = T ma o = N m 2 C -2 me = kg Mass of Neutrons = kg Mass of proton = kg Avogadro s number = per gram mole Boltzmann constant = JK -1

2 Section A 1. How can a galvanometer be converted into voltmeter? 1 2. What happens to then drift velocity of electrons, if length of a conductor is doubled keeping the potential difference constant? 1. The electric field part of an electromagnetic wave in a medium is represented by Calculate the wavelength of the wave What is the rest mass of photon? 1 5. Which type of biasing gives a semiconductor diode very high resistance? 1 Section B 6. Draw a plot of the binding energy per nucleon as a function of mass number for large number of nuclei, 2 A 240. How do you explain the constancy of binding energy per nucleon in the range 0 A 170 using the property that nuclear force is short ranged? 2 7. Give the direction in which the induced current flows in the coil mounted on an insulating stand, when a bar magnet is

3 quickly moved along the axis of the coil from one side to the other as shown in the figure Draw the graph showing the variation of reactance of a capacitor and an inductor with the frequency of an a.c. circuit An object is kept in front of a concave mirror of focal length 15 cm. The image formed is three times the size of the object. Find two possible distances of the object from the mirror. 2 OR What is the polarizing angle of a medium of refractive index In a meter bridge, the balance point is found to be at 9.5 cm from the end A, when the resistor Y is of 12.5 ohm as shown in figure. Determine the resistance of X. What happens if the galvanometer and cell are interchanged at the balance point of the bridge?

4 2 Section C 11. a. Explain, whether an induced current will be developed in a conductor, if it is moved in a direction parallel to magnetic field. b. An electron in an atom revolves around the nucleus in an orbit of radius 0.5 Å. Calculate the equivalent magnetic moment, if the frequency of revolution of electron is MHz. 12. a. Two charges of magnitudes + 4Q and Q are located at points (a, 0) and ( a, 0) respectively. What is the electric flux due to these charges through a sphere of radius '2a' with its centre at the origin? b. Two point electric charges of unknown magnitude and sign are placed at a distance apart. The electric field intensity is zero at a point not between the charges but on the line joining them. Write two essential conditions for this to happen. 1. a. How is the focal length of a spherical mirror affected when

5 the wavelength of the light used is increased? b. If ε0 and μ0 are the electric permittivity and magnetic permeability in free space, ε and μ are the corresponding quantities in a medium, then calculate the index of refraction of the medium. 14. a. Define input resistance of a transistor used in its common emitter configuration. b. Draw the truth table of the following combination of gates: 15. By what percentage will the transmission range of a TV tower be affected, when the height of the tower is increased by 21 %? 16. Two cells of e.m.f. 1.5 V and 2 V and internal resistances 1 Ω and 2 Ω respectively are connected in parallel so as to send current in the same direction through an external resistance of 5 Ω. Draw the circuit diagram. Using Kirchhoff s laws, calculate the current through each branch of the circuit and potential across the 5 Ω resistance. 17. Derive an expression for the average power consumed in a series LCR circuit connected to a.c. source in which the phase difference between voltage and current in the circuit is φ.

6 Define the quality factor in an a.c. circuit. 18. A convex lens made up of glass of refractive index 1.5 is dipped, in turn, in a. medium A of refractive index 1.65 b. medium B of refractive index 1.. Explain giving reason whether it will behave as a converging lens or a diverging lens in each of these two media. OR What is the effect on the interference fringes in a Young s double slit experiment due to each of the following operations: a. The screen is moved away from the plane of the slits b. The separation between the two slits is decreased c. The monochromatic source is replaced by a source of white light. 19. Define resolving power of a compound microscope. How does it change on: a. decreasing the wavelength of light used and b. decreasing the diameter of its object lens? 20. Figure shows the variation of stopping potential V0 with the frequency of incident radiation for two photosensitive metals A and B.

