HBSE Class 12 Physics Important Question Answer 2024-2025

Haryana Board (HBSE) Class 12 Physics Important Question Answer 2024-2025. HBSE Class 12 Physics Important Questions 2024. BSEH Class 12 Physics vv important Questions 2024. Class 12 Physics Most important questions for Board Exam 2024. हरियाण बोर्ड कक्षा 12 भौतिक विज्ञान के अति महत्वपूर्ण प्रश्न 2024.

 

HBSE Class 12 Physics Important Question Answer 2024-2025


 

Objective Questions

1. Wheatstone bridge can not be used for measuring of very ………….. resistances.
(a) high
(b) low
(c) low or high
(d) medium value
Answer – (c) low or high

2. What is the value of speed of X-rays in vacuum ?
(a) 2c
(b) c
(c) c/2
(d) c/√2
Answer – (b) c (speed of light)

3. Lenz’s law is a consequence of law of conservation of :
(a) charge
(b) energy
(c) mass
(d) momentum
Answer – (b) energy

4. By mistake a voltmeter is connected in series and an ammeter is connected in parallel with a resistance in an electrical circuit. What will happen to the instruments ?
(a) voltmeter is damaged
(b) Ammeter is damaged
(c) both are damaged
(d) none is damaged
Answer – (d) none is damaged

5. Ratio of magnitudes of electric force in air and water between an electron and proton is :
(a) k (dielectric constant)
(b) 1/k
(c) 1
(d) 0
Answer – (a) k (dielectric constant)

6. When the distance between two charged particles is made one-fourth, the force between them becomes :
(a) one-fourth
(b) one-third
(c) half
(d) sixteen times
Answer – (d) sixteen times
Force, F = k(q1q2)/r²
New Force, F’ = k(q1q2) / (r/4)² = 16 k(q1q2)/r² = 16F 

7. Which the following relation is called as current density ?
(a) I/A
(b) A/I
(c) I²/A
(d) I³/A²
Answer – (a) I/A (current/area)

8. What is the relative magnetic permeability of substance whose magnetic susceptibility is – 1 ?
(a) 1
(b) – 1
(c) 0
(d) 2
Answer – (c) 0

9. A circuit has a resistance of 12 ohm and an impedance 15 ohm. The power factor of the circuit will be :
(a) 0.8
(b) 0.4
(c) 1.25
(d) 0.125
Answer – (a) 0.8
Power factor = cosФ = R/Z = 12/15 = 0.8

10. Which dopant cannot be used to make Germanium (Ge) as n-type semiconductor ?
(a) P
(b) In
(c) As
(d) Sb
Answer – (b) In (indium)

11. Type of biasing in the given circuit is :

(a) Forward biasing
(b) Reverse biasing
(c) both
(d) can’t say anything
Answer – (a) Forward biasing

12. The magnitude of saturation photoelectric current depends upon :
(a) frequency
(b) intensity
(c) work function
(d) stopping potential
Answer – (b) intensity

13. If h is Planck’s constant, the momentum of a photon of wavelength 0.01 Å is :
(a) 10-² h
(b) h
(c) 10² h
(d) 10¹² h
Answer – (d) 10¹² h
Momentum, p = h/λ = h/(0.01×10-¹⁰) = 10¹² h

14. Volume of a nucleus is directly proportional to :
(a) Mass number
(b) Atomic number
(c) No. of neutrons
(d) Radius
Answer – (a) Mass number

15. Which the following statements is NOT true for nuclear forces ?
(a) They are stronger than Coulomb forces
(b) They have about the same magnitude for different pairs of nucleons
(c) They are always attractive
(d) They saturate as the separation between two nucleons increases
Answer – (d) They saturate as the separation between two nucleons increases

16. In Young’s double-slit experiment, the fringe width is found to be β. If the entire apparatus is immersed in a liquid of refractive index µ. The new fringe width will be :
(a) β
(b) µβ
(c) β/µ
(d) β/µ²
Answer – (c) β/µ

17. CGS unit of electric charge is :
(a) Coulomb
(b) Joule
(c) Faraday
(d) Stat Coulomb
Answer – (d) Stat Coulomb

18. A thin convex lens of focal length 30 cm is placed in contact with a thin concave lens of focal length 20 cm. What is the focal length of the combination ?
(a) + 50 cm
(b) – 60 cm
(c) + 10 cm
(d) – 10 cm
Answer – (b) – 60 cm
Focal length of convex lens, f1 = 30 cm
Focal length of concave lens, f2 = – 20 cm
1/f = 1/f1 + 1/f2 = 1/30 – 1/20 = – 1/60
f = – 60 cm

19. Light of wavelength 6000 Å falls on a plane reflecting surface. The reflected wavelength is :
(a) 6000 Å
(b) < 6000 Å
(c) > 6000 Å
(d) None of these
Answer – (a) 6000 Å

20. Hole is :
(a) an anti particle of electron
(b) a vacancy created when an electron leaves a covalent bond
(c) absence of free electrons
(d) an artificially created particle
Answer – (b) a vacancy created when an electron leaves a covalent bond

21. In a p-type silicon, which of the following statement is true :
(a) Electrons are majority carriers and trivalent atoms are the dopants.
(b) Electrons are minority carriers and pentavalent atoms are the dopants.
(c) Holes are minority carriers and pentavalent atoms are the dopants.
(d) Holes are majority carriers and trivalent atoms are the dopants.
Answer – (d) Holes are majority carriers and trivalent atoms are the dopants.

22. Taking the Bohr radius as ao = 53 pm, the radius of Li++ ion in its ground state, on the basis of Bohr’s model, will be about :
(a) 53 pm
(b) 27 pm
(c) 18 pm
(d) 13 pm
Answer – (c) 18 pm
Atomic number of Lithium atom is 3,
so, r = ao/z = 53/3 = 17.67pm ≈ 18pm (approx.)

23. The stopping potential for photoelectrons depends upon :
(a) Frequency of incident light only
(b) Material of the cathode only
(c) Both the frequency of incident light and the material of the cathode
(d) Intensity of incident light
Answer – (c) Both the frequency of incident light and the material of the cathode

24. Microwaves are the electromagnetic waves with frequency in the range of :
(a) Micro hertz
(b) Mega hertz
(c) Giga hertz
(d) Hertz
Answer – (c) Giga hertz

25. Path of charged particle entering in a uniform magnetic field at an angle of 60° is :
(a) straight line
(b) helical
(c) parabola
(d) circular
Answer – (b) helical

26. A beam of electrons and protons move parallel to each other in same direction, then they :
(a) attract each other
(b) repel each other
(c) no relation
(d) neither attract nor repel
Answer – (b) repel each other

27. In the circuit shown in the figure, the potential difference between X and Y will be :

(a) zero
(b) 20 V
(c) 60 V
(d) 120 V
Answer – (d) 120 V

28. A wire of resistance R is stretched to twice of its original length. Its new resistance will be :
(a) 4 R
(b) 9 R
(c) 3 R
(d) 3 R
Answer – (a) 4 R
Resistance, R = ρL/A
If length increase, then area decrease since volume constant,
New length = 2L
New area = A/2
New resistance, R’ = ρ(2L) / (A/2) = 4 R

29. For high frequency, capacitor offers :
(a) more reactance
(b) zero reactance
(c) less reactance
(d) none of these
Answer – (c) less reactance

30. An electric dipole placed in a non-uniform electric field can experience :
(a) a force but not a torque
(b) a torque but not a force
(c) always a force and a torque
(d) neither a force nor a torque
Answer – (c) always a force and a torque

31. If the net electric flux through a closed surface is zero, then we can infer :
(a) no net charge is enclosed by the surface.
(b) uniform electric field exists within the surface.
(c) electric potential varies from point to point inside the surface.
(d) charge is present inside the surface.
Answer – (a) no net charge is enclosed by the surface.

