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JEE Main Physics Class 12 Current Electricity Part 2 Questions
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© examsnet.com
Question : 20
Total: 100
What equal length of an iron wire and a copper-nickel alloy wire, each of
2
mm
diameter connected parallel to give an equivalent resistance of
3
Ω
?
(Given, resistivities of iron and copper-nickel alloy wire are
12
μ
Ω
cm
and
51
μ
Ω
cm
respectively)
[26 Aug 2021 Shift 1]
82
m
97
m
110
m
90
m
Validate
Solution:
Let the resistance of iron wire be
R
1
and that of copper nickel alloy wire be
R
2
r
1
=
r
2
=
1
mm
=
10
−
3
m
ρ
1
=
12
μ
Ω
cm
=
12
×
10
−
6
Ω
cm
=
12
×
10
−
8
Ω
m
ρ
2
=
51
μ
Ω
cm
=
51
×
10
−
6
Ω
cm
=
51
×
10
−
8
Ω
m
For parallel combination,
R
eq
=
R
1
R
2
R
1
+
R
2
3
=
ρ
1
l
π
r
1
2
ρ
2
l
π
r
2
2
ρ
1
l
π
r
1
2
+
ρ
2
π
r
2
2
⇒
3
=
(
12
×
10
−
8
)
l
π
×
10
−
6
×
51
×
10
−
8
l
π
×
10
−
6
12
×
10
−
8
l
π
×
10
−
6
+
51
×
10
−
8
l
π
×
10
−
6
On solving, l = 97 m
© examsnet.com
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