ISC Class 12th Physics - Paper 1 Exam 2024 : Most Important Very Short Answer Type Questions for Last-Minute Revision

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ISC Class 12 exams have started and you have very little time left for the Physics - Paper 1 exam. Therefore, we are providing Important Very Short Answer Type Questions in this article. You can study them well and score well in your exams.
To help you prepare effectively, we have prepared a list of Important Very Short Answer Type Questions with detailed explanations. This guide aims to make your exam preparation easier by providing information on these questions.
Important Very Short Answer Type Questions are a common format in competitive exams including ISC Class 12 Physics - Paper 1 exam. By practicing "Very Short Answer Type Questions", you not only reinforce your knowledge but also learn to apply it effectively.
These questions cover various topics from the Physics - Paper 1 syllabus. Remember, it is important to understand the logic behind each answer to score well.
ISC Class 12 Physics Very Short Answer Type Questions
Q1. A negatively charged rod is brought close to some small pieces of neutral paper. The positive sides of molecules in the paper are attracted to the rod and the negative sides of the molecules are repelled. Since negative and positive sides are equal in number, why don’t the attractive and repulsive forces cancel out ?
Ans. The positive sides are simply closer to the rod. In accordance to Coulomb’s law, they experience a greater electrical force than the farther-away negative sides. Hence we can say that the closeness wins. This greater force between positive and negative is attractive, so the neutral paper is attracted to the charged rod.
Q2. Will a balloon that is charged either positively or negatively stick to a neutral wall ?
Ans. Yes, because the charge on the balloon induces an opposite charge on the surface of the wall.
Q3. Why insulators are called dielectric and conductors are called non-electric ?
Ans. When insulators are rubbed, they are able to retain the charge placed on them, so they are called dielectric. But in conductors, there is absence of charge as they immediately drain away the charge placed on them, so they are called non-electric.
Q4. What is a conservative field ? Give two examples.
Ans. A conservative field is a field in which the work done on an object by the force due to the field depends only on the initial and final positions of the objects and not on the actual path followed by the object. Gravitational field and electrostatic field are the examples of a conservative field.
Q5. Is electric field a scalar or a vector quantity ?
Ans. It is a vector quantity. Its direction is same as the direction of force.
Q6. What is the locus of an electron, projected perpendicular to a uniform electric field ? (Neglect the effect of gravity)
Ans. Parabola.
Q7. Write the S.I. unit of electric dipole moment.
Ans. Coulomb-metre
Q8. An electric dipole is kept parallel to a ‘non-uniform’ electric field. What will be the motion of the dipole ?
Ans. A resultant force will act on the dipole due to which it will move in the direction of force.
Q9. An electric dipole is kept perpendicular to a ‘non-uniform’ electric field. What will be the motion of the dipole ?
Ans. A torque as well as force will act on the dipole due to which there will be both translatory and rotatory motions in it.
Q10. What is the use of Gaussian surface ?
Ans. By a Gaussian surface, we can easily find the electric field produced by certain sources which are otherwise quite difficult to find by application of coulomb’s law or the principle of superposition.
Q11. What is meant by an equipotential surface ?
Ans. The surface of locus of all points at the same potential is known as equipotential surface.
Q12. Define electric potential. Is it a vector or scalar quantity ?
Ans. The potential at a point in an electric field may be defined as the work done in moving a unit positive test charge from infinity to that point in an electric field. It is a scalar quantity.
Q13. Equipotential surface is perpendicular to the field lines. Why ?
Ans. No work is done in moving a charge from one point to other on an equipotential surface. This indicates that the component of electric field along the equipotential surface is zero. Hence, the equipotential surface is perpendicular to the field lines.
Q14. How much work is done in taking an electron around a nucleus in a circular path ?
Ans. Work done, W = 0.
Q15. A bird perches on a bare high power line and nothing happens to the bird. A man standing on the ground touches the same line and gets a fatal shock. Why ?
Ans. Bird’s whole body is at the same potential, therefore no charge flows since circuits is not completed and no shock is produced in it while man touching the ground maintains a potential difference between different parts of his hand and feet. A large current flows, which electrolyses the blood and causes fatal shock or death.
Q16. In an electric dipole, what is the locus of a point of zero potential ?
Ans. Equatorial line.
ISC Class 12 Study Material
Q17. What is an electric dipole moment ? What is the potential due to an electric dipole at a point in broad side on position ?
Ans. The strength of the dipole is called the dipole moment whose magnitude is equal to the product of the charge q and the separation 2l between the two charges.
The potential due to an electric dipole at a point in broad side on position is zero.