32. Kirchoff’s first rule at a junction in an electrical network, deals with conservation of :
(a) energy
(b) charge
(c) momentum
(d) both energy and charge
Answer – (b) charge

33. The resistance of a metal wire increases with increasing temperature on account of :
(a) decrease in free electron density
(b) decrease in relaxation time
(c) increase in mean free paths
(d) increase in the mass of electron
Answer – (b) decrease in relaxation time

34. The magnetic dipole moment of a current carrying coil does not depend upon :
(a) number of turns of the coil
(b) cross-sectional area of the coil
(c) current flowing in the coil
(d) material of the turns of the coil
Answer – (d) material of the turns of the coil

35. The resolving power of a telescope can be increased by increasing :
(a) wavelength of light
(b) diameter of objective
(c) length of the tube
(d) focal length of eyepiece
Answer – (b) diameter of objective

36. A biconvex lens of glass having refractive index 1.47 is immersed in a liquid. It becomes invisible and behaves as a plane glass plate. The refractive index of the liquid is :
(a) 1.47
(b) 1.62
(c) 1.33
(d) 1.51
Answer – (a) 1.47

37. The variation of slopping potential (Vo) with the frequency (𝛎) of the light incident on two different photosensitive surfaces M1 and M2 is shown in figure. Identify the surface which has greater values of the work function.

(a) M1
(b) M2
(c) both M1 and M2
(d) none of these
Answer – (b) M2

38. How does an increase in doping concentration affect the width of depletion layer of a p-n junction diode ?
(a) decreases
(b) increases
(c) no change take place
(d) can’t say
Answer – (a) decreases

39. Which of the following statements is not correct according to Rutherford model ?
(a) Most of space inside an atom is empty.
(b) The electrons revolve around the nucleus under the influence of coulomb force acting on them.
(c) Most part of the mass of the atom and its positive charge are concentrated at its centre.
(d) the stability of atom was established by the model.
Answer – (d) the stability of atom was established by the model.

40. Two electrons of charge e each move in the same circular path of radius r with a constant speed v each. The magnetic moment associated with motion of these electrons is :
(a) evr
(b) evr/2
(c) ev
(d) 4π²r³e/v
Answer – (b) evr/2

41. Paschen series of atomic spectrum of hydrogen gas lies in :
(a) Infrared region
(b) Ultraviolet region
(c) Visible region
(d) Partly in utlraviolet and partly in visible region
Answer – (a) Infrared region

42. Electromagnetic wave used as diagnostic tool in medicine are :
(a) X-rays
(b) ultraviolet rays
(c) infra red radiation
(d) ultrosonic waves
Answer – (a) X-rays

43. The output of a step-down transformer is measured to 24 V when connected to a 12 W light bulb. The peak value of the current is :
(a) 1/√2 A
(b) √2 A
(c) 2 A
(d) 2√2 A
Answer – (a) 1/√2 A
Vs = 24V, Ps = 12W
Is = Ps/Vs = 12/24 = 1/2 A
Io = √2 Is = √2 × 1/2 = 1/√2 A

44. An electric dipole consisting of charges +q and –q separated by a distance L is in stable equilibrium in a uniform electric field (vector E). The electrostatic potential energy of the dipole is :
(a) qLE
(b) zero
(c) – qLE
(d) – 2qLE
Answer – (c) – qLE

45. For a glass prism, the angle of minimum deviation will be smallest for the light of :
(a) red colour
(b) blue colour
(c) yellow colour
(d) green colour
Answer – (a) red colour

46. Photons of energies 1 eV and 2 eV are successively incident on a metallic surface of work function 0.5 eV. The ratio of kinetic energy of most energetic photoelectrons in the two cases will be :
(a) 1 : 2
(b) 1 : 1
(c) 1 : 3
(d) 1 : 4
Answer – (c) 1 : 3
Kinetic energy = Energy of incident photon – Work function
For the incident photon with energy 1 eV:
Kinetic energy, KE1 = 1 eV – 0.5 eV = 0.5 eV
For the incident photon with energy 2 eV:
Kinetic energy, KE2 = 2 eV – 0.5 eV = 1.5 eV
KE1 : KE2 = 0.5 eV : 1.5 eV = 1 : 3

47. A cell of internal resistance r connected across an external resistance R can supply maximum current when :
(a) R = r
(b) R > r
(c) R = 2r
(d) R = 0
Answer – (d) R = 0

48. In a current carrying conductor the ratio of electric field and the current density at a point is called :
(a) Resistivity
(b) Conductivity
(c) Resistance
(d) Mobility
Answer – (a) Resistivity

49. Above curie temperature, a
(a) ferromagnetic material becomes diamagnetic
(b) ferromagnetic material becomes paramagnetic
(c) paramagnetic material becomes ferromagnetic
(d) paramagnetic material becomes diamagnetic
Answer – (b) ferromagnetic material becomes paramagnetic

50. Displacement current exists only when :
(a) electric field is changing
(b) magnetic field is changing
(c) electric field is not changing
(d) magnetic field is not changing
Answer – (a) electric field is changing

51. An electron is released from rest in a region of uniform electric and magnetic fields acting parallel to each other. The electron will :
(a) move in a straight line
(b) move in a circle
(c) remain stationary
(d) move in helical path
Answer – (a) move in a straight line

52. When two nuclei (A ≤ 10) fuse together to form a heavier nucleus, the
(a) binding energy per nucleon increases
(b) binding energy per nucleon decreases
(c) binding energy per nucleon does not change
(d) total binding energy decreases
Answer – (a) binding energy per nucleon increases

53. At equilibrium, in a p-n junction diode the net current is :
(a) due to diffusion of majority charge carrier
(b) due to drift of majority charge carrier
(c) zero as diffusion and drift currents are equal and opposite
(d) zero as no charge carrier cross the junction
Answer – (c) zero as diffusion and drift currents are equal and opposite

54. In a n-type semiconductor, the donor energy level lies :
(a) at the centre of the energy gap
(b) just below the conduction band
(c) just above the valence band
(d) in the conduction band
Answer – (b) just below the conduction band

55. The electric flux emerging out form IC charge is :
(a) 1/∈o
(b) 4π
(c) 4π/∈o
(d) ∈o
Answer – (a) 1/∈o

56. In Bohr’s model of hydrogen atom, the total energy of the electron in nth discrete orbit is proportional to :
(a) n
(b) 1/n
(c) n²
(d) 1/n²
Answer – (d) 1/n²

