XII Physics Question Bank-pages-45

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Previous Year CBSE Questions Page 441 of 484 
 
SECTION-C ANSWERS 
 
 (1 MARK) 
1. c 
2. c 
3. c 
4. a 
5. d 
6. b 
7. a 
8. a 
9. a 
10. a 
11. CASE BASED STUDY I:(I) b (II) c (III) a (IV) c 
12. CASE BASED STUDY II:(I)b(II) c (III)a (IV) a 
(2 MARKS) 
 
13.In n-type semiconductors, electrons are the major charge carriers. In p-type semiconductors, 
holes are the major charge carriers. 
14. P-type semiconductors are made from germanium impurities that include indium. Impurities 
of a trivalent nature can be added to germanium to generate the P-type material. They are called 
acceptor impurities because they are trivalent. 
15. Because the n-type has more electrons and the p-type 
has more holes when a p-n junction is formed, the 
electrons from the n-side diffuse into the p-side & the 
holes from the p-side diffuse into the n-side. 
 
Potential Barrier 
A potential difference between the two regions is established by the buildup of electric charges of 
opposing polarities in the two regions across the junction. This is referred to as the potential or 
junction barrier. The potential barrier that has formed across the junction prevents charge carriers 
from moving from p to n and vice versa. There is a zone on each side of the intersection where 
Previous Year CBSE Questions Page 442 of 484 
 
mobile charges have depleted and only immobile charges remain. The depletion layer or zone is 
the area around the junction that is devoid of any mobile charge carriers. 
16: Energy bands : In a solid , the energy of electrons lie within certain range. The energy levels 
of allowed energy are in the form of bands, these bands are separated by regions of forbidden 
energy called band gaps. 
Distinguish features : 
 (a) In conductors : valence band and conduction band overlap each other. 
In semiconductors : Valence band and conduction band are separated by a small energy gap. 
In insulators : They are separated by a large energy gap. 
(b) In conductors : Large number of free electrons are available in conduction band. 
In semiconductor : A very small number of electrons are available for electrical conduction. 
In insulators : Conduction band is almost empty i.e., no electron is available for conduction. 
Effect of temperature : (i) In conductors : At high temperature , the collisions of electrons become 
more frequent with the atoms / molecules at lattice site in the metals as a result the conductivity 
decreases (or resistivity increases). 
(ii) In semiconductors : As the temperature of the semiconducting material increases, more 
electrons hole pairs becomes available in the conduction band and valence band , and hence the 
conductivity increases or the resistivity decreases. 
(ii) In insulators : The energy band between conduction band and valence band is very large , so 
it is unsurpassable for small temperature rise. So , there is no change in their behaviour. 
 
 
 
 
 
 
 
 
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( 3 MARKS) 
17. V-I Characteristics 
 
(i) When p-n junction is reverse biased, the majority carriers in p and n region are repelled 
away from the junction. There is small current due to the minority carriers. This current attains 
its maximum or saturation value immediately and is independent of the applied reverse 
voltage. 
(ii) As the reverse voltage is increased to a certain value, called break down voltage, large 
amount of covalent bonds in p and n regions are broken. As a result of this, large electron-
hole pairs are produced which diffuse through the junction and hence there is a sudden rise in 
the reverse current. Once break down voltage is reached, the high reverse current may damage 
the ordinary junction diode. 
18.(a) Bulb-B1 will go as the diode D1is forward biased 
 (b) Reversed biased. 
 (5 MARKS) 
19. 
a) Two important processes involved during the formation of p-n junction are: 
(i) Diffusion and (ii) drift. 
As soon as p-type semiconductor comes in contact with n-type semiconductor due to the 
different concentration gradient to charge carries, the electrons start moving towards p-side 
and the holes start moving from p-side to n-side. This process is called drift. 
Due to diffusion, the positive space charge region is created on the n-side of the junction 
and the negative space charge region is created on the p-side of the junction. Thios charge 
develops an electric field (junction field) from n-side to p-side. This field forces the free 
charges to move. This process is called drift. 
Formation of potential barrier : Electrons are the majority charge carries in n-type 
semiconductor. They move towards p-type semiconductor leaving behind the positive 
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charged ions. Similarly, holes being in majority in the p-type semiconductor, move towards 
the n-type semiconductor. They leave behind the negatively charged ions. This way the 
accumulation of charges takes place near the junction. This stops further diffusion of the 
charges and the potential drop across the function due to these fixed charges is called 
potential barrier. 
 
b) (i) In forward bias, the barrier potential decreases. 
(ii) In reverse bias, the barrier potential increases. 
20. During the first half of the 
cycle, if A is at higher potential 
with respect to centre-tap and B is 
at lower potential, the diode D1 
being forward biased conducts 
and the diodeD2 being reverse 
biased does not conduct. The current flows through the load in thesense H to L.During the second 
half of the cycle, conditions get reversed and only diodeD2 conducts Again, the current flows 
through the load in the sense H to L. 
Thus, in the output we get a unidirectional current. 
 
