The resistance of a wire is $20\, ohms$. It is so stretched that the length becomes three times, then the new resistance of the wire will be ............. $ohms$
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(d) In case of stretching of wire $R \propto {l^2}$
$ \Rightarrow $If length becomes $3$ times so Resistance becomes $9$ times
i.e. $R' = 9 \times 20 = 180\,\Omega $
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