$A$ wire is stretched to increase its length by $2\%$,then the percentage change in its resistance is . . . . . . . (in $\%$)

The resistance of a wire is $20 \, \Omega$. It is stretched such that its length becomes three times its original length. The new resistance of the wire will be ............. $ \Omega$.

Wires $P$ and $Q$ have the same resistance at room temperature. When heated,the resistance of $P$ increases and that of $Q$ decreases. We conclude that:

$A$ copper wire of length $L$ and diameter $D$ is to be reshaped into another wire so as to have minimum resistance. For this,we should:

The resistivity of iron is $1 \times 10^{-7} \, \Omega \cdot m$. The resistance of an iron wire of a particular length and thickness is $1 \, \Omega$. If the length and the diameter of the wire are both doubled, then the resistivity in $\Omega \cdot m$ will be:

$A$ wire of resistance $R$ and radius $r$ is stretched until its radius becomes $r / 2$. If the new resistance of the stretched wire is $x R$,then the value of $x$ is $\qquad$