Chapter 11

NCERT
Class 10
Science
Solutions
18. Explain the following : (a) Why is tungsten used almost exclusively for filament of electric lamps? (b) Why are the conductors of electric heating devices, such as breadtoasters and electric irons, made of an alloy rather than a pure metal?

Question:

Explain the following :

(a) Why is 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 are copper and aluminum wires usually employed for electricity transmission?

Answer:

(a) Tungsten is used in the manufacture of filaments for electric lamps because tungsten has both a high melting point and high resistance. Due to the high resistance, the production of heat is also high in it. Therefore, the filaments shine very brightly after heating.

(b) Alloys are used in making bread toasters and electric irons because alloys have higher melting points and high resistivity than other pure metals. Therefore, they produce a large amount of heat.

(c) Since the resistance is very high in the series and, when the filament of one bulb is broken, the flow of current stops in all the connected bulbs, and thus all are extinguished simultaneously. Due to this, the amount of current flowing in the circuit becomes very less. But, when connected in parallel, the value of resistance is very less, due to which the value of current increases greatly. Therefore, in domestic electric circuits, the parallel is used instead of using the series connection.

(d) Since the resistance of a wire is inversely proportional to the area of the cross-section. When the area of cross-section (increased) is increased, the value of resistance decreases, i.e., R ∝ 1/A

(e) Since copper and aluminum are the best conductors of electricity and they are also cheap. This is why copper and aluminum are used for power transmission.

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