# Grade 12 Physics Model Question English – II

Pre-Board Examinations 2078

Physics

## Candidates are required to give their answers in their own words as far as practicable. The figures in the margin indicate fullmarks.

GROUP A

### 1.       Answer in brief, attempt any FOUR questions. [4  2=8]

a)       Two heating coils one of thin wire and the other of thick wire made of same material and of the same length are connected in parallel to a source of emf, which of the coil will produce more heat? Justify.

b)      Does the thermoelectric effect obey the law of conservation of energy? Explain.

c)       Why is the conductivity of the electrolyte low as compared to that of a metal? Explain.

d)      Distinguish between resistance, reactance and impedance for an A.C circuits.

e)       How does a permanent magnet attract an unmagnetized iron piece? Explain.

f)       A bar magnet is freely falling through the copper ring. Will the acceleration of the magnet be different from acceleration due to gravity? Justify your answer.

### 2.       Answer in brief, attempt any FOUR questions. [4  2=8]

a)    Even a hydrogen atom contains only one electron, its spectrum consists of a large number of lines. Explain why?

b)   When a large nucleus splits during nuclear fission, the daughter nuclei of fission fly apart with enormous kinetic energy. Why does this happen?

c)       Is N-type semiconductor negatively charged? Justify.

d)      Why is clock oil used in Millikan’s experiment? Justify.

e)       What is meant by global warming? Explain.

f)       What is the evidence for the expanding universe?

Answer in brief, attempt any ONE question. [1  2=2]

a)       Solid can propagate both longitudinal and transverse waves but liquid and gas can propagate only longitudinal waves. Explain.

b)      For transverse waves on a string, is the wave speed the same as the speed of any part of the string? Explain the difference between these two speeds.

### 4.       Answer in brief, attempt any ONE question. [1×2=2]

a)       Explain why diffraction of sound is quite common but not light.

b)      Define wave front. How is it related to the direction of the corresponding rays?

### GROUP B

5.       Answer any THREE questions. [3  4=12]

a)       What is meant by shunt? How can you convert a galvanometer into a voltmeter of suitable range?

b)      What is Hall Effect? Derive an expression of Hall voltage.

c)       Derive an expression for the emf induced in rectangular coil rotating in a magnetic field.

d)      What is meant by magnetic hysteresis? Sketch a typical hysteresis curve and explain what can be deduced from this about the magnetic properties of the material.

### 6.       Answer any THREE questions. [3  4=12]

a)       Describe an experiment to determine electronic charge of electron.

b)      State and explain Bragg’s law.

c)       What is rectification? Explain, with diagram, how semiconductor diodes act as full wave rectifier.

d)      Define one curie? State radioactive disintegration law and obtain the relation between half life and decay constant.

### 7.       Attempt any ONE question. [1  4=4]

a)       What are stationary waves and how are they formed? State their important properties.

b)      Describe sound wave as pressure wave and deduce an expression for the pressure amplitude.

### 8.       Attempt any ONE question. [1  4=4]

a)       Prove Snell’s law using wave theory of light.

b)      What is polarizing angle? Derive the relation between the polarizing angle and refractive index of a medium.

### GROUP C

9.       Solve, any TWO, numerical problems. [2  4=8]

a)       The e.m.f. of a battery A is balanced by a length of 75.0 cm on a potentiometer wire. The e.m.f. of a standard cell, 1.02 V, is balanced by a length of 50.0 cm. What is the e.m.f. of A?

Calculate the new balance length if A has an internal resistance of 2 and a resistor of 8 is joined to its terminals.

b)      A soft iron ring has a mean diameter of 0.2m and an area of cross section 5×10-4 m2. It is uniformly wound with 2000 turns carrying a current of 2.0A and the magnetic flux in the iron is 8.0 ×10-3 Wb. What is the permeability of the iron?

c)       A variable frequency 220 V alternating voltage source is connected across a series combination of L = 5.0H, C = 80μF and R = 40Ω. Calculate (i) the angular frequency of the source which drives the circuit in resonance, (ii) the impedance of the circuitand the amplitude of the current at resonance.

### 10.   Solve, any TWO, numerical problems. [2  4=8]

a)       In photoelectric effect experiment with light of a certain frequency, you find that a reverse potential difference of 1.25V is required to reduce the current to zero. Find a) the maximum KE and b) maximum speed of the emitted photo-electrons. (me= 9.1×10-31 Kg, e =1.6 ×10

19C)

b)      An electron is confined within a region of width 5.000 ×10-11m. (a) Estimate the minimum uncertainty in the x-component of the electron’s momentum. (b) What is the kinetic energy of an electron with this magnitude of momentum? [Planck’s constant = 6.610-34 Js]

c)       Assuming that about 200 MeV energy is released per fission of 92U235 nuclei, what would be the mass U235 of consumed per day in a nuclear rector of power 1MW approximately?

11.   A railroad train is traveling at 30.0 m/s in still air. The frequency of the note emitted by the train whistle is 262 Hz. What frequency is heard by a passenger on a train moving in the opposite direction the first at 18.0m/s and a) approaching the first; and receding from the first?

(Given speed of sound air = 344m/s) 

12.   Coherent light with wavelength 450nm falls on a double slit. On a screen

1.80 m away, the distance between dark fringes is 4.20 mm. what is the separation of slits?