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Class 12 NEET Physics Atoms and Nuclei Part 1 Questions
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© examsnet.com
Question : 1
Total: 100
Two radioactive substances
A
and
B
have decay constants
5
λ
and
λ
respectively. At
t
=
0
they have the same number of nuclei. The ratio of number of nuclei of
A
to those of
B
will be
(
1
e
)
2
after a time interval
(2007)
4
λ
2
λ
1
2
λ
1
4
λ
Validate
Solution:
Given
:
λ
A
=
5
λ
,
λ
B
=
λ
At
t
=
0
,
(
N
0
)
A
=
(
N
0
)
B
N
A
N
B
=
(
1
e
)
2
According to radioactive decay,
N
N
0
=
e
−
λ
t
∴
N
A
(
N
0
)
A
=
e
−
λ
A
t
....(i)
N
B
(
N
0
)
B
=
e
−
λ
B
t
.......(ii)
Divide (i) by (ii), we get
N
A
N
B
=
e
−
(
λ
A
−
λ
B
)
t
or,
N
A
N
B
=
e
−
(
5
λ
−
λ
)
t
or,
(
1
e
)
2
=
e
−
4
λ
t
or,
(
1
e
)
2
=
(
1
e
)
4
λ
t
or,
4
λ
t
=
2
⇒
t
=
2
4
λ
=
1
2
λ
© examsnet.com
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