57. In an a.c. circuit, the applied voltage and flowing current are E = Eo sinωt and I = Io sin(ωt + π/2) respectively. What is the average power consumed in one cycle in this circuit ?
(a) Ev Iv
(b) zero
(c) ∞
(d) – 1
Answer – (b) zero
Paverage = VI cosϕ = VI cos(π/2) = VI × 0 = 0

58. The electric potential on the axis of an electric dipole at a distance ‘r’ from its centre is V. Then the potential at a point at the same distance on its equatorial line will be :
(a) 2V
(b) -V
(c) V/2
(d) Zero
Answer – (d) Zero

59. Resistance of conductor does not depend on :
(a) Length of conductor
(b) Nature of material
(c) Radius of cross section of conductor
(d) Potential difference applied across the conductor
Answer – (d) Potential difference applied across the conductor

60. The temperature (T) dependence of resistivity of materials A and material B is represented by fig.(i) and fig.(ii) respectively. Identify material A and material B.

(a) material A is copper and material B is germanium
(b) material A is germanium and material B is copper
(c) material A is nichrome and material B is germanium
(d) material A is copper and material B is nichrome
Answer – (b) material A is germanium and material B is copper

61. If the magnetizing field on a ferromagnetic material is increased, its permeability is :
(a) decreases
(b) increases
(c) remains unchanged
(d) first decreases and then increases
Answer – (a) decreases

62. An iron cored coil is connected in series with an electric bulb with an AC source as shown in figure. When iron piece is taken out of the coil, the brightness of the bulb will :

(a) decrease
(b) increase
(c) remain unaffected
(d) fluctuate
Answer – (b) increase

63. A ray of light passing from air through an equilateral glass prism undergoes minimum deviation when the angle of incidence is 3/4 of the angle of prism. Speed of light in the prism is :
(a) c
(b) c/2
(c) c/4
(d) none
Answer – (d) none
A = 60°, i = ¾A = ¾(60°) = 45°
In the position of minimum deviation,
r = A/2 = 60°/2 = 30°
μ = sini/sinr = sin45°/sin30° = (1/√2)/√2 = √2
As, μ = c/v
v = c/μ = (3×10⁸)/√2 = 2 × 10⁸ m/s

64. Which of the following statement is NOT true about the properties of electromagnetic waves ?
(a)These waves do not require any material medium for their propagation.
(b) Both electric and magnetic field vectors attain the maxima and minima at the same time.
(c) The energy in electromagnetic wave is divided equally between electric and magnetic fields.
(d) Both electric and magnetic field vectors are parallel to each other.
Answer – (d) Both electric and magnetic field vectors are parallel to each other.

65. In two positions convex lens produces magnified image of given object. The positions are :
(a) At f at 2f
(b) Between f and 2f, between optical center and f
(c) Beyond 2f, between c and f
(d) At 2f, between optical centre and f
Answer – (b) Between f and 2f, between optical center and f

66. If Young’s double slit experiment is immersed in water, then fringe width :
(a) decreases
(b) increases
(c) remain same
(d) none
Answer – (a) decreases

67. The work function for a metal surface is 4.14 eV. The threshold wavelength for this metal surface is :
(a) 4125 Å
(b) 2062.5 Å
(c) 3000 Å
(d) 6000 Å
Answer – (c) 3000 Å
Work Function, w = hc/λ
hc/λ = 4.14eV
Threshold wavelength, λ = (6.6 × 10-³⁴ × 3 × 10⁸) ÷ (4.14 × 1.6 × 10-¹⁹) = 3 × 10-⁷ m = 3000 Å

68. The radius of the inner most electron orbit of a hydrogen atom is 5.3 × 10-¹¹ m. The radius of the n = 3 orbits :
(a) 1.01 × 10-1¹⁰ m
(b) 1.59 × 10-¹ m
(c) 2.12 × 10-¹⁰ m
(d) 4.77 × 10-¹⁰ m
Answer – (d) 4.77 × 10-¹⁰ m
r3 = (n)²r1 = (3)² × 5.3 × 10-¹¹ = 4.77 × 10-¹⁰ m

69. Which of the following statements about nuclear forces is not true ?
(a) The nuclear force between two nucleons falls rapidly to zero as their distance is more than a few femto meters (fm).
(b) The nuclear force is much weaker than the Coulomb force.
(c) The force is attractive for distances larger than 0.8 fm and repulsive if they are separated by distance less than 0.8 fm.
(d) Then nuclear force between neutron-neutron, proton-neutron and proton-proton is approximately the same.
Answer – (b) The nuclear force is much weaker than the Coulomb force.

70. Power of lens is 10 diopters, which of following is correct :
(a) Convex lens of focal length 10 metre
(b) Convex lens of focal length 10 cm
(c) Concave lens of focal length 10 metre
(d) Concave lens of focal length 10 cm
Answer – (b) Convex lens of focal length 10 cm
Power = 1/F
F = 1/P = 1/10 = 0.1 m = 10 cm


 

Assertion Reason Based Questions

1. Assertion (A) : Light travels faster in glass than in air.
Reason (R) : Air is denser than glass.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (d) A is false and R is also false.

2. Assertion (A) : p-type semiconductors is a positive type crystal.
Reason (A) : p-type semiconductor is an negative type crystal.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (d) A is false and R is also false

3. Assertion (A) : No two electric lines of force can intersect each other.
Reason (R) : Tangent at any point of electric line of force gives the direction of electric field.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

4. Assertion (A) : Colours can be seen in thin layers of oil on the surface of water.
Reason (R) : White light is composed of several colours.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (b) Both A and R are true and R is not the correct explanation of A.

5. Assertion (A) : Impact parameter for scattering of α particles by 180° is zero.
Reason (R) : Zero impact parameter means α particles tends to hit the centre the nucleus.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

6. Assertion (A) : In a simple battery circuit, the point of lowest potential is positive terminal of the battery.
Reason (R) : The current flows towards the point of the higher potential, as it does in such a circuit from the negative to positive terminal.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (d) A is false but R is also false.

7. Assertion (A) : At resonance, the inductive reactance is equal and opposite to the capacitive reactance.
Reason (R) : In series LCR circuit, the inductive reactance is equal and opposite to capacitive reactance.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (c) A is true but R is false.

8. Assertion (A) : Photo electric effect demonstrates the wave nature of light.
Reason (R) : The number of photoelectrons is proportional to the frequency of light.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (d) A is false but R is also false.

9. Assertion (A) : Balmer series lies in visible region of electromagnetic spectrum.
Reason (R) : Balmer means visible, hence series lies in visible region.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (c) A is true but R is false.

10. Assertion (A) : The temperature coefficient of resistance is positive for metals and negative for semiconductors.
Reason (R) : The charge carriers in metals are negatively charged whereas in semiconductors they are positively charged.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (b) Both A and R are true and R is not the correct explanation of A.

