Explain the following.
Why is the tungsten used almost exclusively for filament of electric lamps?
Tungsten is used almost exclusively for the filament of electric lamps because it has a very high melting point (3380°C). The filament gets very hot when electricity passes through it, and a high melting point prevents it from melting. It also has high resistivity.
Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?
The conductors of electric heating devices are made of an alloy rather than a pure metal because alloys have high resistivity (or resistance) which helps in producing more heat. They also have a high melting point and do not oxidize (burn) readily at high temperatures.
Why is the series arrangement not used for domestic circuits?
The series arrangement is not used for domestic circuits because if one appliance is switched off or fuses, the circuit breaks and all other appliances will also stop working. Additionally, in a series circuit, the voltage is divided across appliances, so they would not operate at their required potential difference.
How does the resistance of a wire vary with its area of cross-section?
The resistance of a wire is inversely proportional to its area of cross-section. This means that as the area of cross-section increases (thicker wire), the resistance decreases. Conversely, a thin wire has higher resistance.
Why are copper and aluminium wires usually employed for electricity transmission?
Copper and aluminium wires are usually employed for electricity transmission because they are good conductors of electricity with low resistivity. This minimizes the loss of energy as heat during transmission. They are also ductile and cost-effective.
Explanation
The provided context lists the questions (e.g., Intext Question 4 matches part b, Intext Question 2 relates to part d) but does not contain the explanatory text. However, based on the standard NCERT Class 10 Science curriculum concepts implied by these questions:
- (a) Tungsten's property of a high melting point is essential for filaments.
- (b) Alloys are chosen for heating devices due to high resistivity and oxidation resistance.
- (c) Series circuits are impractical for domestic use due to the disadvantage that a single fault breaks the entire circuit.
- (d) The relationship between resistance and area is inverse, as implied by the comparison of thick and thin wires in Intext Question 2.
- (e) Copper and aluminium are preferred for transmission due to low resistivity, minimizing power loss.