Get 65% Discount on Preventive Health Checkup
Career in India        

MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Atoms Molecules and Nuclei Page 5

‹‹ Previous  |  Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |   Next ››



7.

Radioactivity

 

  • Radioactivity is the phenomenon of spontaneous disintegration of an unstable nucleus
    (
    Z > 83) where a  or b particles are emitted, accompanied by g rays.

Properties of a, b particles and g rays:

 

Property

a

b

g

1. 

Charge

+2e

- e

0

2.

Mass

- 4mp where mp = mass of proton

me where me = mass of electron

Radiation

3.

Speed when emitted

107 m/s

Upto 99% of c

c

4.

Ionization of gas/air

Intense

Moderate

Low

5.

Penetrating Power

Least

Moderate

Highest

6.

Deflection by

Deflected

Deflected

Not deflected

7.

Change in Z, A of emitting nucleus

A A - 4
Z Z - 2

Z Z + 1

No change

8.

Photographic plate

Affect

Affect

Affect

9.

Produce fluorescence?

Yes

Yes

Yes


c : speed of light

 

Rutherford-Soddy Laws or Group Displacement Laws:

(Laws of radioactive transformation)

  1. When a particle is emitted, ZXA Z - 2YA - 4 + 2He 4
  2. When b particle is emitted, ZXA Z + 1YA + - 1e0
  3. When g  rays are emitted, ZXA ZXA

 

 

8.

Law of Radioactive Decay

 

Statement: The rate of radioactive disintegration at any instant is directly proportional to the number of radioactive nuclei of the parent element present at that time.

dN

a  N 

 

dt

\

dN

= - lN 

 

dt

where l is a constant called the decay constant. Negative sign indicates that as time increases, no. of atoms decreases.

Integrating the equation

dN

= - lN, we get,

 

dt

N = N0 e -lt

This expression is called exponential law of radioactive decay.

 

Graph:

This graph shows the variation of the number of radioactive atoms (N) with time t.
Further, it shows that law of decay is exponential in nature.

 

Half Life

Half life (T) of a radioactive element is defined as the time required for half the initial number of radioactive nuclei of the element to disintegrate.

i.e. N =

N0

at t = T (half life)

 

2

Using this in the equation N = N0 e -lt we get,

T

=

0.693

 

l

t = 0 No. of atoms = N0 = N0/20

t = T

=

N0

= N0/21

 

2

t = 2T

=

N0

= N0/22

 

4

t = 3T

=

N0

= N0/23

 

8

\ t = nT

N =

N0

 

2n

 

\

N0

= 2n

, when t = nT

 

N

 

Decay Constant l = -

dN/ N

 

dt

Hence the decay constant l can be defined as the fraction of radioactive nuclei (dN/ N) disintegrating per unit time. The SI unit of l is (second)-1.

l is a measure of stability of a radioactive material.

 

 




 

‹‹ Previous  |  Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |   Next ››


Career in India | Jobs in India
© All Rights Reserved, indicareer.com | partners | Sitemap