11. Assertion (A) : Inductance coil ar e made of copper.
Reason (R) : Induced current is more in wire having less resistance.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

12. Assertion (A) : Distance of closest approach of alpha particle to the nucleus is always greater than the size of the nucleus.
Reason (R) : Strong nuclear repulsion does not allow alpha particle to reach the surface of nucleus.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

13. Assertion (A) : The de-Broglie wavelength equation has significance for any microscopic or submicroscopic particles.
Reason (R) : The de-Broglie wavelength is inversely proportional to the mass of the object if velocity is constant.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

14. Assertion (A) : A dentist uses a concave mirror to examine a small cavity.
Reason (R) : A dentist uses a concave mirror so as to form a magnified, virtual image of an object.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

15. Assertion (A) : In a series combination of capacitors, charge on each capacitor is same.
Reason (R) : In such a combination, charge can not move only along one route.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

16. Assertion (A) : The bending of an insulated wire increases the resistance of wire.
Reason (R) : The drift velocity of electrons in bent wire decreases.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (d) A is false and R is also false.

17. Assertion (A) : Large angle scattering of alpha particles led to the discovery of atomic nucleus.
Reason (R) : Entire positive charge of atom is concentrated in the central core.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.

18. Assertion (A) : The electrical conductivity of a semiconductor increases on doping.
Reason (A) : Doping always increases the number of electrons in the semiconductor.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (c) A is true but R is false

19. Assertion (A) : In an interference pattern observed in Young’s double slit experiment, if the separation (d) between coherent sources as well as the distance (D) of the screen from the coherent sources both are reduced to 1/3rd, then new fringe width remains the same.
Reason (R) : Fringe width is proportional to (d/D).
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is the correct explanation of A

20. Assertion (A) : The photo electrons produced by a mono chromatic light beam incident on a metal surface have a spread in their kinetic energies.
Reason (R) : The energy of electrons emitted from inside the metal surface, is lost in collision with the other atoms in the metal.
(a) Both A and R are true and R is correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
Answer – (a) Both A and R are true and R is correct explanation of A.


 

Case Study Based Questions

1. According to Ohm’s law, the current flowing through a conductor is directly proportional to the potential difference across the end of the conductors i.e. I ∝ V ⇒V/I = R, where R is resistance of the conductor.
Electrical resistance of a conductor is the obstruction posed by the conductors to the flow of electric current through it. It depends upon length, area of cross-section, nature of material and temperature of conductor.
We can write, R ∝ L/A or R = ρ (L/A), where ρ is the electrical resistivity of material of the conductor.
Questions :
(i) Dimensions electric resistance is :
(a) [ML² T-² A-²]
(b) [ML² T-³ A-²]
(c) [M-¹ L-² T-¹ A]
(d) [M-¹ L² T² A-¹]
Answer – (b) [ML² T-³ A-²]

(ii) If 1 µA current flows through a conductor when potential difference of 2 volt is applied across its ends, then the resistance of the conductors is :
(a) 2 × 10⁶ Ω
(b) 3 × 10⁵ Ω
(c) 1.5 × 10⁵ Ω
(d) 5 × 10⁷ Ω
Answer – (a) 2 × 10⁶ Ω
R = V/I = 2/(1×10-⁶) = 2 × 10⁶ Ω

(iii) Specific resistance of a wire depends upon :
(a) length
(b) cross-sectional area
(c) mass
(d) none of these
Answer – (d) none of these
Specific resistance of a wire only depends on nature of material and is independent of dimensions.

(iv) The slope of graph between potential difference and current through a conductor is :
(a) a straight line
(b) curve
(c) first curve then straight line
(d) first straight line then curve
Answer – (a) a straight line

OR

The resistivity of the material of a wire 1.0 m long, 0.4 mm diameter and having a resistance of 2.0 ohm is :
(a) 1.57 × 10-⁶ Ωm
(b) 5.25 × 10-⁷ Ωm
(c) 7.12 × 10-⁵ Ωm
(d) 2.55 × 10-⁷ Ωm
Answer – (d) 2.55 × 10-⁷ Ωm
A = πD²/4 = π(4×10-⁴)²/4 = 4π×10-⁸ m²
ρ = RA/L = (2×4π×10-⁸)/1 = 2.55 × 10-⁷ Ωm

2. Power (P) of a lens is given by reciprocal of focal length (f) of the lens i.e. P = 1/f, where f is in metre and P is in diopter.
For a convex lens, power is positive and for a concave lens power is negative. When number of thin lenses of power P1, P2, P3, ……… are held in contact with one another, the power of combination is given by algebric sum of the powers of all the lenses i.e. P = P1 + P2 + P3 + ………..
Questions :
(i) A convex and a concave lens separated by distance d are then put in contact. The focal length of combination is :
(a) becomes 0
(b) remains the same
(c) decreases
(d) increases
Answer – (d) increases

(ii) If two lenses of power +1.5 D and +1.0 D are placed in contact, then the effective power of combination will be :
(a) 2.5 D
(b) 1.5 D
(c) 0.5 D
(d) 3.25 D
Answer – (a) 2.5 D
P = P1 + P2 = 1.5 + 1.0 = 2.5 D

(iii) If the power of a lens is +5 D, what is the focal length of the lens ?
(a) 10 cm
(b) 20 cm
(c) 15 cm
(d) 5 cm
Answer – (b) 20 cm
f = 1/P = 1/5 = 0.2 m = 20 cm

(iv) Two thin lenses of focal lengths +10 cm and –5 cm are kept in contact. The power of the combination is :
(a) – 10 D
(b) – 20 D
(c) 10 D
(d) 15 D
Answer – (a) – 10 D
P = P1 + P2 = 1/f1 + 1/f2 = 100/10 – 100/5 = – 10 D

OR

A convex lens of focal length 25 cm is placed coaxially in contact with a concave lens of focal length 20 cm. The system will be :
(a) converging in nature
(b) diverging in nature
(c) can be converging or diverging
(d) none of these
Answer – (b) diverging in nature
P = P1 + P2 = 1/f1 + 1/f2 = 100/25 – 100/20 = -1 D

3. Neutrons and protons are identical particles in the sense that their masses are nearly the same and the force called nuclear force, does distinguish them. The nuclear force is the strongest force. The stability of the nucleus is determined by the neutron proton ratio mass defect. The shape of the nucleus is calculated by quadruple moment and the spin of the nucleus depends on the mass number. The whole mass of the atom (nearly 99%) is centered at the nucleus.
Questions :
(i) What do you understand by nuclear forces ?
Answer – Nuclear force is the force which exists between the protons and neutrons present in a nucleus to keep them together.

(ii) A force between two protons is same as the force between proton and neutron. The nature of force is :
(a) electric force
(b) weak nuclear force
(c) gravitational force
(d) strong nuclear force
Answer – (d) strong nuclear force

(iii) All the nucleons in an atom are held by :
(a) tensor forces
(b) magnetic forces
(c) nuclear forces
(d) coulomb’s forces
Answer – (c) nuclear forces

(iv) How is the radius of nucleus related to the mass number ?
Answer : R = Ro A^⅓ where Ro is empirical constant.

