### Chapter 3: Electricity

Q
##### Electricity

Question:

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 are copper and aluminium wires usually employed for electricity transmission?

(a) The tungsten is used almost exclusively for filament of electric lamps because it has a very high melting point (3300°C). On passing electricity through a tungsten filament, its temperature reaches to 2700°C and it gives heat and light energy without being melted.

(b) The conductors of electric heating devices such as bread-toasters and electric irons, are made of an alloy rather than a pure metal because the resistivity of an alloy is much higher than that of pure metal which helps to produce high heat and an alloy does not undergo oxidation (or burn) easily even at high temperature.

(c) The series arrangement is not used for domestic circuits because in a series circuit, if one electrical appliance stops working due to some defect, then all other appliances also stop working because the whole circuit is broken and in series each device will get different voltage. So, the parallel connection is preferred over the series connection.

(d) The resistance of a wire is inversely proportional to its area of cross-section, i.e., Resistance $R \propto \frac{1}{A}$. If the area of a cross-section of a conductor of fixed length is increased, then resistance decreases because there are more free electrons for movement in the conductor.

(e) Copper and aluminium wires usually employed for electricity transmission because they have very low resistances. So, they do not become too hot on passing electric current. They are good conductors of electricity.

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