# Physics Paper V Atomic Physics Previous Year Question Paper Year 2006 : IIIrd Year

**Q. 1. **Answer any six questions :

(i) How many electrons are present in one Coulomb? 1

(ii) What is quantisation of charge ? 1

(iii) If the work function of a metal is 2.5 eV, calculate the stopping potential.

(iv) What is Planck’s constant ? Give its SI units. 1

(v) There are two photons having wavelength 2000 A and 3000 A respectively. Which one posseses more energy ? 1

(vi) Explain binding energy. I

(vii) Calculate the rest mass energy of an electron in MeV.

(viii) What is an atomic mass unit ?

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**Q. 2.** State the fundamental postulates of Bohr’s theory of hydrogen atom. On the basis of these postulates, derive expresÂ¬sions for the radius and energy of an an electron in the n th orbit.

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**Q. 3. (a) **Describe Millikan’s oil drop experiment for the determination of charge of an electron giving necessary theory. 5

**(b) **A charged oil drop is made stationary by a uniform electric field of 3 x 104 volt/m. Find the charge on the drop of its mass if its mass is 10~14 kg.

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**Q. 4. **What do you understand by Compton effect ? Show that the change in wavelength of an X-ray photon on scattering from a free electron is independent of the wavelength of the incident photon.

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**Q. 5. (a)** Derive the law of radioactive decay. Explain radioactive constant, half life and mean life period. Establish a relation between them. 4

**(b)** Explain the conditions under which secular and transient radioactive equilibrium takes place.

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**Q. 6. (a)** Distinguish between fission of a nucleus and fusion of two nuclei, illustrating each process by an example. 2

**(b)** Describe chain reaction and its uses in reactors. 2

**(c )** Calculate the energy released by fission of 1 kg of Ura-nium-235 in kWh. Assume the energy per fission event is 200 MeV.

Useful constants :

Electronic charge e = 1,6 x 10-19 C

Electronic mass m = 9.1 x 10~31 kg

Planck constant h = 6.62 * 10*34 J-s

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**Q. 7. ** Write notes on any two of the following:

(i) Rutherford scattering; 4

(ii) Aston’s mass spectrograph; 4

(iii) Zeeman effect.

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