OR

What is the ratio of the nuclear densities of two nuclei having mass number in the ratio 1 : 3 ?
Answer : Here A1 : A2 = 1 : 3
ρ1 : ρ2 = 1 : 1
(the nuclear density is same for all nuclei and is independent of the mass ratio)

4. Static electricity is build up of an electrical charge on the surface of an object. We see static electricity everyday. When our dry hair are dressed with a plastic comb, hairs get charged. Lightning is a powerful form of static electricity. Atoms are made up of tiny particles called neutrons, protons and electrons. The neutrons and protons together form the nucleus. The electrons revolve around the outside of the nucleus. A static charge is formed when two surface are rubbed against each other and the electrons move from one object to another.
Questions :
(i) Which atomic particle move from one surface to another in order to form static charge ?
(a) Electrons
(b) Protons
(c) Neutrons
(d) All of the above
Answer – (a) Electrons

(ii) What is static electricity ?
(a) Electricity that flows in one direction
(b) Electricity that constantly changes direction
(c) An electric charge on the surface of an object
(d) Electricity that is sent over the air
Answer – (c) An electric charge on the surface of an object.

(iii) When a charged rod is brought near a neutral paper piece, then charged rod :
(a) Attracts the paper piece
(b) Repels the paper piece
(c) Neither attract nor repel the paper piece
(d) None of the above
Answer – (a) Attracts the paper piece

(iv) Which of the following is the practical application for static electricity ?
(a) Air filters
(b) Photocopier
(c) Laser printers
(d) All of the above
Answer – (d) All of the above

OR

Which of the following is an example of static electricity ?
(a) Electricity for a light bulb
(b) An electric socket in your home
(c) Your pants sticking to your legs
(d) None of the above
Answer – (c) Your pants sticking to your legs

5. Silicon and Germanium in pure form are intrinsic semiconductor. In such semiconductors, valence band and conduction band are separated by energy gap known as forbidden energy gap. This energy gap is larger for silicon than for germanium. An intrinsic semiconductor will behave like an insulator at T = 0 K. It is the thermal energy at higher temperature which excites some electrons from the valence band to the conduction band. When a small amount say, a few parts per million of suitable impurity is added to the pure semiconductors, the conductivity of the semiconductor is increased manifold.
Questions :
(i) In intrinsic semiconductor at room temperature the number of electrons and holes are :
(a) equal
(b) zero
(c) unequal
(d) infinite
Answer – (a) equal

(ii) With the increase of temperature, the resistivity of a semiconductor :
(a) decreases
(b) increases
(c) may increase or decrease
(d) does not change
Answer – (a) decreases

(iii) Which dopant cannot be used to make germanium (Ge) a p-type semiconductor ?
(a) P
(b) In
(c) B
(d) Al
Answer – (a) P (Phosphorus)

(iv) What is the net charge on n-type semiconductor ?
Answer – Net charge = 0

OR

What is doping?
Answer – The process of adding impurity atom to the pure semiconductors is called doping.

6. The large scale transmission and distribution of electrical energy over long distance is done with the use of transformer. The voltage output of the generator is stepped-up (so that current is reduced and consequently the I²R loss is cut down). It is then transmitted over long distances to an area sub-station near the consumers. There the voltage is stepped down. It is further stepped down at distributing sub-stations and utility poles before a power supply of 240 V reaches our home.
Questions :
(i) A transformer works on the principle of :
(a) Converter
(b) Inverter
(c) Mutual Induction
(d) Self induction
Answer – (c) Mutual Induction

(ii) Quantity that remains unchanged in a transformer is :
(a) voltage
(b) current
(c) frequency
(d) none of these
Answer – (c) frequency

(iii) The best material for the core of a transformer is :
(a) stainless steel
(b) mild steel
(c) hard steel
(d) soft iron
Answer – (d) soft iron

(iv) In a transformer, number of turns in the primary is 140 and that in secondary is 280. If current in primary is 4A, then that in the secondary is :
(a) 4A
(b) 2A
(c) 6A
(d) 10A
Answer – (b) 2A  (N1/N2 = I2/I1)

OR

The core of any transformer is laminated, so as to :
(a) reduce the energy loss due to eddy currents
(b) make it light weight
(c) make it robust and strong
(d) increase the secondary voltage
Answer – (a) reduce the energy loss due to eddy currents

7. Photoelectric effect thus gave evidence to the strange fact that light in interaction with matter behaved as if it was made of quanta or packets of energy, each of energy hv. Is the light quantum of energy to be associated with a particle? Einestein arrived at the important result that the light quantum can also be associated with momentum (h𝛎/c). This particle was later named photon. Each photon moves with speed of light c. Photons are electrically neutral and are not deflected by electric and magnetic fields.
Questions :
(i) Which one among the following shows particle nature of light ?
(a) Photoelectric effect
(b) Interference
(c) Refraction
(d) Polarization
Answer – (a) Photoelectric effect

(ii) Which of the following statement about photon is incorrect ?
(a) photons exert no pressure
(b) Momentum of photon is (h𝛎/c)
(c) Energy of photon is (h𝛎)
(d) Photons are electrically neutral
Answer – (a) photons exert no pressure

(iii) The rest mass of photon is :
(a) h𝛎/c
(b) h𝛎/c²
(c) h𝛎/λ
(d) zero
Answer – (d) zero

(iv) In a photon-particle collision (such as photon-electron collision), which of the following may not be conserved ?
(a) Total energy
(b) No. of photons
(c) Total momentum
(d) both (a) and (b)
Answer – (b) No. of photons

OR

Calculate number of photons in 6.62 J of radiation energy of frequency 10¹² Hz. Given h = 6.62 × 10-³⁴ Js.
Answer : E = nh𝛎
n = E/h𝛎 = 6.62/(6.62×10-³⁴×10¹²) = 10²²

8. Smallest charge that can exist in nature is the charge of an electron. During friction it is only the transfer of electron which makes the body charged. Hence net charge on any body is an integral multiple of charge of an electron (1.6 × 10-¹⁹C) i.e., q = ±ne where n = 1, 2, 3, 4, ……
Hence no body can have a charge represented as 1.8e, 2.7e, 2e/5, etc. Recently, it has been discovered that elementary particles such as protons or neutrons are elemental units called quarks.
Questions :
(i) If a charge on a body is 1nC, then how many electrons are present on the body ?
Answer : Charge (Q) = Ne, where, N is number of electrons present on the body, e is the charge on an electron.
-1×10-⁹C = N × (-1.6×10-¹⁹C)
N = (10-⁹)/(1.6×10-¹⁹) = 6.25 × 10⁹ electrons

(ii) Charge is scalar or vector ?
Answer – Scalar

(iii) A polythene piece rubbed with wool is found to have a negative charge of 3.2 × 10-⁷ C. Calculate the number of electrons transferred.
Answer – Given, Charge (Q) = 3.2 × 10-⁷ C
Charge on the electron, e = 1.6 × 10-¹⁹ C
Therefore,
Number of electron transferred is given by,
N = Q/e = (3.2×10-⁷)/(1.6×10-¹⁹) = 2 × 10¹² electrons

OR

What is charge ?
Answer – Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges; positive and negative.

