CBSE Sample Papers for Class 12 Physics – CBSE Physics Questions and Answers
1. What is the function of a ‘Repeater’ used in communication system?
A repeater is an amplifying device used to increase the range of the transmission in communication systems with the help of a set of receiver and transmitter.
2. What is ground wave communication? Explain why this mode cannot be used for long distance communication using high frequencies.
Ground waves are the radio waves that travel along the surface of the Earth. The propagation of the wave is guided along the Earth’s surface and follows the curvature of the Earth. The propagation of high frequency wave is not possible through ground waves for long distance communication because while progressing, ground waves induce current in the ground and bend round the corner of the objects on the Earth due to which the energy of the ground waves of high frequency is almost absorbed by the surface of the Earth after travelling a small distance. Thus, ground wave communication is not suited for high frequency.
3. (a) What are the three basic units in communication systems?
Write briefly the function of each of these,
(b) Write any three applications of the Internet used in communication systems.
(a) A communication system consists of three basic units:
(i) Transmitter: This unit is used for transmitting the information after modifying it to a suitable form. It basically consists of a transducer that converts signal in any physical form to electrical signal for transmission. After that, the signal is modulated to transmit over long distances.
(ii) Communication channel: This unit carries the modulated signal from the transmitter to the receiver. E.g free space, transmission lines acts as a communication channel.
(iii) Receiver: This receives the signal, followed by a demodulator, an amplifier and a transducer. The demodulator demodulates the modulated signal, the amplifier boosts up its intensity and the transducer converts it back again electrical form to the needed physical form.
(b) The list of uses of Internet is given below.
(i) E-banking: It is an electronic payment system that enables customers of a financial institution (usualy a bank) to proceed for financial transactions on a website operated by that institution. To bank online with an institution, it is required that the customer is a member of that institution and has the access of Internet.
(ii) Internet surfing: Navigation over World Wide Web (www) from one webpage/website to another is called Internet surfing. It is an interesting way of searching and viewing information on any topic of interest.
(iii) E-shopping (E-commerce): It is a form of electronic commerce, by which the consumers can buy the products or the services over Internet. Apart from the above internet is used for sending e-mail, e-ticketing, etc.
4. State the underlying principle of a potentiometer. Write two factors by which current sensitivity of a potentiometer can be increased. Why is a potentiometer preferred over a voltmeter for measurirg the emf of a cell?
(a) Principle of potentiometer: When a steady current flows through the potentiometer wire then, the potential difference across the uniform wire is directly proportional to the length of the part across which the potential is measured.
(b) Current sensitivity can be increased by
(i) increasing the length of the wire
(ii) decreasing the current in the wire using a rheostat
(c) Potentiometer is preferred over voltmeter because it measures accurate emf of the cell. It uses null method, so no current is drawn by the galvanometer from the cell in balanced condition of otentiometer and a voltmeter measures the voltage across the terminals of a cell when the cell is in closed circuit. This voltage is called terminal voltage of a cell not Emf.
5. A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens?
When a concave lens made up of certain material is placed in a medium of refractive index less than the refractive index of the material of lens it behaves as a diverging lens and when it is placed in a medium of refractive index greater than the refractive index of the material of lens it behaves as a converging lens. So, if a concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65 it will behave as a converging lens.
6. Name the parts of the electromagnetic spectrum which is
(a) suitable for radar systems used in aircraft navigation
(b) used to treat muscular strain
(c) used as a diagnostic tool in medicine.
Write the brief, how these waves can be produced.
(a) Microwaves are used in RADAR system for aircraft navigation. These are Produced by special vacuum tubes, namely Klystrons, magnetrons.
(b) Infrared waves are used to treat muscular strain. These are produced by hot bodies and molecules.
(c) X rays are used as a diagnostic tool in medicine. These rays are produced when high energy electrons are stopped suddenly on a metal of high atomic mass.
7. A group of students while coming from the school noticed a box marked “Danger H.T. 2200 V” at a substation in the main street. They did not understand the utility of a such a high voltage, while they argued, the supply was only 220 V. They asked their teacher this question the next day. The teacher thought it to be an important question and therefore explained to the whole class.
Answer the following questions :
(i) What device is used to bring the high voltage down to low voltage of a.c. current and what is the
principle of its working?
(ii) Is it possible to use this device for bringing down the high dc voltage to the low voltage? Explain.
(iii) Write the values displayed by the students and the teacher.
(i) Transformer is used to bring the high voltage down to low voltage of a.c. current. It works on the principle of mutual induction. When current in one circuit changes, emf is produced in the neighbouring circuit.
(ii) The transformer cannot convert d.c. voltages because it works on the principle of mutual induction. When the current linked with the primary coil changes, the magnetic flux linked with the secondary coil also changes. This change in flux induces emf in the secondary coil. If we apply a direct current to the primary coil, the current will remain constant. Thus, there is no mutual induction, and hence no emf is induced.