7 a. Which of the metals has larger threshold wavelength? Give reason. b. Which metal gives out electrons having larger kinetic energy for the same wavelength of incident radiation? Explain why? c. If the distance between the light source and metal A is halved, how will the kinetic energy of electrons emitted change? Why? 21. Explain briefly, with the help of circuit diagram, how a p-n junction diode work as half wave rectifier? 22. Draw a simple demodulation circuit and explain its working. Section D 2. Muthuswami a resident of Kundakulam was all set to leave everything and shift to another place in view of the decision of Govt. to start nuclear thermal power plant at Kundakulam. His granddaughter Prachi, a science student was really upset on the ignorant decision of her grandfather. She could finally convince him not to shift, since adequate safety measures to

8 avoid any nuclear mishap have already been taken by the Govt. before starting nuclear thermal plants. a. What is the value displayed by Prachi in convincing her grandfather? b. What is the principle behind working of nuclear reactor? c. Why is heavy water used as moderator? 4 Section E 24. a. A parallel plate capacitor of capacitance C is charged to a potential difference V and then the battery is disconnected. Now a dielectric slab of the dimensions equal to the spacing between the plates is inserted between the plates. What are the changes, in the capacitance, charges, potential difference, electric field and the energy stored? b. A parallel plate capacitor of capacitance C is charged to a potential V. It is then connected to another uncharged capacitor having the same capacitance. Find out the ratio of the energy stored in the combined system to that stored initially in the single capacitor. 5 OR a. Derive an expression for the maximum torque acting on an electric dipole, when held in a uniform electric field. b. Consider two hollow concentric spheres, S1 and S2, enclosing charges 2Q and 4Q respectively as shown in the figure.

9 1. Find out the ratio of the electric flux through them. 2. How will the electric flux through the sphere S1 change if a medium of dielectric constant 'εr' is introduced in the space inside S1 in place of air? Deduce the necessary expression State Biot-Savart law and give the mathematical expression for it. Use this law to derive the expression for the magnetic field due to a circular coil carrying current at a point along its axis. How does a circular loop carrying current behave as a magnet? 5 OR a. Draw a labelled diagram of a moving coil galvanometer. Describe briefly its principle and working. b. Answer the following : 1. Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer? 2. Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason. 5

10 26. a. With the help of ray diagram, explain the phenomenon of total internal reflection. b. Obtain lens makers formula using the expression Here the ray of light propagating from a rarer medium of refractive index to a denser medium of refractive index is incident on the convex side of spherical refracting surface of radius of curvature R. 5 OR a. Deduce the expression for the refractive index of glass in terms of angle of prism and angle of minimum deviation. b. Out of blue and red light, which is deviated more by a prism? Give reason. 5

11 ANSWERS 1. By connecting a suitable high resistance in series to the coil of a galvanometer. 2. The drift velocity will become half of the initial value and resistance will become double m 4. Zero 5. Reverse biasing 6. The constancy of binding energy per nucleon in the range 0 A 170 shows that a nucleon in the nucleus is not attracted by all the remaining nucleons. It is attracted only by its some close neighbours and hence the nuclear force is short ranged.

12 7. a. When magnet moves towards the coil, the current flows in clockwise direction and b. When magnet moves away from the coil, the current flows in anticlockwise direction a. Use the relations: b. For real image object distance is 20 cm and for virtual image object distance is 10 cm. Use the relation OR We get i = 60

13 10. Use the relation: We get, X = 8.16 ohm If the galvanometer and cell are interchanged at the balance point of the bridge, the galvanometer will not show any deflection as the condition for a balanced bridge is still satisfied. 11. a. Since Lorentz force on the free electrons in the conductor is zero, no potential difference is produced across the ends of the conductor and hence no induced current will be developed. b. Using the relations: We get, M = Am a. b. Both the charges are of opposite sign or nature. The point where electric field intensity is zero is closer to the smaller charge. 1. a. No change in the focal length of a spherical mirror on increasing the wavelength of light used. b. The index of refraction of the medium is