9. Electromagnetic Induction : When a current I flows through a coil, flux linked with it is Φ = LI, where L is a constant known as self inductance of the coil.

Any charge in current sets up an induced emf in the coil. Thus, self inductance of a coil is the induced emf set up in it when the current passing through it changes at the unit rate. It is a measure of the opposition to the growth or the decay of current flowing through the coil. Also value of self inductance depends on the number of turns in the solenoid, its area of cross-section and the permeability of its corematerial.
Questions :
(i) What is self inductance ?
Answer – Self inductance is the tendency of a coil to resist changes in current in itself.

(ii) State the factors on which self inductance of a long solenoid depends ?
Answer – Self inductance depends on: Size of coil, Shape of the coil, Material of the coil and Medium.

(iii) What is the induced emf in a coil of 10 henry inductance in which current varies from 9 A to 4 A in 0.2 second.
Answer – Induced emf, e = -di/dt
Given, L = 10 H, ∆i = 9A – 4A = 5A, dt = 0.2s
emf, e = -L(di/dt) = 10 × (5/0.2) = 250 V

OR

What is Lenz’s law ?
Answer – Lenz’s law states that the current induced in a circuit due to a change or a motion in a magnetic field is so directed as to oppose the change in flux and to exert a mechanical force opposing the motion.


 

Subjective Questions

1. What is nuclear fusion ?
Answer – It is the phenomenon of fusing two or more lighter nuclei into a bigger one.

2. What is displacement current? How is it different from a conduction ?
Answer – Displacement current is due to changing electric field. Conduction current is due to flow of electrons in the circuit.

3. Write Farday’s laws of Electromagnetic induction.
Answer : Faraday’s 1st law – Whenever there is a change in the magnetic flux linked with the circuit changes an e.m.f is induced in it. It lasts so long as the change in the flux continues.
• Faraday’s 2nd law – The rate of change of magnetic flux linked with a circuit is directly proportional to emf. induced.

4. The ground state energy of hydrogen atom is -13.6 eV. Find the kinetic energy and potential energy of the electron in this state.
Answer – Total Energy, E = – 13.6 eV
Kinetic Energy = – E = 13.6 eV
Potential Energy = – 2 K.E = – 2 × 13.6 = – 27.2 eV

5. What will be the effect of (i) forward biasing and (ii) reverse basing on the width of depletion layer in a p-n junction diode.
Answer – (i) In forward biasing, the forward voltage opposes the potential barrier. As a result of it potential barrier reduced and width of depletion layer decreases.
(ii) In reverse biasing the reverse voltage supports the potential barrier. As a result of it potential barrier and width of depletion layer increases.

6. Define magnetic dipole moment and give its S.I. unit. Also mention the direction of magnetic dipole moment.
Answer – Magnetic dipole moment is the product of strength of either pole (m) and the magnetic length (2l) of the magnet. It’s S.I. unit is joule/tesla or Am².
The direction of magnetic dipole moment is from South to North pole of the magnet.

7. What is Lenz’s law ?
Answer – Lenz’s law states that the current induced in a circuit due to a change or a motion in a magnetic field is so directed as to oppose the change in flux and to exert a mechanical force opposing the motion.

8. Why no work is done in moving a test charge from one point to another point on equipotential surface. Give proof.
Answer – If points A and B lie on equipotential surface then VB = VA
VB – VA = WAB/q i.e. WAB = 0
Hence no work is done in moving the test charge from one point of equipotential surface to the other.

9. Two similar charges repel each other with a force of 44.1 N when placed 2 cm apart in air. Calculate the strength of charge.
Answer : F = 44.1 N, r = 2 cm = 2 × 10-² m
F = 1/4π∈o × q1q2/r²
44.1 = (9×10⁹×q×q) ÷ (2×10-²)²
q = 14 × 10-⁷ C

10. What is polarisation of light ?
Answer – The phenomenon of restricting the vibrations of light in a particular direction perpendicular to the direction of wave motion is called polarization of light.

11. State Law of Malus.
Answer – When a beam of completely plane polarised light is incident on an analyser, the resultant intensity of transmitted light is directly proportional to square of the cosine of the angle (θ) between plane of transmission of analyser and polariser.

12. Define drift velocity? Establish the relation between current and drift velocity.
Answer – Drift velocity is defined as the average velocity with which the free electrons get drifted towards the positive end of the conductor under the influence of an external electric field.
Current (I) = neAvd

13. Name the series of hydrogen spectrum which line in the visible region of electromagnetic spectrum ?
Answer – Balmer series lies in visible part of spectrum.

14. What is charge ?
Answer – Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges; positive and negative.

15. What do you mean by electromagnetic waves? Give two uses of gamma rays.
Answer – Electromagnetic waves are the waves in which electric field and magnetic field vectors are perpendicular to each other as well as perpendicular to direction of propagation.
Two uses of gamma (γ) rays :
(i) γ-rays are used in the treatment of cancer and tumours.
(ii) γ-rays are used to produce nuclear reactions.

16. Give any two reasons of energy loss in transformer.
Answer – (i) Copper loss is the energy loss in the form of heat in the copper coils of transformer.
(ii) Iron loss is in the form of heat in the iron core of transformer.

17. Which of the following electromagnetic waves has (a) minimum wavelength (b) minimum frequency? Write one use of each of these two waves.
Infrared waves, microwaves, γ-rays and X-rays.
Answer – (a) minimum wavelength : γ-rays
(b) minimum frequency : Microwaves
Use : γ-rays are used to treat cancer and Microwaves are used in communication.

18. Define current sensitivity and voltage sensitivity of a moving coil galvanometer. How can a galvanometer be made more sensitive.
Answer – Current sensitivity is deflection produced in galvanometer when unit current is passed through it.  Is = θ/I
Voltage sensitivity is the deflection produced in galvanometer when unit potential difference is applied across it.  Vs = θ/V = Is/R
Senstivity of galvanometer can be increased by decreasing the twisting constant (k). It can be done by using a phosphor-bronze wire for suspension.

19. What is p-n junction diode.
Answer – A p-n junction diode is a basic semiconductor device that controls the flow of electric current in a circuit.

20. State laws of photoelectric effect ?
Answer – Laws of photoelectric emission :
(i) There is a definite cut off value of frequency below which electrons cannot be ejected by any substance.
(ii) Number of emitted electrons are directly proportional to the intensity of light incident.
(iii) Kinetic energy of emitted electrons depends on the frequency of incident light on substance.
(iv) There is no time logging between the incident of light and emission of electrons.

21. The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is Bo = 510 nT. What is the amplitude of the electric field part of the wave ?
Answer – Here, Bo = 510 nT = 510 × 10-⁹ T
Eo = CBo
Eo = 3 × 10⁸ × 510 × 10-⁹ = 153 NC-¹

22. What is nuclear binding energy ?
Answer – Binding energy of a nucleus is the energy with which nucleons are bound in the nucleus.

