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Question
1 kg piece of copper is drawn into a wire of cross-sectional area 1 mm2, and then the same wire is remoulded again into a wire of cross-sectional area 2 mm2. Find the ratio of the resistance of the first wire to the second wire.
Options:
A .  2:1
B .  4:1
C .  8:1
D .  16:1
Answer: Option B
:
B
Volume of a solid is the product of length and cross-sectional area. V=AL
Since volume is same in both the cases, the length of second wire will be half of the length of 1st wire.
Accordingly, let length of 1st wire = L
area of 1st wire= A = 1mm2
So resistance R=ρLA=ρL1=ρL
Length of the second wire =L2 and
area of 2ndwireA=2mm2
So resistance of 2nd wire R=ρL2A
R=ρL2×2
R=14(ρL)
R=14(R)
RR=41
R : R' = 4 : 1
we get the resistance of the 1st wire is 4 times the value of resistance of the 2nd wire.

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