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Q.
The temperature coefficient of resistance of a wire is $0.00125/K$. At $300\,K$ its resistance is 1 $\Omega$ .The resistance of the wire will be 2 $\Omega$ at
Step 1: Applying Relation between Temperature and Resistance
$
R = R _0\left(1+\alpha\left( T _2- T _1\right)\right)
$
Given Initial resistance at $300 K$ is $1 \Omega$.
Let temperature be T when final resistance becomes $2 \Omega$
$
\begin{array}{l}
\Rightarrow 2 \Omega=1 \Omega(1+0.00125( T -300 K )) \\
\Rightarrow 1=\frac{5}{4} \times 10^{-3}( T -300 K ) \\
\therefore T =1100 K
\end{array}
$