Q18. What will be the nature of the electric potential at a place, where the electric field E = 0 ?
Ans. The place where E = 0, is an equipotential surface where electric potential can be zero or remain constant throughout.
Q19. The electric potential is zero at a point in an electric field. Will the electric field be necessarily zero at that point ?
Ans. No, zero potential at a point does not necessarily mean that the electric field is zero at that point. At a point on the equatorial line of a dipole, the potential is zero, but the electric field is not certainly zero.
Q20. If the electric field intensity is zero at a point, will the electric potential be necessarily zero at that point ?
Ans. No, at a point midway between two equal and similar charges, the electric field intensity is zero but the electric potential is not zero.
Q21. What will happen if the plates of a charged capacitor are suddenly connected by metallic wire ?
Ans. The capacitor will be discharged immediately and its energy would be converted into heat energy.
Q22. The distance between the plates of a parallel plate capacitor is d. A metal plate of thickness d/2 is placed between the plates. What will be its effect on the capacitance ?
Ans. After introducing the metal plate of thickness d/2, the arrangement is equivalent to two capacitors connected in series. The distance between the plates of each capacitor is d/4. Hence, the capacitance will be doubled.
Q23. What is energy density ? What is its S.I. unit ? Give its formula.
Ans. Energy density is defined as the energy stored per unit volume of the medium between the plates of the capacitor. Its S.I. unit is J m–3.
Energy density = 1/2e0E2
where E is the electric field intensity and e0 the electric permittivity of the free space.
Q24. Discuss how capacitance is affected when :
(i) a dielectric slab is introduced.
(ii) a conducting slab is inserted between the plates of a parallel plate capacitor. Assume the slab thickness to be one-half of the plate separation.
Ans.
(i) On introducing dielectric slab, the capacitance increases.
(ii) On introducing conducting slab, the capacitance increases.
Q25. A capacitor is charged by using a battery, which is then disconnected. A dielectric slab is then slipped between the plates. Describe qualitatively what happens to the charge, capacitance, the potential difference, the electric field strength and the stored energy.
Ans.
(a) Charge will remain same.
(b) Capacitance will increase.
(c) Potential difference will decrease.
(d) Electric field will decrease.
(e) Energy stored will decrease.
Q26. When a capacitor remains connected to a battery, a dielectric slab is slipped between the plates. Describe qualitatively what happens to the charge, the capacitance, the potential difference, the electric field strength and the stored energy.
Ans.
(a) Charge will increase.
(b) Capacitance will increase.
(c) Potential difference will remain constant.
(d) Electric field strength will remain constant.
(e) Energy will increase.
Q27. The free electrons of a metal can move freely within the metal but cannot come out of its surface. why ?
Ans. Free electrons in metals cannot come out due to the attraction of positive ions present in the metal.
Q28. What is relaxation time of free electrons in metals ?
Ans. Relaxation time is the average time interval between two successive collisions.
Q29. What is the cause of resistance of a conductor ?
Ans. While drifting, the free electrons collide with the ions and atoms of the conductor, i.e., motion of the electrons is opposed due to the collisions. This is the basic cause of resistance in a conductor.
Q30. What is the order of the drift velocity of electrons ?
Ans. 10–6 m/s.
Q31. What do you understand by average relaxation time ?
Ans. It is defined as the time for which an electron moves freely in between two collisions (with conductor atoms). It is denoted by symbol t (Tau).
Q32. A current flowing in a copper wire is sent in another copper wire of double the radius. How will the drift velocity of free electrons be affected ?
Ans. vd (= i/neA) will be reduced to one-fourth.
Q33. What causes an electric current ?
Ans. An electric current flows between two points only when there is a potential difference between them. The greater the potential difference between them, the greater is the flow of electric current, i.e.,
i ∝ V
Q34. What is the direction of flow of current ? How can the current be kept continuous ?
Ans. The conventional direction of electric current is taken as opposite to the direction of flow of electrons. Current can be kept continuous by maintaining constant potential difference between two points. This is done by connecting an electric cell or battery between the two points.
Q35. Name the carriers of electric current in :
(i) a metallic rod,
(ii) an electrolytic cell,
(iii) a hydrogen discharge tube,
(iv) a semiconductor,
(v) a superconductor.
Ans.
(i) Free electrons.
(ii) Positive and negative ions of the electrolyte.
(iii) Electrons and protons.
(iv) Electrons and holes.
(v) Electron pairs.
Q36. Although the drift velocity of electrons is very small, yet an electric bulb lights up almost instantly when switched on, whatever be the distance of the bulb from the switch. Why ?
