# Physics Paper VI Quantum Physics And Electronics Previous Year Question Paper Year 2005 : IIIrd Year

**PART ‘A’**

**Q. 1. (a)**What is thermionic emission ? Define thermionic work-function. 1,2

**(b)**What is space charge effect ? Discuss how does it limit the plate current. 1, 2

**(c )**Describe with the help of a neat circuit diagram how the I-V characteristics of a pentode can be obtained. Compare these characteristics with output characteristic curves of a transistor.

**Answer :**

**Q. 2. (a) **Discuss how a barrier potential is formed in a p-n junction diode. How does it depend upon forward and reverse biasing ? 2, 2

**(b) **Describe the working of a full-wave rectifier. Obtain expressions for its ripple factor and efficiency.

**Answer : **

**Q. 3. (a) **What is a class-A amplifier ? Obtain an expression for gain of a class A voltage amplifier using a triode. 1, 7

**(b)** In a two-stage amplifier, using identical stages an a.c. signal of 0.2 V produces an a.c. output of 12.8 V. Determine the gain at each stage.

**Answer : **

**Q. 4. (a)** What do you understand by feedback in amplifiers? Obtain an expression for producing sustained oscillations. 1,4

**(b)** What are p- and n-type semiconductors ? Explain transistor action in an NPN transistor.

**Answer : **

**PART ‘B’**

**Q. 5. (a)**Obtain an expression for wavelength of de Broglie waves. 2

**(b)**What is the wavelength of matter-waves asociated with an electron accelerated (from rest) through a potential difference of 1.21 kV ? 2

**(c )**Describe an experiment to establish the concept of de Broglie wavelength of an electron.

**Answer :**

**Q. 6.** Obtain an expression for Planck’s formula and deduce from it Rayleigh-Jeans law and Wien’s law.

**Answer : **

**Q. 7. (a)** Derive Schrodinger wave equation and obtain the eigenvalues for a square-well potential of infinite height. 5, 2

**(b)** Explain the significance of wave-function.

**Answer : **

**Q. 8.** Write notes on any two of the following :

**(a) ** Heisenberg’s uncertainty principle;

**(b) ** Harmonic oscillator and its eigenvalues;

**(c ) ** Group and phase velocity.

**Answer : **