Conductivity increases in the order of

Write the equation representing the relation between resistivity and temperature.

$A$ silver wire has a resistance of $2.1 \Omega$ at $27.5^{\circ} C$ and a resistance of $2.7 \Omega$ at $100^{\circ} C$. Then the temperature coefficient of the resistivity of silver will be . . . . . . .

$A$ strip of copper and another of germanium are cooled from room temperature to $80\, K$. The resistance of

The length of a given cylindrical wire is increased by $100\,\%$. Due to the consequent decrease in diameter,the percentage change in the resistance of the wire will be ................ $\%$.

$A$ wire of circular cross-section has an inner portion of radius $R$ made of a material of resistivity $\rho$ and is surrounded by an outer portion of thickness $R$ made of a material of double resistivity. Find the resistance of a length $l$ of such a wire.