Ans. As soon as the switch is made on, an electric impulse is transmitted through the circuit with the speed of light (3 × 108 ms–1). Thus an electric field is established in the circuit instantly and electrons start drifting at every point in the circuit at the same time. Hence, electric current begins to flow almost instantly in the circuit.
Q37. State the conditions under which ohm’s law is not obeyed in a conductor.
Ans. (a) When temperature changes. (b) When the conductor is under any strain.
Q38. What is the main cause of death due to electric shock ?
Ans. The main cause of death due to electric shock is electrolysis of blood which takes place even with very small currents.
Q39. What do you mean by linear resistor ?
Ans. A linear resistor is a resistor whose resistance does not change with the variation of current flowing through it.
Q40. A steady current is flowing in a cylindrical conductor. Is there any electric field within the conductor ?
Ans. Yes, because current flows in a conductor only when the electric field established within the conductor exerts force on the free electrons.
Q41. What are the different effects of electric current ?
Ans. Electric current has three main effects :
(a) Heating effect.
(b) Chemical effect.
(c) Magnetic effect.
Q42. What are the physical conditions which must be constant in Ohm’s law ?
Ans. Physical conditions which must be constant in Ohm’s law are :
(a) Area of cross-section of wire.
(b) Length of the wire.
(c) Temperature of the wire.
(d) Pressure.
(e) No stress.
(f) No strain.
Q43. Explain the difference between ohmic and non-ohmic resistances. Give examples.
Ans. If temperature and other physical states of a conductor remains unchanged, the resistance of the conductor remains constant whatever potential difference is applied between the ends of the conductor, then it is called ‘ohmic’ resistance, e.g. metallic wires, mercury. If the resistance changes with change in the potential difference, then it is called ‘non-ohmic’ resistance, e.g. diode valve, thermistor.
Q44. Why does the resistance of a metal increase with increase in temperature ?
Ans. If the temperature of the metal is raised, the atoms vibrate more strongly and the electrons make more violent collisions with them and the resistance of the metal increases.
Q45. Write down the metals silver, aluminium, copper and iron in increasing order of their electrical conductivity.
Ans. Iron, aluminium, copper, silver.
Q46. Name two physical conditions on which the resistivity of a metal depends.
Ans. Two physical conditions are :
(a) Temperature.
(b) Strain.
Q47. What is the effect of rise in temperature on the electrical conductivity of a (i) metal, (ii) semiconductor, (iii) electrolyte ?
Ans. (a) It decreases. (b) It increases. (c) It increases.
Q48. Which metal has the lowest resistivity ?
Ans. Silver.
Q49. Give reasons why the electrical conductance of electrolytes is less than that of metals ?
Ans. This is because charge carriers in electrolytes, i.e., ions have less mobility as compared to free electrons in metals.
Q50. The radius of a copper wire is doubled what will be its new specific resistance ?
Ans. It remains unchanged because specific resistance depends only on the material of a conductor at a given temperature.
Q51. Does the relation V = IR apply to non-ohmic resistors ?
Ans. Yes, because the formula V = IR defines resistance, not the Ohm’s law.
Q52. Is the filament resistance lower or higher in a 500 watt light bulb than in a 100 watt bulb? Both bulbs are designed to operate on 110 volt.
Ans. As the potential difference across each bulb is constant so, resistance is inversely proportional to the power therefore, 500 watt bulb will have low resistance.
Q53. What is kilowatt hours (kWh) ? What is it commonly known as ?
Ans. It is the unit of electrical energy. 1 kWh is commonly known as 1 unit.
Q54. Why an ammeter is put in series while the voltmeter is connected in parallel with the rest of the circuit ?
Ans. Ammeter is always connected in series because current in series remains the same. Voltmeter is connected in parallel because potential difference in the parallel circuit remains constant.
Q55. When a high power heater is connected to electric mains, the bulbs lightening in the house becomes dim. Why ?
Ans. The heater draws a heavy current from the mains and so there is an appreciable potential drop in the line. As a result, the potential difference across the bulbs fall and they become dim.
Q56. Under what condition the e.m.f. of a cell will be equal to its potential difference ?
Ans. When the cell is in open circuit condition.
Q57. Define internal resistance of a cell.
Ans. The resistance offered by the electrolyte of a cell is called internal resistance of the cell.
Q58. What is the most important property of a standard cell ?
Ans. The most important property of a standard cell is to maintain a constant potential for a long period of time.
Q59. What is the resistance of an ideal voltmeter ?
Ans. Resistance of an ideal voltmeter is infinite.
Q60. On increasing the current drawn from a cell, the potential difference across cell terminals is further lowered. Why ?
Ans. This happens due to internal resistance r of the cell. Because V = E – ir, as i is increased, V is lowered.