(iii) The value of gaining knowledge and curiosity about learning new things is being displayed by the students. The value of providing good education and undertaking the doubts of students has been displayed by the teacher.
8. Give the ratio of velocities of light rays of wavelengths 4000Å and 8000 Å in vacuum.
The velocity of light rays of different wavelengths in vacuum is same and hence the ratio of their velocities is 1.
Class 12th CBSE Physics Solved Sample Papers
9. How is the intensity of the central maximum affected, if the width of slit is doubled in a single slit diffraction experiment?
The intensity of the central maximum increased by four times.
10. What is the significance of the negative energy of the electron in the orbit?
The negative energy of the electron in the orbit signifies the fact that the nucleus and the electron form a bound system.
11. State two disadvantages of semiconductor devices.
1. The semiconductor devices are temperature sensitive. Even a small overheating may cause damage to them.
2. The semiconductor devices cannot handle as much power as ordinary vacuum tubes can do.
12. How are electromagnetic waves produced?
Electromagnetic waves are produced by accelerated charges. An oscillating charge produces an oscillating electric field in space, which produces an oscillating magnetic field. Both the fields regenerate each other as the wave propagates through the space.
13. Descirbe in brief about the sky wave propagation.
Long distance communication of frequencies (few MHz to 40 MHz) is done by means of reflection of these waves from ionosphere of earth. This is sky wave propagation and is used for short wave broad -cast services. The density of ionised particle in ionosphere varies with height. There is a layer where ionised particle density is maximum (~300 km) which reflects these high frequency waves.
14. A current is set up in a long copper pipe. Is there a magnetic field (a) inside (b) outside the pipe?
Inside the pipe, the magnetic field is zero, but there is a finite value of the magnetic field outside the pipe.
15. A train is moving with uniform speed from north to south. Will any induced e.m.f. appear across the ends of its axle?
Yes, it will appear as the train is intercepting vertical component of earth’s magnetic field.
16. What evidence is there that sound wave is not an electromagnetic wave?
Electromagnetic waves can travel through vacuum while the sound waves cannot.
Cbse Class 12 Physics Sample Paper 2018 Questions and Answers
17. When can a charge act as a source of electromagnetic waves? How are the directions of electric and magnetic field vectors, in an electromagnetic wave related to each other and to the direction of propagation of the wave?
An accelerated or retarded charge or an oscillating LC circuit can be a source of electromagnetic waves. Electric and magnetic field vectors are perpendicular to each other and perpendicular to the direction of propagation of the wave.
The speed of all electromagnetic waves is same in vacuum.
18. On what factors, does the ground wave propagation depend?
The ground wave propagation depends on
(i) the frequency of radio waves and
(ii) Power of transmitter.
19. Is the electrostatic potential necessarily zero at a point, where the electric field strength is zero? Give an example to illustrate your answer.
If the electric field strength is zero at a point, the electrostatic potential is not necessarily zero at that point.
For example, at a point midway between two equal and similar charges, the electric field strength is zero, but the electrostatic potential is twice that due to a single charge.
20. (a) What is the significance of modulation index? (b) What are T.V. signals? How can they be received?
(a) Modulation index determines the strength and quality of transmitted signal. Higher modulation index ensures better quality and better strength.
(b) T.V. signals are those signals which have frequency range 80 to 200 MHz. Since these waves neither follow curvature of earth, nor get reflected by ionosphere their reception is possible by using tall antennas for direct transmission /reception or by geostationary satellite communication.
21. Ms. Preeti and her mother live in Delhi. One day she went to the market with her mother and decided to come back home by metro. At metro station they were made to pass through a gate way for security check. Preeti passed through it and was waiting for her mother to come. She heard a long beep when her mother passed through metal detector. Preeti was confused why metal detector beeped in case of her mother. She asked the duty staff, who explained her in detail. Both were satisfied with the security system.
(i) What values are displayed by Preeti?
(ii) Name the components present in the metal detector.
(iii) Write the Principle on which a metal detector works.
(i) Curiosity to know the reason, care for her mother.
(ii) Inductor and capacitor because resonance is exhibited only if both L and C are present in the circuit.
22. What is the cause of displacement current?
The charging electric field between the plates of a capacitor causes displacement current.
23. If the frequency of incident light is double, what will be the k.E. of the electrons emitted?
K.E. will be more than doubled
24. Which property of a p–n junction is used in rectification of a.c. voltages?
The forward bias resistance is low as compared to reverse bias resistance. This property is used in rectification.
25. What is role of sky wave propagation in transmission of high frequency signals? Explain.
The electromagnetic waves of frequency up to 30 MHz get reflected by the ionosphere. However, when the frequency of electromagnetic waves is above 40 MHz, they are no longer reflected by the ionosphere but undergo refraction.
Therefore, high frequency signals (in the frequency range from a few MHz to 30 MHz) are transmitted via reflection from the ionosphere. It is called sky wave propagation.
Whereas the transmission of signal by surface wave propagation is limited to a distance of 100 km or so, sky wave propagation enables to transmit the signal practically all over the surface of the earth.