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Physics
SET-II
Q.1. Write the relation for the force acting on a charged particle q moving with velocity 
Answer:
Q.2. Draw the pattern of electric field lines due to an electric dipole.
Answer:
Q.3. Identify the semiconductor diode whose I-V characteristics are as shown.
Answer: Photodiode/Solar cell.
Section – B
Q.4. How is the equation for Ampere’s circuital law modified in the presence of displacement current? Explain.
Answer:
Case-1
Let the case-1 where a point P is considered outside capacitor charging from Ampere’s law. Magnetic field at point will be
Case-2
Now take case-2 where shape surface under consideration covers capacitor’s plate as we consider there is no current through capacitor then this value of B will be zero.
Hence, there is a contradiction. Therefore this Ampere’s law was modified with addition of displacement current inside capacitor.
Where, Id displacement current.
During charging of capacitor, outside the capacitor, Ic (conduction current) flows and inside Id displacement current flows.
Q.5. Under what conditions does the phenomenon of total internal reflection take place? Draw a ray diagram showing how a ray of light deviates by 90° after passing through a right-angled isosceles prism.
Answer: The phenomenon of total internal reflection occurs when,
- Angle of incidence is equal or greater than the critical angle. i ≥ C
- When light travels from a denser medium to less denser medium.In case of right angle isosceles triangle if light rays fall normally on AB then light incident face AC with angle of incidence > critical angle.
Hence, total internal reflection will occur with normal to the surface of BC.
Q.6. A beam of light converges at a point P. Draw ray diagrams to show where the beam will converge if
(i) a convex lens, and
(ii) a concave lens is kept in the path of the beam.
Answer:
Section – C
Q.7. (a) How is the stability of hydrogen atom in Bohr model explained by de-Broglie’s hypothesis?
(b) A hydrogen atom initially in the ground state absorbs a photon which excites it to n = 4 level. When it gets de-excited, find the maximum number of lines which are emitted by the atom. Identify the series to which these lines belong. Which of them has the shortest wavelength?
Answer: (a) From Bohr’s model-An atom has a number of stable orbits in which an electron can reside without the emission of radiant energy. Each orbit corresponds to a certain energy level. Electron revolves is circular orbit
The motion of an electron in circular orbits is restricted in such a manner that its angular momentum is an integral multiple h/2Π
Q.8. What is the reason to operate photo-diodes in reverse bias? A p-n photodiode is fabricated from a semiconductor with a band gap of range of 2.5 to 2.8 eV. Calculate the range of wavelengths of the radiation which can be detected by the photodiode.
Answer : Photo diode are reverse biased for working in photo-conductive mode. This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction capacitance. The reverse bias also increases the dark current without much change in the photo current.
The range of wavelengths of radiation which can be detected by the photodiode is 443.97 nm to 497.25 nm.
Q.9. A ray of light incident on the face AB of an . isosceles triangular prism makes an angle of incidence (i) and deviates by angle P as shown in the figure. Show that in the position of minimum deviation ∠β = ∠α . Also find out the condition when the refracted ray QR suffers total internal reflection.
Answer:
Q.10. A 100 μF parallel plate capacitor having plate separation of 4 mm is charged by 200 V dc. The source is now disconnected. When the distance between the plates is doubled and a dielectric slab of thickness 4 mm and dielectric constant 5 is introduced between the plates, how will
(i) its capacitance,
(ii) the electric field between the plates, and
(iii) energy density of the capacitor get affected? Justify your answer in each case
Answer:
As dielectric of 4 mm is inserted between the plates of capacitor and the spacing between the plates is doubled then it will acts as following Fig-A and Fig-B.
Physics
SET-III
Q.1. Draw a pattern of electric field lines due to two positive charges placed a distance d apart.
Answer : Electric field lines due to two positive charge placed at a distance d apart:
Q.2. When a charge q is moving in the presence of electric (E) and magnetic (B) fields which are perpendicular to each other and also perpendicular to the velocity v of the particle, write the relation expressing v in terms of E and B.
Answer : v = E/B
Q.3. Draw the I-V characteristics of a Zener diode.
Answer:
Section – B
Q.3. State, with the help of a ray diagram, the working principle of optical fibers. Write one important use of optical fibers.
Answer:
Optical fiber works on the principle of total internal reflection. When the angle of incidence is greater than Critical angle then incident rays are totally reflected back in the same media.
When, θi < θc , Total internal reflection occurs and if θi < θc , refraction occurs.
Application: Optical fiber are used for communication due to very high bandwidth of media.
Q.4. How are electromagnetic waves produced by oscillating charges ? What is the source of the energy associated with the em waves.
Answer : Oscillating charges are responsible for the generation of periodically varying electric field in the space. The oscillating charges generate varying electric current which in turn is responsible for the generation of periodical varying magnetic field. This way the electromagnetic waves are generated.
Q.5. The wavelength of light from the spectral emission line of sodium is 590 nm. Find the kinetic energy at which the electron would have the same deBroglie wavelength.
Section – C
Q.6. (a) Draw the energy level diagram for the line spectra representing Lyman series and Balmer series in the spectrum of hydrogen atom.
(b) Using the Rydberg formula for the spectrum of hydrogen atom, calculate the largest and shortest wavelengths of the emission lines of the Balmer series in the spectrum of hydrogen atom. (Use the value of Rydberg constant R = 1.1 x 107 m-1
Answer: a) Energy level diagram showing lyman and balmer series :
Spectrum wavelengths of both series for hydrogen atom
Q.7. In a network, four capacitors C1, C2, C3 and C4 are connected as shown in the figure.
(a) Calculate the net capacitance in the circuit.
(b) If the charge on the capacitor C1 is 6 μC,
(i) calculate the charge on the capacitors C3 and C4 and,
(ii) net energy stored in the capacitors C3 and C4 connected in series.
Answer: (a)
Q.8.Draw the circuit diagram of a full wave rectifier. Explain its working principle. Show the input waveforms given to the diodes D1 and D2 and the corresponding output waveforms obtained at the load connected to the circuit.
Answer:
During the first half of input sinusoidal ac signal diode D1 conducts as it is forward bias and during the second half of input ac signal diode D2 conducts as it is forward bias now. D2 and D1 are inverse bias conditions during first and second half respectively and doesn’t conduct. Due to this output appears as waveform (b).
Q.9. (a) When a convex lens of focal length 30 cm is in contact with a concave lens of focal length 20 cm, find out if the system is converging or diverging.
(b) Obtain the expression for the angle of incidence of a ray of light which is incident on the face of a prism of refracting angle A so that it suffers total internal reflection at the other face. (Given the refractive index of the glass of the prism is μ).
Answer:
