if length of wire becomes double and diameter becomes one fourth of its original value then find resistance of wire
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R = р l/ A
d= 2r
A = π r²
Now when the value changed
R' = p L/ A'
d'= 1/4 d
r' =d'/2
r'= 1/ 8 d
r'=1/4 r
A' = π r'²
A'= π r² / 16
A' = A /16
L = 2l
put values of L AND A'
R' = p L / A'
R' = p ×2 ×l×16 / A
R' = 32 p l/ A
R' = 32 R
d= 2r
A = π r²
Now when the value changed
R' = p L/ A'
d'= 1/4 d
r' =d'/2
r'= 1/ 8 d
r'=1/4 r
A' = π r'²
A'= π r² / 16
A' = A /16
L = 2l
put values of L AND A'
R' = p L / A'
R' = p ×2 ×l×16 / A
R' = 32 p l/ A
R' = 32 R
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