VITEEE 2014 :: Physics :: General questions

• Physics - General questions

If the electron in the hydrogen atom jumps from the third orbit to second orbit , the wavelength of the emitted radiation in term  of Rydberg constant

Answer:

Explanation:

 As we know, $\frac{1}{\lambda}=R\left(\frac{1}{2^{2}}-\frac{1}{3^{2}}\right)=R\left(\frac{1}{4}-\frac{1}{9}\right)$

$\frac{1}{\lambda}=R\left(\frac{9-4}{36}\right)=\frac{5R}{36}$

$\therefore$    $\lambda=\frac{36}{5R}$

The activity of a radioactive sample is measured as $N_{0}$ counts per minute at t=0 and $N_{0}/C$ counts per minute at t=5 min. The time , (in minute) at  which the activity reduces to half  its value is 

Answer:

Explanation:

After  n half -lives

$\frac{N}{N_{0}}=\left(\frac{1}{2}\right)^{n}=\left(\frac{1}{2}\right)^{t/7}$

$N=\frac{N_{0}}{e}=\frac{cN_{0}}{cN_{0}}=\left(\frac{1}{2}\right)^{5/7}$

$\Rightarrow$   $\frac{1}{e}$=$\left(\frac{1}{2}\right)^{5/7}$

Taking log on both sides , we get

$\log 1-\log e= \frac{5}{7} \log \frac{1}{2}$

$-1=\frac{5}{7}(-\log 2)$

T=$5 \log_{e}2$

Now, let  t be the time after which activity reduces to half 

$\left(\frac{1}{2}\right)=\left(\frac{1}{2}\right)^{1'/5 log_{e}2}$

$t'=5 log_{e}2$

A Zener diode, having a breakdown voltage equal to 15 V  is used in a voltage regulator circuit shown in the figure. The current  through the diode is 

Answer:

Explanation:

For R=$1k \Omega$

$i_{1}=\frac{15}{1}mA=15 mA$

$R=250 \Omega$

$i_{250}=\frac{20-15}{250}=\frac{5}{250}=20mA$

$i_{Zener}=20-15=5mA$

If 1000 droplets each of potential 1V and radius r are mixed to form a big drop. Then, the potential of the drop as compared to small droplets , will be 

Answer:

Explanation:

Potential  of big drop =$q \times n^{2/3}=100 V$

In a semiconductor, separation between conduction and valence band  is of the order of 

Answer:

Explanation:

In conductor separation between conduction and valence bands is zero and in the insulator, it is greater than 1eV.

Hence, in semiconductor, the separation between conduction and valence band is $1eV'$

• Physics - General questions