VITEEE 2015 :: Physics :: General questions

• Physics - General questions

 The angular momentum of an electron in Bohr 's hydrogen atom whose energy is -3.4 e V , is 

Answer:

Explanation:

Energy of electron on the nth orbit of hydrogen atom

$E_{n}=-\frac{13.6}{n^{2}}eV$

 $\Rightarrow 3.4=-\frac{13.6}{n^{2}}\Rightarrow n^{2}=4$

 or n=2

 Angular momentum  of electron

 $L=\frac{nh}{2\pi}=\frac{2h}{2\pi}=\frac{h}{\pi}$

A nuclear transformation  is given by   $Y(n,\alpha)\rightarrow _{3}Li^{7}$.The nucleus of elements Y is 

Answer:

Explanation:

Y(n, $\alpha$)  the nucleus splits in to $\alpha$- particle and neutrons

 i.e,  $_{Z}Y^{A}+_{0}n^{1}\rightarrow_{3}Li^{7}+_{2}He^{4}$

 and Z+0=3 +2 or Z=5

 So, A+1=7+4

A=10
Hence , the nucleus of element Y is boron 

 $_{5}Y^{10}=_{5}B^{10}$

The half-life for $\alpha$- decay of uranium  ₉₂ U²²⁸ is  047 x10⁸ yr. If a rock contains 60% of original ₉₂ U²²⁸  atoms , then its age is [ take log 6= 0.7787 , log 2= 0.3]

Answer:

Explanation:

Given:  T_1/2 =4.47 x 10⁸

 $\Rightarrow \frac{N}{N_{0}}=\frac{60}{100}=\left(\frac{1}{2}\right)^{n}\Rightarrow 2^{n}=\frac{10}{6}$

 Apply  logarithm on both sides

 n log 2= log 10-log 6

 $\Rightarrow$   n x 0.3 = 1-0.778=0.22

$\Rightarrow n=\frac{0.222}{0.3}=0.74$

 So,  t = n T_1/2  =0.74 x 4.47x 10⁸

 or t = 3.3 x 10⁸ yr

 When a hydrogen atom is raised from ground  energy level  to excited energy level, then

Answer:

Explanation:

As r increase, the potential energy increases. Thus, it decreases the kinetic energy of the hydrogen atom. So, when an atom jumps from one energy level to a higher level, its potential energy increases and kinetic energy  decreases

• Physics - General questions