Question

1. The temperature of a superconductor is gradually lowered. At the critical temperature, how ( potnt) does the resistivity of the superconductor change? OIt reaches a constant positive value. OIt suddenly drops to zero. OIt begins a gradual decrease. OIt experiences a spike to its highest value. 2. A ductile metal wire has resistance R. Then the wire is stretched to three times its origina point) length. The stretching shrinks the diameter of the wire, but does not affect its density or resistivity. What is the resistance of the stretched wire? O9R O3R

Answer 1

1.

It suddenly drops to zero.

2.

$\rho=\frac {RA}{l}=\frac {R'A'}{l'}$ (Resistivity)

$\frac {RA}{l}=\frac {R'A'}{l'}$

$\frac {1}{ D^2l}=\frac {1}{D'^23l}$ (Density)

$D'^2=\frac {D^2}{3}$  

$\frac {R\pi D^2}{l}=\frac {R'\pi D'^2}{l'}$

$\frac {RD^2}{l}=\frac {R' D^2}{9l}$

$\mathbf{R'=9R}$