23. Write the relation, connecting current density (J) and conductivity (σ) of the conductor, when an electric field (E) is applied.
Answer – Current density, J = σ E

24. What is quantisation of charge? Why it has no practical consequence at macroscopic level ?
Answer – Total charge present on a body is fixed. Total charge is integral multiple of smallest unit of charge i.e. electronic charge.
Q = ne
At macroscopic level total charge present on a body is much larger than the charge on an electron. Granary nature of charge on electron vanishes. So it has no practical consequence at macroscopic level.

25. What is the shape of the wavefront in each of the following cases :
(a) Light diverging from a point source
(b) The portion of the wavefront of light from a distant star intercepted by the Earth.
Answer – (a) The geometrical shape of the wave front would be diverging spherical wave front.
(b) As the star (i.e., source of light) is very far off i.e. at infinity, the wave front intercepted by earth must be a plane wave front.

26. An object is kept 20 cm in front of a concave mirror of radius of curvature 60 cm. Find the nature and position of the image formed.
Answer : f = -R/2 = -60/2 = – 30 cm, u = – 20 cm
1/f = 1/v + 1/u
1/v = 1/f – 1/u = -1/30 + 1/20 = 1/60
v = + 60 cm
Nature of image : virtual, erect and magnified

27. Explain the terms ‘depletion layer’ and ‘potential barrier’ in a p-n junction diode. How are the (a) width of depletion layer and (b) nature of potential barrier affected when the p-n junction is forward biased ?
Answer – The small region near the junction which is depleted of free charge carrier and has only immobile ions is called depletion layer.
The accumulation of –ve charges in p-side and +ve charges in n-side acts as a barrier across junction is called barrier potential.
In forward biasing (a) width of depletion layer decreases (b) potential barrier decreases

28. What are electric field lines? Draw these for a point charges +Q and –Q.
Answer – Electric field lines are imaginary lines, can be straight or curved, tangent to it at any point gives the direction of electric field.

29. What are coherent sources? Why two identical bulbs do not act as coherent sources ?
Answer – The source of light which emit continuous light of same wavelength, same frequency and in same phase or having a constant phase difference are called coherent sources.
Two independent sources of light cannot emit wave continuously. Two identical sources of light will not have same phase or constant phase difference between them.

30. Define self-inductance of a coil? Write its dimensions.
Answer – Self inductance of a coil is equal to emf induced in the coil when rate of change of current through the coil is unity.
Dimension, [L] = [ML²T-²A-²]

31. Two identical bars, one of paramagnetic material and other of diamagnetic material are kept in a uniform external magnetic field parallel to it. Draw a diagrammatically the modifications in the magnetic field pattern in each case.
Answer –

32. What is internal resistance of a cell? On what factors does it depends ?
Answer – Internal resistance of a cell is the resistance offered by the electrolyte and electrodes of a cell. It depends upon nature of electrodes and electrolyte, distance between the electrodes and area of electrodes immersed in the electrolyte.

33. Electromagnetic waves with wavelength :
(i) λ1 is suitable for radar systems used in air craft navigation.
Answer – Microwave

(ii) λ2 is used to kill germs in water purifiers.
Identify and name the part of the electromagnetic spectrum to which these radiations belong.
Answer – Ultraviolet

34. State ampere’s circuital law.
Answer – Ampere’s circuital law states that, the line integral of the magnetic field surrounding closed loop equals to the number of times the algebraic sum of currents passing through the loop.

35. A narrow slit is illuminated by a parallel beam of monochromatic light of wavelength λ equal to 6000 Å, separation between the slit is 2 cm. What is the angular width of the central maxima.
Answer – Given λ = 6000 Å, d = 2 cm = 2 × 10-² m
Angular width 2θ = 2λ/d = (2×6000)/(2×10-²)  = 600000 Å

36. Define distance of closest approach in Rutherford alpha scattering experiment. Write mathematical formula.
Answer – The minimum distance between the centre of the nucleus and the alpha particle just before it gets reflected back through 180° is defined as the distance of closest approach ro (also known as contact distance).
ro = 1/4πεo × 2Ze²/½mv² = 1/4πεo × 2Ze²/Ek

37. Explain Rutherford alpha scattering experiment.
Answer – Rutherford’s alpha (α) particles scattering experiment resulted into the discovery of nucleus of an atom. That is, during his experiment, he found that, most space of an atom is empty, and he could find a small positively charged center in an atom which is called as the nucleus.

38. Two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have surface charge densities of opposite signs and of magnitude 17.7 × 10-²² C/m². What is electric field intensity E :
(a) in the outer region of the first plate
Answer – In the outer region of first plate, electric field intensity E is Zero.

(b) between the plates ?
Answer – Electric field intensity E in between the plates is given by relation E = σ/εo
Where, εo = Permittivity of free space = 8.85 × 10-¹² N-¹ C² m-²
E = (17.7×10-²²)/(8.85×10-¹) = 2 × 10-¹⁰ N/C
Therefore, electric field between the plates is 2 × 10-¹⁰ N/C.

39. What is Radioactivity ?
Answer – Radioactivity is the property by virtue of which a heavy nuclei disintegrates itself without any external agent.

40. Discuss the formation of depletion layer in p-n junction diode.
Answer – When p-type semiconductor is joined with n-type semiconductor, electron e from the n-side diffuse towards p-side and holes from p-side diffuse towards n-sides leaving behind a layer of immobile +ve ions on n-side and immobile –ve ions on p-side leading to formation of depletion layer.

41. Arrange the following electromagnetic radiation in the ascending order of their frequencies :
X-rays, microwaves, gamma rays, radiowaves
Write two uses of microwaves.
Answer – Radiowaves < microwaves < X-rays < gamma rays
Microwaves use in microwave oven and in rader system.

42. Define the SI unit of power of a lens.
Answer – One Dioptre is the power of a lens whose focal length is one metre.

43. Bar magnet acts as an equivalent solenoid. How ?
Answer –

44. State and prove Gauss’s law in Electrostatics.
Answer – Gauss’s Theorem : The surface integral of electric field over a closed path is equal to 1/∈o times the total charge enclosed by surface.

45. Discuss three basic postulates of Bohr’s model of atom.
Answer – Postulates of Bohr’s model of atom :
(i) Every atom consists of a central core called nucleus in which entire mass and positive charge are concentrated. Electrons revolved around the nucleus in circular orbits. The necessary centripetal force is provided by the electrostatic force of attraction between the electron and nucleus. mv²/r = kze²/r²
(ii) Electron can revolve only in certain discrete non-radiating orbits called stationary orbits for which total angular momentum is an integral multiple of, h/2π where h is planck’s constant. mvr = nh/2π
(iii) The emission/absorption of energy occurs only when an electron jumps from one orbit to another. Energy is absorbed when the electron jumps from an inner to an outer orbit and energy is emitted when electron jumps from outer to inner orbit. E2 – E1 = h𝛎

46. Draw the circuit diagram of a full wave rectifier and explain its output waveform.
Answer – Full wave rectifier :

During +ve half cycle diode D1 is forward biased and diode D2 is reverse biased. The forward current flows due to D1. During –ve half cycle, diode D1 is reverse biased and diode D2 is forward biased. The forward current flows due to D2. The output waveforms is shown in figure.