21. 
i) (d) Reason : In reverse biased p-n junction, potential difference across a junction 
becomes (V+VB) 
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ii) (c) 
iii) (b) Reason : D3 is in R.B. and D1 is in F.B. 
∴2 Ω and 4 Ω are in series and are connected to 12 V. 
 
iv) (d) 
 
 
 
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Question Paper for 10 Years 
CBSE Board Questions March 2020, March 2022 
March 2020 
Section A-1 Marks Questions 
1) Define the term threshold frequency in photoelectric emission. 
2) What is the impedance of a capacitor of capacitance C in an ac circuit using source of frequency 
n Hz? 
3) What is the value of impedance of a resonant series LCR circuit? 
4) The threshold frequencies of two photosensitive surfaces are v1 and v2, respectively. What is 
the ratio of the velocities of the photoelectrons emitted from these surfaces when light of frequency 
v is incident on them and photo emissions occurs? 
5) Draw the graph showing variation of the value of the induced emf as a function of rate of change 
of current flowing through an ideal inductor. 
Section B-2 Marks Questions 
1) Which of the following electromagnetic waves has a) minimum wavelength and b) minimum 
frequency? Write one use of each of these two waves. 
 Infrared waves, Microwaves, y ray, X rays 
2) A circular loop carrying a current 5A, produce a magnetic field of π mT, at its centre. Find the 
value of magnetic moment of the loop. 
3) Draw V-I characteristics of PN junction diode. Explain why current under the reverse bias is 
almost independent of applied voltage up to the critical voltage. 
4) Define the term mobility of charge carriers in a current carrying conductor. Obtain the relation 
for mobility in terms of relaxation time. 
5) Define the term drift velocity of electron in a current carrying conductor. Obtain the relationship 
between the current density and drift velocity of electrons. 
6) An object is placed in a front of convex mirror of focallength 15 cm. It produces an image that 
is half the size of the object. Find a) Position of the object, b) nature of the image and c) draw the 
ray diagram of image formation. 
7) a) Explain the term 'sharpness of resonance ' in ac circuit. 
b) In a series LCR circuit, Vl = Vc ≠ Vr. What is the value of power factor for this circuit? 
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8) An ac source of emf V = Vo sinwt is connected to a capacitor of capacitance C. Deduce the 
expression for current (I) flowing in it. Plot the graph of i) V vs wt ii) I vs wt. 
Section C-3 marks questions 
1) A capacitor of 4 uF is charged by a battery of 12 V. The battery is disconnected and a dielectric 
slab of dielectric constant 8 is inserted in between the plates of the capacitor to fill the space 
completely. Find the change in the a) charge store in the capacitor, b) potential difference between 
the plates of capacitor, and c) energy stored in the capacitor. 
2) What is the difference in the construction of an astronomical telescope and a compound 
microscope? The focal lengths of the objective and eyepiece of a compound microscope are 1.25 
cm and 5.0 cm, respectively. Find the position of the object relative to the objective in order to 
obtain an regular magnification of 30 when the final image is formed at the near point. 
3) The maximum kinetic energy of the photoelectrons emitted I doubled when the wavelength of 
light incident on the photosensitive surface changes from λ1 to λ2. Deduce the expression for the 
threshold wavelength and work function for the metal surface in terms of λ1and λ2. 
4) a) Draw the graph of radius of orbit (rn) in hydrogen atom as a function of orbit number (n). 
b) In a hydrogen atom, find the ratio of time taken by the electron to complete one revolution in 
the first excited and in the second excited states. 
5) a) Derive the condition balance for Wheatstone meter bridge. 
b) Draw the circuit diagram of a meter bridge to explain how it is based on Wheatstone bridge. 
Section D-5 marks questions 
1) a) Derived lens maker's formula for a biconvex lens. 
b) A point object is placed at a distance of 12 cm on the principal axis of a convex lens of focal 
length 10 cm. A convex mirror is placed Coaxially on the other side of the lens at a distance of 10 
cm. If the final image coincides with the object, sketch the ray diagram and find the focal length 
of the convex mirror. 
2) a) What is the wavefront? How does it propagate? Using Huygens principle, explain the 
reflection of plane wavefront from a surface and verify the laws of reflection. 
b) A parallel beam of light of wavelength 500 nm falls on a narrow slit and the resulting diffraction 
pattern is obtained on a screen 1 m away. If the first minimum is formed at a distance of 2.5 mm 
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from the centre of the screen, find the i) width of the slit, and ii) distance of first secondary 
maximum from the centre of the screen. 
3)a) Use gauss's law to show that due to a uniformly charge spherical shell of radius R, the electric 
field at any point situated outside the shell at a distance r from its centre is equal to the electric 
field at the same point, when the entire charge on the shell were concentrated at its centre. Also 
plot the graph showing the variation of electric field with r, for r ≤ R and r≥ R. 
b) Two point charges of + 1 μ C and + 4 μ C are kept 30 cm apart. How far from the +1 μ C charges 
on the line joining the two charges, will the net electric field be zero? 
4) a) A circular loop of radius R carries a current I. Obtain an expression for the magnetic field at 
a point on its axis at a distance x from its centre. 
b) A conducting rod of length 2 m is placed on a horizontal table in north South direction. It carries 
a current of 5A from south to north. Find the direction and magnitude of magnetic force acting on 
the rod. Given that the Earth magnetic field at the place is 0.6 × 10 ^ -4 T and angle of dip is π/6. 
5)a) Obtain the expression for the deflecting torque acting on the current carrying rectangular coil 
of a galvanometer in a uniform magnetic field. Why is radial magnetic field employed in the 
moving coil galvanometer? 
b) Not In syllabus 
 
