Explain the following. a. Why is the tungsten used almost exclusively for filament of electric lamps? b. Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal? c. Why is the series arrangement not used for domestic circuits? d. How does the resistance of a wire vary with its area of cross-section? e. Why copper and aluminium wires are usually employed for electricity transmission?

Solution:

1. The resistivity and melting point of tungsten is very high. Due to this property, it doesn’t burn readily when heated. Electric lamps operate at high temperature. Hence, tungsten is a choice of metal for the filament of electric lamps.
2. The conductors of electric heating devices are alloys because of their high resistivity. Alloys have higher resistivity than pure metals. Due to its high resistivity, a large amount of heat is produced when current passes through it.
3. The series arrangement is not used for domestic circuits due to the following reasons:

• The overall voltage gets distributed in a series circuit. As a result, electric appliances may not get the rated power for their operation.
• All the connected appliances cannot be operated independently. If one device is defective, then the entire circuit will not function.
• The total resistance becomes large, and as a result, the current is reduced.

1. Resistance is inversely proportional to the area of cross section. When the area of cross section increases the resistance decreases and vice versa.
2. Copper and aluminium are good conductors of electricity and have low resistivity, because of which they are usually employed for electricity transmission. Due to low resistivity, the power losses in the form of heat are also significantly less when electricity is transmitted through them.