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XII PHYSICS S.NO OPT ANSWER S.NO OPT ANSWER 1 d 10 1 C 27.93 MeV 2 c 98.425 MHz and 98.575 MHz 2 C -27.2 eV 3 b d,p,e 3 C 240 V , 120 V and 120 V 4 d small couple per unit twist 4 A its input current is zero 5 c 240 V , 120 V and 120 V 5 C 1 6 a AC only 6 C 98.425 MHz and 98.575 MHz 7 b 5 A rms current 7 A AC only 32 8 c P 8 B d,p,e 15 9 a is a constant 9 D quantum theory of light 10 b 5900 Ao and 5880 Ao 10 B 5900 Ao and 5880 Ao 11 d quantum theory of light 11 A isotope 12 c inside the sphere 12 A 1.732 13 c Ic will increase 13 D an atomic transition in the target 14 a isotope 14 A capacitive feedback is used 15 c -27.2 eV 15 C Ic will increase 16 c 1 16 B 2→1 17 b high frequency carrier waves 17 C 15 P 32 18 c isotones 18 A is a constant 19 a capacitive feedback is used 19 B high frequency carrier waves 20 b 2→1 20 D electric potential 21 c 27.93 MeV 21 D 10 22 c 100 Ω ± 2 % 22 D brushes 23 a Does not vary 23 A Sodium vapour lamp 24 b 9 x 10 4 NC -1 24 C inside the sphere 25 d electric potential 25 B 5 A rms current 26 a its input current is zero 26 D small couple per unit twist 27 d an atomic transition in the target 27 C 100 Ω ± 2 % 28 a 1.732 28 A Does not vary 29 d brushes 29 C isotones 30 a Sodium vapour lamp 30 B 9 x 10 4 NC -1 31 Electric Dipole & Dipole moment: Two equal and opposite charges separated by a very small distance constitute an electric dipole. The magnitude of the dipole moment is given by the product of the magnitude of the one of the charges and the distance between them. 32 The metal body of the bus provides electrostatic shielding, where the electric field is zero. During lightning the electric discharge passes through the body of the bus. 33 Ohm’s law: At a constant temperature, the steady current flowing through a conductor is directly proportional to the potential difference between the two ends of the conductor. 34
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First Law : The mass of a substance liberated at an electrode is directly proportional to the charge passing through the electrolyte.
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Second Law : The mass of a substance liberated at an electrode by a given amount of charge is proportional to the chemical equivalent of the substance. Characteristics of heating element: (1) It has high specific resistance (2) It has high melting point (3) It is not easily oxidized Fleming’s right hand rule: The forefinger, the middle finger and the thumb of the right hand are held in the three mutually perpendicular directions. If the forefinger points along the direction of the magnetic field and the thumb is along the direction of motion of the conductor, then the middle finger points in the direction of the induced current. This rule is also called generator rule.
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Uses of IR: (i) Infrared lamps are used in physiotherapy. (ii) Infrared photographs are used in weather forecasting. (iii) As infrared radiations are not absorbed by air, thick fog, mist etc, they are used to take photograph of long distance objects. (iv) Infra red absorption spectrum is used to study the molecular structure.
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Medical applications of X-rays: (i) X–rays are being widely used for detecting fractures, tumours, the presence of foreign matter like bullet etc., in the human body. (ii) X–rays are also used for the diagnosis of tuberculosis, stones in kidneys, gall bladder etc. (iii) Many types of skin diseases, malignant sores, cancer and tumours have been cured by controlled exposure of X-rays of suitable quality. (iv) Hard X–rays are used to destroy tumours very deep inside the body.