Q61. A 4 W resistance wire is bent in the middle by 180° and both the halves are twisted with each other. Find its new resistance.
Ans. On bending by 180°, the resistance of each part will be 2 W. On twisting these parts, they will be connected in parallel. Hence the new resistance will be 1 W.
Q62. Write the formula of equivalent e.m.f. and internal resistance of n cells of unequal e.m.f. connected in series.
Ans. Equivalent e.m.f., Eeq. = E1 + E2 + E3 + …… + En.
Equivalent internal resistance, req = r1 + r2 + …… + rn
Q63. Why is a current loop considered a magnetic dipole ?
Ans. A bar magnet (magnetic dipole) suspended in a uniform magnetic field experiences a torque and so it sets with its axis parallel to the field. A current loop also experiences a torque in a magnetic field due to which it sets with axis parallel to the field.
Q64. What is a Lorentz force ?
Ans. The force exerted by a magnetic field on a moving electric charge is called Lorentz force.
Q65. Give the position of a magnetic dipole held in a magnetic field, where its potential energy is minimum ?
Ans. When the dipole is placed parallel to the magnetic field, the dipole has minimum potential energy.
Q66. Why are the pole pieces of a horseshoe magnet in a moving coil galvanometer made cylindrical in shape ?
Ans. The pole pieces of a horseshoe magnet in a moving coil galvanometer are made cylindrical in shape to create a uniform radial magnetic field such that in every position, the plane of the coil remains parallel to the lines of force, i.e., in every position, q = 90°.
Q66. Which has a greater resistance, a galvanometer or a galvanometer changed into a voltmeter ?
Ans. A voltmeter (galvanometer changed into a voltmeter) has greater resistance than that of the galvanometer so a very high resistance is connected in series with the coil of the galvanometer.
G + R > G.
Q67. How do we connect a galvanometer, an ammeter and a voltmeter in a circuit ?
Ans.
(a) A galvanometer simply detects current and can be connected in the circuit (in series or parallel) wherever current is to be detected.
(b) An ammeter is connected in series.
(c) A voltmeter is connected in parallel.
Q68. What happens when we connect (i) an ammeter in parallel, and (ii) a voltmeter in series in a circuit ?
Ans. (i) If an ammeter is connected in parallel, it will draw almost the entire current as it has extremely low resistance; this large current may even burn the ammeter. (ii) a voltmeter has very large resistance. Hence, when connected in series, it will reduce the circuit current to almost zero.
Q69. What is the function of shunt in an ammeter ?
Ans. The shunt is a very small resistance connected in parallel to a galvanometer to convert it in ammeter and thus reduces the combined resistance of the ammeter. It allows very small current in the coil and thus prevents excessive heating of the coil.
Q70. What happens if a bar magnet is cut into two pieces : (i) perpendicular to its length. (ii) along its length.
Ans. (i) Each piece is a magnet with north and south poles of same pole strength as before but of reduced length.
(ii) Each piece is a magnet with same length but reduced pole strength.
Q71. An isolated magnetic pole is not possible. How can we have it, if we need it for some practical purposes ?
Ans. If we take a sufficiently long magnet, one of its pole can be considered to be located at a large distance from the other. Hence, both of its poles can be treated as isolated magnetic poles for all practical purposes.
Q72. What will you do if you want to save a sensitive instrument from a magnetic field ?
Ans. The instrument should be enclosed inside a hollow conductor of soft iron, since there cannot be any magnetic lines of force inside the conductor.
Q73. A magnetic needle is placed on a cork floating on a still lake in the northern hemisphere. What will happen to the system ?
Ans. There is no net translatory motion of the system, since it experiences a torque only. Due to the torque, the system rotates in such a way that the axis of the needle becomes parallel to the magnetic meridian.
Q74. A copper loop and an aluminium loop are removed from a magnetic field in the same time-interval. In which loop will the induced e.m.f. and induced current be greater ?
Ans. The induced e.m.f. will be same in both, but the induced current will be greater in copper loop because its resistance is smaller than that of aluminium loop.
Q75. How can the concept of electromagnetic induction be used to apply breaks to fast moving automobiles ?
Ans. To retard the motion of a fast moving automobile, a strong magnetic field should be applied across the wheels. The consequent change in magnetic flux induces an e.m.f., which according to Lenz’s law, opposes the cause (motion), thus, retarding the motion.
Q76. Does the change in magnetic flux induce e.m.f. or current ?
Ans. The change in magnetic flux always induces e.m.f. However, the current is induced only when the circuit is closed.
Q77. Name the part of electromagnetic spectrum of wavelength 10–12 m and mention its one application.
