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Old Wednesday, October 10, 2007
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FEDERAL PUBLIC SERVICE COMMISSION
COMPETITIVE EXAMINATION FOR RECRUITMENT TO POSTS
IN BPS – 17 UNDER THE FEDERAL GOVERNMENT, 2007.

PHYSICS, PAPER - II

TIME ALLOWED: THREE HOURS MAXIMUM MARKS:100

NOTE: (i) Attempt FIVE questions in all including question No. 8 which is compulsory. All questions carry EQUAL marks.
(ii) Extra attempt of any question or any part of the attempted question will not be considered
(iii) Candidate must draw two straight lines (==================) at the end to separate each question attempted in Answer Books.

Q.1.
Write an expression giving the capacity of parallel plate capacitor and derive an equation giving the energy stored in it. How the capacity changes when it is filled with a dielectric material? (4,10,6)

Q.2.
(a) Explain Ampere’s circuital law. Use it to find the magnetic field in a
solenoid. (6,8)
(b) A solenoid 1.33cm long and 2.6cm in diameter carries a current of 17.8A. The magnetic field inside the solenoid is 22.4mT. Find the length of the wire forming the solenoid. (6)

Q.3.
(a) Explain the term radioactivity. Write the equation for decay rate and find the expression for half life of radioactive element. How will you find the age of the radioactive sample? (2,3,6,5)
(b) A rock is formed to contain 4.2mg of U and 2mg of Pb. Assume that the rock contained no lead at formation, with all the lead now present arising from the decay of uranium. Find age of the rock. The half life of U is 4.47x10 9 years.(4)

Q.4.
(a) What is thermionic emission? Name and explain some of its important applications. (4,6)
(b) How can you use triode as detector in radio wave receivers? (10)

Q.5.
(a) What is P-N junction? Explain its working in forward and reverse biasing conditions. (4,8)
(b) Draw bridge circuit for rectification and explain how it rectifies an AC. (8)

Q.6.
(a) Explain de Broglie hypothesis and write an equation giving de Broglie wavelength. Give an experiment to prove it. (4,4,6)
(b) Calculate the de Broglie wavelength of an electron whose kinetic energy is 120eV. (6)

Q.7.
Write notes on any TWO of the following: (10 each)
(a) Pauli’s exclusion principle
(b) Zeeman effect
(c) Elementary particles

COMPULSORY QUESTION

Q.8. Write only the correct answer in the Answer Book. Do not reproduce the question.
(20)

(1) Four equal charges of +10 m C are placed at four corners of a square. The electric intensity at the center of the square is:
(a) Zero NC–1
(b) 5 NC–1
(c) 2.5 NC–1
(d) None of these

(2) A current of 200mA is flowing in a conductor. How many electrons are passing
through it in one minute?
(a) 75X1020
(b) 750X1018
(c) 75X1015
(d) None of these

(3) A particle having charge q and moving with velocity v enters in the magnetic field
B at right angles to the field. The force acting on the particle is:
(a) Bqv
(b) Bq/v
(c) Bv/q
(d) None of these

(4) Maximum voltage is induced in the conductor when it moves in the magnetic field
in the direction:
(a) Perpendicular to the field
(b) Parallel to the field
(c) At an angle of 45°to the field
(d) None of these

(5) Lines of magnetic field have the property:
(a) Lines do not terminate at the magnetic poles
(b) Lines continue to pass through the bar magnet
(c) Both (a) and (b)
(d) None of these

(6) Bohr atomic theory cannot explain:
(a) The intensity of spectral lines
(b) Atomic interaction
(c) Both (a) and (b)
(d) None of these

(7) Electron-positron annihilation gives out:
(a) Alpha-rays
(b) Beta-rays
(c) Gamma-rays
(d) None of these

(8) In photoelectric effect the energy of the photon in excess of work function of the
metal is:
(a) Absorbed by the metal
(b) Reflected back
(c) Taken by the emitted electron
(d) None of these

(9) The probability that a particle will leak through a potential barrier is known as:
(a) Tunnel effect
(b) Zeeman effect
(c) Quantum effect
(d) None of these

(10) A narrow wave group helps to determine accurately, the:
(a) Position of a particle
(b) Momentum of a particle
(c) Both (a) and (b)
(d) None of these

(11) The relationship between the atomic number (A), mass number (M), and number of neutrons (N) is:
(a) A=MN
(b) A=M–N
(c) A=N–M
(d) None of these

(12) In 1896, the radioactivity was discovered by:
(a) Curies
(b) Becqueral
(c) Dalton
(d) None of these

(13) The emission of one alpha particle from the nucleus of an atom produces a change of:
(a) –1 in atomic number
(b) –1 in atomic mass
(c) –2 in atomic number
(d) None of these

(14) Fermions have half integral spin and:
(a) Obey exclusion principle
(b) Do not obey exclusion principle
(c) May or may not obey exclusion principle
(d) None of these

(15) The conversion of atoms of a given element into atoms of a different isotope or of a different element, as in radioactive disintegration or by nuclear bombardment, is
known as:
(a) Radioisotope
(b) Transmutation
(c) Transformation
(d) None of these

(16) According to Band theory of solids, free electrons are present in:
(a) Valance band
(b) Conduction band
(c) Both (a) and (b)
(d) None of these

(17) The forbidden energy gap of a material is 0.01eV, the material is:
(a) Semiconductor
(b) Insulator
(c) Conductor
(d) None of these

(18) In a semiconductor which is heavily doped with arsenic, electrons are also called:
(a) Majority carriers
(b) Minority carriers
(c) Fast carriers
(d) None of these

(19) Among the vacuum tube devices, the ‘suppresser grid’ is present only in:
(a) Triode
(b) Tetrode
(c) Pentode
(d) None of these

(20) In a p-n-p transistor, the emitter-base junction is always forward biased while the collector-base junction is reversed biased. As a result, any changes in base current will effect the:
(a) emitter current
(b) base current
(c) collector current
(d) None of these
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