First slide

Ohm's law

Question

If a copper wire is stretched to make it 0.1% longer, the percentage increase in resistance will be

Moderate

A

0.2
B

2
C

1
D

0.1

Solution

$R = ρ (\frac{l}{A}) = ρ (\frac{l^{2}}{Al}) or R \propto l^{2}$
for such small percentage changes $\frac{∆ R}{R} \times 100 = 2 \frac{∆ L}{L} \times 100$
( $∴$ Volume A l remains same)

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