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Postulates of special theory of relativity: (i) The laws of Physics are the same in all inertial frames of reference. (ii) The velocity of light in free space is a constant in all the frames of reference. Curie: Curie is defined as the quantity of a radioactive substance which gives 3.7 × 1010 disintegrations per second or 3.7 × 1010 becquerel. This is equal to the activity of one gram of radium. Properties of neutrons: (i) Neutrons are the constituent particles of all nuclei, except hydrogen. (ii) Neutrons are neutral particles with no charge and mass slightly greater than that of protons. Hence,
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they are not deflected by electric and magnetic fields. (iii) Neutrons are stable inside the nucleus. But outside the nucleus they are unstable. The free neutron decays with an emission of proton, electron and antineutrino, with half life of 13 minutes. 0n1 → 1H1 + −1e0 + ν (iv) As neutrons are neutral, they can easily penetrate any nucleus. (v) Neutrons are classified according to their kinetic energy as (a) slow neutrons and (b) fast neutrons. Bandwidth of an amplifier: Bandwidth is defined as the frequency interval between lower cut off and upper cut off frequencies. Summing amplifier:
Intrinsic semiconductor: A semiconductor which is pure and contains no impurity is known as an intrinsic semiconductor. In an intrinsic semiconductor, the number of free electrons and holes are equal. Common examples of intrinsic semiconductors are pure germanium and silicon.
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Types of radio wave propagation: (i) Ground (surface) wave propagation (ii) Space wave propagation (iii) Sky wave (or) ionospheric propagation
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OR
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Properties of Cathode rays (i) They travel in straight lines. (ii) Cathode rays possess momentum and kinetic energy. (iii) Cathode rays produce heat, when allowed to fall on matter. (iv) Cathode rays produce fluorescence when they strike a number of crystals, minerals and salts. (v) When cathode rays strike a solid substance of large atomic weight, X-rays are produced. (vi) Cathode rays ionize the gas through which they pass. (vii) Cathode rays affect the photographic plates. (viii) The cathode rays are deflected from their straight line path by both electric and magnetic fields. The direction of deflection shows that they are negatively charged particles. (ix) Cathode rays travel with a velocity upto (1/10)th of the velocity of light. (x) Cathode rays comprises of electrons which are fundamental constituents of all atoms.
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Applications of photo electric cells (i) Photoelectric cells are used for reproducing sound in cinematography. (ii) They are used for controlling the temperature of furnaces. (iii) Photoelectric cells are used for automatic switching on and off the street lights. (iv) Photoelectric cells are used in the study of temperature and spectra of stars. (v) Photoelectric cells are also used in obtaining electrical energy from sunlight during space travel. (vi) These cells are used in instruments measuring light illumination. (vii) These cells are used in opening and closing of door automatically. (viii) Photoelectric cells are used in burglar alarm and fire alarm.
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Because of the high electric field near the comb D, the air gets ionised due to action of points, the negative charges in air move towards the needles and positive charges are repelled on towards the belt. These positive charges stick to the belt, moves up and reaches near the comb E. As a result of electrostatic induction, the comb E acquires negative charge and the sphere acquires positive charge. The acquired positive charge is distributed on the outer surface of the sphere. The high electric field at the comb E ionises the air. Hence, negative charges are repelled to the belt, neutralises the positive charge on the belt before the belt passes over the pulley. Hence the descending belt will be left uncharged. Thus the machine, continuously transfers the positive charge to the sphere. As a result, the potential of the sphere keeps increasing till it attains a limiting value (maximum). After this stage no more charge can be placed on the sphere, it starts leaking to the surrounding due to ionisation of the air. The leakage of charge from the sphere can be reduced by enclosing it in a gas filled steel chamber at a very high pressure. The high voltage produced in this generator can be used to accelerate positive ions (protons, deuterons) for the purpose of nuclear disintegration. 64
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pulse of red light of wavelength 6943 Å emerges through the partially silvered end of the crystal.
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When an ionising radiation enters the counter, primary ionisation takes place and a few ions are produced. These ions are accelerated with greater energy due to the high potential difference and they cause further ionisation and these ions are multiplied by further collisions. Thus an avalanche of electrons is produced in a short interval of time. This avalanche of electrons on reaching the anode generates a current pulse, which when passing through R develops a potential difference. This is amplified by electronic circuits and is used to operate an electronic counter. The counts in the counter is directly proportional to the intensity of the ionising radiation.
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The ionisation of the gas is independent of the type of the incident radiation. Hence, G.M. counter does not distinguish the type of radiation that enters the chamber. Feedback in amplifiers: Feedback is said to exist in an amplifier circuit, when a fraction of the output signal is returned or fed back to the input and combined with the input signal.
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Answer Key by R.MASILAMANI PG ASST IN PHYSICS K. KAMARAJAR MAT HR SEC SCHOOL SIVAKASI
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