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Previous Year CBSE Questions Page 449 of 484 
 
March 2022 
Section A-1 marks questions 
1) Give reason: The resistance of a pn junction is low when it is forward biased and is high when 
it is reversed biased. 
2) Doping of intrinsic semiconductors is a necessity for making electronic devices. 
Section B-2 marks questions 
1) What is mean by energy band gap in solid? Draw the energy band diagrams for a conductor, an 
insulator and a semiconductor. 
2) Name the spectral series for hydrogen atom which lies in the visible region. Find the ratio of 
maximum to the minimum wavelength of this series. 
3) What are matter waves? A proton and an alpha particle are accelerated through a same potential 
difference. Find the ratio of the de Broglie wavelength associated with the portion to that with the 
alpha particle. 
4) Not in syllabus 
Section C-3 Marks Questions 
1) Differentiate between nuclear fission and nuclear fusion. 
2) a) In Geiger Marsden experiment calculate the distance of closest approach for an alpha particle 
with energy 2.56 × 10^ -12 joule. Consider that the particle approaches gold nucleus (Z= 79) in 
head on position. 
b) In the above experiment is repeated with a proton of the same energy, then what will be the 
value the distance of closest approach? 
3) Briefly explain how bright and dark fringes are formed on the screen in Young’s double slit 
experiment. Hence derived the expression for the fringe width. 
4) i) Draw a labelled ray diagram showing the formation of the image at infinity by an astronomical 
telescope. 
ii) A telescope consists of an objective of focal length 150 cm and an eyepiece of focal length 6 
cm. If the final image is formed at infinity, then calculate: 
i) the length of the tube in this adjustment and 
ii) the magnification produced. 
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5) i) Draw a labelled ray diagram showing the formation of the image at least distance of distinct 
vision by a compound microscope. 
ii) A small object is placed at a distance of 3cm from a magnifier of focal length 4cm Find 
I) the position of the image formed, and 
II) the linear magnification produced. 
6) a) Use Einstein’s photoelectric equation to depict the variation of the maximum kinetic energy 
(Ek) of electrons emitted, with the frequency (v) of the incident radiation. 
b) A photosensitive surface is illuminated with a beam of i) yellow light, and ii) red light both have 
the same intensity. 
In which case will 
I) photoelectrons have more Ek? 
Ii) more numbers of electrons be emitted? Justify your answer in each case. 
7) a) Identify electromagnetic waves which 
i) are used in radar system 
ii) affect a photographic plate 
iii) are used in surgery. 
Write their frequency range. 
8) A plane wavefront is propagating from a rarer into a denser medium. Use Huygens principle to 
show the refracted wavefront and verify Snell’s law. 
 
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