14 14. a. The input resistance of a transistor used in its common emitter configuration is defined as the ratio of small change in base-emitter voltage to the small change produced in base current at constant collector-emitter voltage. b. A B y y Y Use the formula: The percentage increase in the transmission range of the TV tower is 10 %. 16. a. I1 = , I2 = and b. Potential difference across R = V

15 17. a. The average power consumed in a series LCR circuit connected to a.c. source in which the phase difference between voltage and current in the circuit is φ is given by b. The quality factor of a resonant LCR-circuit is defined as the ratio of the voltage drop across inductor to the applied voltage. 18. Use the formula: a. In medium A lens will behave as a diverging lens. b. In medium B lens will behave as a converging lens. OR a. Fringe width will increase as D increases. b. Fringe width will increase as d decreases. c. The colours of white light will produce interference pattern. The central bright fringe is white in colour and the fringes closest on either side of it is blue and farthest is red. 19. Resolving power of a microscope is defined as the reciprocal of the least separation between two close objects so that they appear just separated when seen through the microscope. a. Resolving power of a microscope will increase on decreasing the wavelength of light.

16 b. Resolving power of a microscope will decrease on decreasing the diameter of its object lens. 20. a. The metal A has larger threshold wavelength as it has smaller threshold frequency. b. The metal A gives out electrons having larger kinetic energy for the same wavelength of incident radiation because it has smaller threshold frequency and hence smaller work function. c. If the distance between the light source and metal A is halved, the kinetic energy of electrons emitted will remain unchanged because the change in intensity of the incident radiation does not affect the kinetic energy of the emitted electrons. 21.

17 When an alternating voltage is applied across a diode in series with a load resistance, a pulsating output voltage appears across the load only during the half cycles of the a.c. when the diode is under forward bias. The corresponding circuit is called a halfwave rectifier circuit as shown in the image. When the secondary of the transformer supplies a.c. voltage across the terminals A and B, then in the positive half-cycle of a.c. there is a current through the load RL when the diode is under forward bias. In the negative half-cycle of a.c. there is no current through the load RL when the diode is under reverse bias. The output voltage is in the form of varying pulses restricted to only one direction. 22. Working of Demodulation Circuit: The current flows through the diode only during the positive half cycle of the AM wave. The envelope of the rectified wave is the information signal (fm). The

18 signal fm is separated from the carrier wave using the RC filter and given to an amplifier. The output voltage of amplifier is connected to a loud speaker which produces the original sound waves. 2. a. Awareness, social responsibility b. Controlled fission chain reaction c. Ordinary water absorb neutron at a fast rate. To overcome this difficulty, Heavy water is used as a moderator which has negligible cross sections for neutron absorption. 24. a. The capacitance increases, charge remains the same, potential difference decreases, electric field decreases and energy stored in the capacitor also decreases. b. Using the formula: We get, the ratio of the energy stored in the combined system to that stored initially in the single capacitor as 1:2.

19 OR a. b. 1. Using Gauss theorem, 2. If S1 is filled with medium of dielectric constant 'εr', then 25. Biot Savart law states that the magnetic field produced due to a small conductor of length dl carrying a current I at a point at distance r is given by

20 Where k is the constant of proportionality and θ is the angle between the current and the line joining the small conductor to the observation point. The magnetic field due to a circular coil carrying current at a point along its axis is given by, Where a is the radius of the circular coil, x is the distance from the centre of the coil, n is the number of turns in the coil and I is the current flowing in the coil. The magnetic field pattern produced by a circular loop is similar to that of a magnet. a. OR Principle: When a current carrying coil is placed in magnetic field, it experiences a torque.

21 b. 1. It is necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer to increase the sensitivity and hence magnetic field becomes radial. 2. We know that If the resistance of the coil is constant then voltage sensitivity is proportional to current sensitivity. But if resistance of the coil also increases then in that case on increasing the current sensitivity the voltage sensitivity remains constant. 26. a. b.

22 OR a. b. Blue light will suffer more deviation because the refractive index of blue light is more than red and hence the angle of deviation for blue is more.