Ans. Wavelength 10–12 m belongs to X-rays. This is used in radio therapy.
Q78. Which of the following has the longest wavelength ? Microwaves, X-rays, ultraviolet rays.
Ans. Microwaves.
Q79. Which part of electromagnetic spectrum has largest penetrating power ?
Ans. Gamma rays.
Q80. Identify the part of the electromagnetic spectrum which is :
(i) produced by bombarding a metal target by high speed electrons,
(ii) produced in nuclear reaction.
Ans. (i) X-rays, (ii) Gamma-rays
Q81. Arrange the following radiations in descending order of their frequencies :
X-rays, Radio waves, Ultraviolet rays, Gamma rays, Infrared rays, Visible light, Microwaves.
Ans. Gamma rays, X-rays, Ultraviolet rays, Visible light, Infrared rays, Microwaves, Radio waves.
Q82. Name the constituent radiation of electro-magnetic spectrum which :
(i) is used in ovens for cooking purposes.
(ii) is used in scientific research.
(iii) is used in treatments of cancer and tumours.
(iv) is used for sterilizing the surgical instruments.
(v) is used in solar water heaters.
(vi) is used to broadcast radio programmes to long distances.
(vii) is used in electric bulbs.
Ans.
(i) Microwaves
(ii) X-rays
(iii) Gamma rays
(iv) Ultraviolet rays
(v) Infrared waves
(vi) Radio waves
(vii) Visible light
Q83. What is photometry ?
Ans. Photometry is a branch of physics that deals with the measurement of light emitted by or reflected from objects.
Q84. Write any one use for each of the following mirrors : (i) Convex (ii) Concave
Ans. (i) Convex mirrors are used as reflector in street lights. (ii) Concave mirrors are used as shaving mirrors.
Q85. A concave mirror is held in water. What change do you expect in the focal length ?
Ans. The focal length of a concave mirror has to do nothing with the external medium. Hence, it will remain unchanged.
Q86. Does a beam of white light give a spectrum on passing through a hollow prism ?
Ans. No, because no dispersion takes place in a hollow prism.
Q87. Does each and every incident photon essentially eject an electron ?
Ans. No, it may be absorbed in some other way. Only about 1% of incident photons eject out electrons.
Q88. What is the value of Planck’s constant ?
Ans. Planck’s constant, h = 6·62 × 10 –34 joule-sec.
Q89. What is photoelectric emission ?
Ans. The phenomenon of emission of electrons from the surface of certain substances, mostly metals, when illuminated with visible, ultraviolet light or X-rays is called ‘photoelectric emission’.
Q90. Name the physical quantity whose dimensional formula is the same as that of Planck’s constant ?
Ans. Angular momentum.
Q91. Is photoelectric emission a spontaneous process ?
Ans. No, it is not a spontaneous process.
Q92. Is photoelectric emission an instantaneous process ?
Ans. Yes, it is an instantaneous process.
Q93. What is cut off or stopping potential ?
Ans. Cut off or stopping potential is that minimum negative potential applied to the anode which completely stops the photoelectric current.
Q94. Every metal has a definite work function. Why do photoelectrons not come out with the same energy if incident radiation is monochromatic ? Why is there an energy distribution of photo-electrons ?
Ans. Electrons are present in a continuous band of levels, different electrons require different energy to get out of the atom. For this reason, the electrons knocked off by a monochromatic radiation possesses different energies.
Q95. Why are alkali metals most suitable for photoelectric emission ?
Ans. Alkali metals exhibit photoelectric effect with visible light because the work function of an alkali metal is quite low. Therefore, they emit photoelectrons even when visible light falls on them.
Q96. It is harder to remove a free electron from copper than from sodium. Which metal has greater work function? Which metal has higher threshold wavelength ?
Ans. Since, it is harder to remove an electron from copper than sodium, thus work function of copper is greater than sodium. Threshold wavelength will be more for sodium.
Q97. No photoelectrons can be liberated from a metal surface if the wavelength of the incident light exceeds a certain value. Why ?
Ans. If the energy of the incident light photon, which decreases with increasing wavelength (E = hc/l), is less than the work function of the metal, photoelectric emission cannot take place.
Q98. What happens to the resistance and capacitance of a semiconductor on heating ?
Ans. Resistance decreases but capacitance increases.
Q99. What is doping ?
Ans. The process of deliberately adding a suitable impurity atom in a controlled manner to increase the electrical conductivity of an intrinsic semiconductor is called doping.
100. What type of semiconductor is obtained when a crystal of silicon is doped with a trivalent element ?
Ans. p-type semiconductor.