1

### AIPMT 2008

If the lattice parameter for a crystalline structure is 3.6 $\mathop A\limits^ \circ$, then the atomic radius in fcc crystal is
A
2.92 $\mathop A\limits^ \circ$
B
1.27 $\mathop A\limits^ \circ$
C
1.81 $\mathop A\limits^ \circ$
D
2.10 $\mathop A\limits^ \circ$

## Explanation

The atomic radius in a f.c.c. crystal is = ${a \over {2\sqrt 2 }}$

$\therefore$ Atomic radius = ${{3.6} \over {2\sqrt 2 }}$ = 1.27 $\mathop A\limits^ \circ$
2

### AIPMT 2008

The voltage gain of an amplifier with 9% negative feedback is 10. The voltage gain without feedback will be
A
1.25
B
100
C
90
D
10

## Explanation

Negative feedback is applied to reduce the output voltage of an amplifier. If there is no negative feedback, the value of output voltage could be very high. In the options given, the maximum value of voltage gain is 100. Hence it is the correct option.
3

### AIPMT 2008

A p-n photodiode is made of a material with a band gap of 2.0 eV. The minimum frequency of the radiation that can be absorbed by the material is nearly
A
1 $\times$ 1014 Hz
B
20 $\times$ 1014 Hz
C
10 $\times$ 1014 Hz
D
5 $\times$ 1014 Hz

## Explanation

Energy gap Eg = 2.0 eV

Eg = 2.0 × 1.6 × 10–19 J

Eg = hf

$\therefore$ f = ${{{E_g}} \over h}$

= ${{3.2 \times {{10}^{ - 19}}} \over {6.625 \times {{10}^{ - 34}}}}$

= 5 $\times$ 1014 Hz
4

### AIPMT 2007

For a cubic crystal structure which one of the following relations indicating the cell characteristics is correct ?
A
a $\ne$ b $\ne$ c  and  $\alpha$ = $\beta$ = $\gamma$ = 90o
B
a = b = c  and  $\alpha$ $\ne$ $\beta$ $\ne$ $\gamma$ = 90o
C
a = b = c  and  $\alpha$ = $\beta$ = $\gamma$ = 90o
D
a $\ne$ b $\ne$ c  and  ;$\alpha$ $\ne$ $\beta$ $\ne$  and  $\gamma$ = 90o

## Explanation

In a cubic crystal structure
a = b = c  and  $\alpha$ = $\beta$ = $\gamma$ = 90o