# Physics Class 12 NCERT Solutions: Chapter 3 Current Electricity Part 7 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 16. Two wires of equal length, one of aluminium and the other of copper have the same resistance. Which of the two wires is lighter? Hence explain why aluminium wires are preferred for overhead power cables.

Answer:

Resistivity of aluminium,

Relative density of aluminium,

Let be the length of aluminium wire and be its mass.

Resistance of the aluminium wire

Area of cross-section of the aluminium wire

Resistivity of copper,

Relative density of copper,

Let be the length of copper wire and be its mass.

Resistance of the copper wire

Area of cross-section of the copper wire

The two relations can be written as.

It is given that,

And,

Mass of the aluminium wire,

Mass of the copper wire,

Dividing equation (3) by equation (4) , we obtain

For

For

It can be inferred from this ratio that is less than . Hence, aluminium is lighter than copper.

Since aluminium is lighter, it is preferred for overhead power cables over copper.

Q: 17. What conclusion can you draw from the following observations on a resistor made of alloy manganin?

Current A | Voltage V | Current A | Voltage V |

Answer:

It can be inferred from the given table that the ratio of voltage with current is a constant, which is equal to . Hence, manganin is an ohmic conductor i.e.. , the alloy obeys Ohm՚s law. According to Ohm՚s law, the ratio of voltage with current is the resistance of the conductor. Hence, the resistance of manganin is .