2013
PHYSICS (Theory)
Full Marks: 70
Time: 3 hours
The figures in the margin indicate full marks for the questions
NEW COURSE
1. Answer following questions: 1x8=8
- Define one electron volt.
- State Gauss’s Law of electrostatics.
- Write the expression for Lorentz force acting on a charged particle.
- What step can be taken to achieve high resolving power of a telescope?
- Find the dimension of Planck’s constant.
- What is the change of atomic number Z of a nucleus when it emits a β-particle?
- What is the difference between analog and digital signals?
- What is a transducer?
2. What is an electric dipole? Find an expression for the torque acting on an electric dipole placed in an external uniform electric field. 1+1=2
3. If electric field E = 0 in a region do you think potential at the region should also be zero? Justify your answer. 2
Or
What is electrostatic shielding? How can it be achieved? 1+1=2
4. Find an expression for potential at a point due to a point charge. 2
Or
A 400 pF capacitor is charged by a 100 V Battery. How much electrostatic energy is stored by the capacitor? 2
5. State Kirchoff’s laws of current electric. 2
6. Establish the following relation for current following through a circuit containing external resistance R, a battery of e.m.f E and internal resistance r.
7. A charged particle enters a magnetic field with velocity v in a direction perpendicular to the field. Find an expression for the radius of the circular path of the particle. 2
8. What is Curie temperature of a ferromagnetic material? Give one example of a ferromagnetic material. 1+1
Or
What are hard ferromagnetic and soft ferromagnetic materials? Give one example of each.
9. Draw a ray diagram to show formation of virtual image by a concave mirror. 2
10. A coil of self inductance 20mH is connected to an a.c. source of 220 V and of frequency 50Hz. What is the inductive reactance and r.m.s current in the circuit? 2
11. Discuss the basic processes involved in the generation of e.m.f in a solar cell when light falls on it. 2
Or
What is energy band gap of a semiconductor? What range of energy band gap of semiconductors is suitable for using in solar cells? 1+1=2
12. How do you define mobility of a charge carrier in a conductor? Establish the following relation for mobility 1+2
13. Show that the angular frequency of a charged particle moving in a circular path in a magnetic field is independent of its velocity. 3
Or
Find the following expression for the magnetic moment of an electron moving in a circular path
Where 
is the angular momentum of the electron about the nucleus, 
and 
are its charge and mass 3
is the angular momentum of the electron about the nucleus, and are its charge and mass 3
14. Write down the four Maxwell’s equations. 3
Or
The electric field of an e.m. wave is given by
Where E is in 
, t in seconds, and x is in meters. 3
, t in seconds, and x is in meters. 3
Find (i) Propagation vector K (ii) Wavelength 
and
and
(iii) Frequency of the e.m. wave.
15. How is a wave front defined? State Huygens’ Principle of propagation of light wave. 1+2=3
16. Establish the following relation for total deviation 
of a ray of light refracted through a triangular glass prism.
of a ray of light refracted through a triangular glass prism. 
Where 
the angle of incidence; 
is the angle of emergence and 
is the angle of the prism. 3
the angle of incidence; is the angle of emergence and is the angle of the prism. 3
Or
In a Young’s double slit experiment two slits are made one millimeter apart and the screen is placed one meter away. What is the fringe separation when a monochromatic light of wavelength 500nm is used? 3
17. In a series LCR circuit in which 
a sinusoidal a.c. voltage of peak value 250V is applied. Find
a sinusoidal a.c. voltage of peak value 250V is applied. Find - Frequency at which resonance occurs.
- Current in the circuit at resonant condition.
A rectangular coil of turns 
and area 
is rotating with angular velocity 
in a uniform magnetic field B. Find an expression for the e.m.f generated in the coil. 3
and area is rotating with angular velocity in a uniform magnetic field B. Find an expression for the e.m.f generated in the coil. 3
18. What is step up and step down transformer? To transfer electric energy from generating station initially step up transformer is used. Why? 1+1+1=3
19. What is photo electric effect? Why is photo electric current proportional to the intensity of incident radiation? 2+1=3
20. Why is modulation necessary? What is amplitude modulation? Draw a block diagram of a simple modulator circuit for obtaining A. M. signal. 1+1+1=3
Or
What is modulation index? A message signal of frequency 
and peak voltage 
is used to modulate a carrier wave of frequency 
and peak voltage
. Determine its modulation index. 2+1=3
and peak voltage is used to modulate a carrier wave of frequency and peak voltage. Determine its modulation index. 2+1=3
21. Establish the following lens maker’s formula: 5
Or
Describe Young’s double slit experiment and determine the conditions for obtaining bright and dark fringes. 1+4=5
22. Draw diagrams to show how a p-n junction is (a) forward biased and (b) reverse biased. Draw V-I characteristics of the diode in both conditions and show in it break down voltage. Mention one important use of Zener diode. 1+1+1+1+1=5
Or
Draw a circuit diagram of a transistor amplifier in CE configuration. Find an expression for its a.c current gain. 2+3=5
23. State Bohr’s postulates regarding Bohr’s model of the hydrogen atom. Find an expression for the radii of the orbits of the electron of the hydrogen atom. 3+2=5
Or
State one drawback of Rutherford’s model of the atom. What modifications of the Rutherford’s model were suggested by Bohr? State two limitations of Bohr’s model of the atom. 1+2+2=5