47. Derive an expression for the force between two long straight parallel conductors carrying current in same direction. Hence define one ampere.
Answer –

48. State Biot-Savart’s law. Use it to determine magnetic field at axial point of current carrying coil.
Answer –

49. Draw a labelled ray diagram showing image formation in an astronomical telescope. Derive expression for its magnifying power.
Answer – Astronomical Telescope: It consists of two lenses, objective and eye piece. Objective lens is of large aperture and large focal length. Eye-piece of small aperture and focal length is used.
In normal adjustment: final image is formed at infinity.

50. State Huygen’s Principle. Using Huygen’s construction, explain reflection of a plane wave by plane surface.
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51. What is A.C. generator? With the help of circuit diagram explain its principle and working.
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52. Write any two differences between interference of light waves and diffraction.
Answer –

Interference Diffraction
1. It is the redistribution of light energy waves coming from two coherent sources. 1. It is the bending of light around corners of an obstacle.
2. All the bright fringes are of same intensity in interference pattern. 2. All the bright bands are not of the same intensity in diffraction pattern.

 

53. Derive an expression to find out an unknown resistance by plotting a labelled diagram of meter-bridge.
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54. State and prove Ampere’s circuital law.
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55. Differentiate between step-up and step-down transformer.
Answer –

Step-up transformer Step-down transformer
1. It converts low input voltage into high output voltage. 1. It converts high input voltage into low output voltage.
2. The no. of turns in its secondary coil is more than in its primary. i.e. ns > np 2. The no. of turns in its secondary coil is less than in its primary. i.e. ns < np

 

56. Write Einstein’s photoelectric equation. Find a relation between cut-off potential, frequency of the incident light and threshold frequency.
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57. Draw the circuit arrangement for studying V-I characteristics of a p-n junction diode in (i) forward biasing and (ii) reverse biasing. Draw the typical V-I characteristics of a silicon diode. Describe briefly the breakdown voltage in reverse biasing.
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58. Name two important processes involved in the formation of a p-n junction diode. With the help of circuit diagram, explain the working of junction diode as a half wave rectifier. Draw its input and output waveforms.
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59. Draw a ray diagram to show the working of a compound microscope. Obtain the expression for the total magnification for the final image to be formed at the near point.
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60. What is Total Internal Reflection? State the essential conditions for the phenomenon of total internal reflection to take place. Explain briefly with the help of a necessary diagram, how the phenomenon of total internal reflection is used in optical fibres.
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61. Using Gauss’s law deduce the expression for electric field due to a uniformly charged spherical conducting shell of radius R at a point (i) outside and (ii) inside the shell.
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62. What is a capacitor? Derive an expression for the capacitance of a parallel plate capacitor in which a dielectric medium of dielectric constant K partially fills the space between the plates. Draw the necessary diagram.
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63. Derive mathematically the conditions for constructive and destructive interference at an arbitrary point due to coherent sources in terms of phase difference ϕ.
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64. Derive lens maker formula using a convex lens.
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65. Obtain an expression for magnetic field due to a long solenoid at a point inside the solenoid and on the axis of solenoid.
Answer – Magnetic field inside a solenoid is uniform.
Let n be the no. of turns per unit length of solenoid I be the current flowing through it.
Consider a rectangular loop PQRS such that PQ = L
Total current flowing through rectangle PQRS = nLI

66. State Huygen’s Principle. Using Huygen’s construction, explain refraction of a plane wave by a plane surface.
Answer –

67. What is refraction of light?
Prove that µ1/(-u) + µ2/v = (µ21)/R, when refraction occurs from rarer to denser medium at a convex spherical refracting surface.
Answer – Refraction of light is the phenomenon of change in path of light, when it goes from one medium to another.

68. What is drift velocity? Derive expression for it? What is the relation between drift velocity and mobility ?
Answer –  Drift velocity is the average velocity with which free electrons are drifted towards +ve end of conductor under the influence of external electric field.

69. Derive expression for total energy of hydrogen atom. Draw the energy level diagram for hydrogen atom and show the different transitions in its spectrum.
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70 Derive expression for energy stored in a capacitor.
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71. Derive an expression for electric field at a point on the equitorial line of an electric dipole.
Answer – Let us consider two charges –q and +q separated by certain distance 2a form a dipole of moment p = q(2a)

72. Explain principle, construction and working of moving coil galvanometer by drawing its diagram.
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73. Derive mirror formula for concave mirror when image formed is real ?
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74. Explain the processes of nuclear fission and nuclear fusion by using the plot of binding energy per nucleon (BE/A) versus the mass number A.
Answer –

When we move from the heavy nuclei region to the middle region of the plot, there is a gain in the overall binding energy and hence release of energy. This indicates that energy can be released when a heavy nucleus (A ≈ 240) breaks into two roughly equal fragments. This process is called nuclear fission.
Similarly, when we move from lighter nuclei to heavier nuclei, we again find that there is a gain in the overall binding energy and hence release of energy. This indicates that energy can be released when two or more lighter nuclei fuse together to form a heavy nucleus. This process is called nuclear fusion.

75. Explain principle and working of p-n junction diode in fullwave rectifier ?
Answer –

Full-wave rectifier circuit

Working of Full Wave Rectifier :
During the positive half cycle, diode D1, is forward biased as it is connected to the top of the secondary winding while diode D2 is reverse biased as it is connected to the bottom of the secondary winding. Due to this, diode D1 will conductacting as a short circuit and D2, will not conductacting as an open circuit.
During the negative half cycle, the diode D1 is reverse biased and the diode D2 is forward biased because the top half of the secondary circuit becomes negative and the bottom half of the circuit becomes positive. Thus in a full wave rectifiers, DC voltage is obtained for both positive and negative half cycle.

76. State the two Kirchhoff’s rules used in then analysis of electric circuits and explain them.
Answer : Kirchhoff’s first rule (the junction rule) – The sum of all currents entering a junction must equal the sum of all currents leaving the junction.
Kirchhoff’s second rule (the loop rule) – The algebraic sum of changes in potential around any closed circuit path (loop) must be zero.

78. Derive the equation of the balanced state in a Wheatstone bridge using Kirchhoff’s laws.
Answer – In balanced condition, Ig = 0
So, VB = VD or P/Q = R/S. This is called condition of balance.

79. Prove prism formula.
Answer –

A = Prism angle
δ = Angle of deviation
i1 = Angle of incidence
i2 = Angle of emergent
In the case of minimum deviation, ∠r1 = ∠r2 = ∠r
A = ∠r1 + ∠r2
So, A = ∠r + ∠r = 2∠r
∠r = A/2
Now, again
A + δ = i1 + i2  (:. In the case of minimum deviation i1 = i2 = i and δ = δm)
So, A + δm = i + i = 2i
Now, i = A + δm/2
Now, from snell’s rule,
μ = sini/sinr
μ = sin[(A+δm)/2] ÷ sin(A/2)

 

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