# Optionals IAS Mains Physics: Questions 150 - 155 of 306

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## Question 150

Appeared in Year: *2016*

### Describe in Detail

Essay▾Show that in the nuclear shell model, the level spacing between major oscillator shells is approx. .

### Explanation

The radial density distribution is given by

Malenka, Levinger and Kent have carried out calculation of for square wells of infinite and finite depth. The density falls off to zero smoothly at the edge for infinite walls, but faster than experimental results indicate. In the harmonic oscillators well the density is peaked at the centre and decreases

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## Question 151

Appeared in Year: *2016*

### Describe in Detail

Essay▾The equation of a progressive wave moving on a string is . In this equation, are in centimeters and is in seconds. Calculate amplitude, frequency and velocity of the wave. It two particles at any instant are situated apart, what will be the phase difference between these particles?

### Explanation

For any type of wave, transverse or longitudinal.

We have the equation .

This can be put in the form

Since and , it takes the form

Comparing it with given equation, we find it to be of the form, where (amplitude),

Since and therefore

There, amplitude of the wave , frequency , velocity and

wavelength .

In a distance , the phase difference is given by

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## Question 152

Appeared in Year: *2007*

### Describe in Detail

Essay▾Give the Boolean expression and the truth table for an XOR gate. Why NAND gates are called universal gates? Realise OR and XOR gates employing only NAND gates.

### Explanation

The circuit of an exclusive – OR gate is shown in Fig (1a). As obvious, there are two inputs and one output. The exclusive – OR operation is denoted by . Hence the expression for the output of the circuit is give as

- The symbol of an exclusive OR gate is shown

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## Question 153

Appeared in Year: *2016*

### Describe in Detail

Essay▾Why are NAND and NOR gates called universal gates? Give logic diagram, Boolean equation and truth table of a NAND gate.

### Explanation

- NAND and NOR gates are called universal gates. A universal gate is a gate which can implement any Boolean function without need to use any other gate type. In practice, it is advantageous since NAND and NOR gates are economical and easier to fabricate and the basic gates used in all IC digital logic families. In fact, an AND gate is typically imple

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## Question 154

Appeared in Year: *2007*

### Describe in Detail

Essay▾Describe Stern – Gerlach experiment. Explain how it demonstrate the discrete nature of the magnetic moment of atom.

### Explanation

- In this experiment, silver (Ag) atoms are heated in an oven, the oven has a small hole through which some of the silver atoms escape. As shown in fig (1) the beam goes through a collimator and is subjected to an inhomogeneous magnetic field produced by a pair of pole piece, one of which has a very sharp edge.
- We must now work out on the effect of th

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## Question 155

Appeared in Year: *2007*

### Describe in Detail

Essay▾Describe briefly the concept of effective mass in semiconductors and explain the significance of negative effective mass.

### Explanation

The dynamical effective mass is given by the relation:

Insofar as the motion in an electric field is concerned, the block electron behaves like a free electron whose effective mass is given by (1)

The mass is inversely proportional to the curvature of the band; where the curvature is large – that is is large - the mass is small; a small curvature

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