Identical pieces of Ge and Cu are taken and cooled, then:

1. Resistivity of both increases

2. Resistivity of both decreases

3. Resistivity of Cu increases and Ge decreases

4. Resistivity of Cu decreases and Ge increases

A potential difference of 5 V is applied across a conductor of length 10 cm. If the drift velocity of electrons is $2.5\times {10}^{-4}$ m/s, then the electron mobility will be:

1. $5\times {10}^{-4} {\mathrm{m}}^2 {\mathrm{V}}^{-1} {\mathrm{s}}^{-1}$

2. $5\times {10}^{-6} {\mathrm{m}}^2 {\mathrm{V}}^{-1} {\mathrm{s}}^{-1}$

3. $5\times {10}^{-2 }{\mathrm{m}}^2 {\mathrm{V}}^{-1} {\mathrm{s}}^{-1}$

4. Zero

The resistance of a rectangular block of copper of dimensions $2 \mathrm{mm}\times 2 \mathrm{mm}\times 5 \mathrm{m}$ between two square faces is $0.02\mathrm{\Omega }$. What is the resistivity of copper?

1. $1.6\times {10}^{-8} \mathrm{\Omega }$

2. $1.6\times {10}^{-6} \mathrm{\Omega }-\mathrm{m}$

3. $1.6\times {10}^{-8} \mathrm{\Omega }-\mathrm{m}$

4. Zero

Two wires A and B of the same material, having radii in the ratio of 1: 2 carry currents in the ratio of 4: 1. The ratio of the drift speed of electrons in A and B is:

1. 16: 1

2. 1: 16

3. 1: 4

4. 4: 1

If a copper wire is stretched to make its radius decrease by 0.1%, then the percentage increase in resistance is nearly:

1. 0.1%

2. 0.8%

3. 0.4%

4. 0.2%

A certain piece of copper is to be shaped into a conductor of minimum resistance. Its length and diameter should respectively be:

1. L, D

2. 2L, $\frac{\mathrm{D}}{2}$

3. $\frac{\mathrm{L}}{2}$, 2D

4. $\mathrm{L}, \frac{\mathrm{D}}{2}$

A wire of resistance x ohm is drawn out so that its length is increased to twice its original length and its new resistance becomes $20\mathrm{\Omega }$, then x will be:

1. 5 $\mathrm{\Omega }$

2. 10 $\mathrm{\Omega }$

3. 15 $\mathrm{\Omega }$

4. 20 $\mathrm{\Omega }$

A piece of wire is cut into four equal parts and the pieces are bundled together side by side to form a thicker wire. Compared with that of the original wire, the resistance of the bundle is:

1. The same

2. $\frac{1}{16}$ as much

3. $\frac{1}{8}$ as much

4. $\frac{1}{4}$ as much

The temperature coefficient of resistance of a wire at $0°\mathrm{C}$ is $0.00125°{\mathrm{C}}^{-1}$. At $25°\mathrm{C}$, its resistance is one ohm. The resistance of the wire will be 1.2 ohms at:

1. 225 K

2. $190°\mathrm{C}$

3. $260°\mathrm{C}$ 

4. 185 K

At conductor behaves as a superconductor

1. Above the critical temperature

2. At the critical temperature

3. At $100°\mathrm{C}$

4. At the boiling point of that metal