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JEE Mains 06-Sep-2020 Shift 2 Solved Paper
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© examsnet.com
Question : 2
Total: 75
Three rods of identical cross-section and lengths are made of three different materials of thermal conductivity
K
1
,
K
2
,
and
K
3
,
respectively. They are joined together at their ends to make a long rod (see figure). One end of the long rod is maintained at
100
°
C
and the other at
0
°
C
(see figure). If the joints of the rod are at
70
°
C
and
20
°
C
in steady state and there is no loss of energy from the surface of the rod, the correct relationship between
K
1
,
K
2
and
K
3
is :
K
1
:
K
3
=
2
:
3
;
K
2
:
K
3
=
2
:
5
K
1
<
K
2
<
K
3
K
1
:
K
2
=
5
:
2
;
K
1
:
K
3
=
3
:
5
K
1
>
K
2
>
K
3
Validate
Solution:
Rods are identical have same length
(
l
)
and area of cross-section (A)
Combination are in series, so heat current is same for all Rods
(
Δ
Q
Δ
t
)
AB
=
(
Δ
Q
Δ
t
)
BC
=
(
Δ
Q
Δ
t
)
CD
=
Heat current
(
100
−
70
)
K
1
A
l
=
(
70
−
20
)
K
2
A
l
=
(
20
−
0
)
K
3
A
l
30
K
1
=
50
K
2
=
20
K
3
3
K
1
=
2
K
3
K
1
K
3
=
2
3
=
2
:
3
5
K
2
=
2
K
3
K
2
K
3
=
2
5
=
2
:
5
© examsnet.com
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