# 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 aluminum wires usually employed for electricity transmission?

(a) Tungsten is used as filament in electric bulbs

The melting point and resistivity of Tungsten are very high. At high temperatures, it does not burn quite well. The temperature of the electric lamps is extremely high. Hence, tungsten is mainly used as the heating element of electric bulbs. Tungsten has a very high melting point and very high resistivity so does not burn easily at a high temperature.

(b) Conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy

The conductors of electric heating devices such as bread toasters and electric irons are made of alloy because the resistivity of an alloy is more than that of metals which produces a large amount of heat.

• At high temperatures, the alloys do not oxidize.
• Alloy does not melt readily and get deformed.
• Alloys have a low-temperature coefficient of resistance

Hence alloys are used in electric heating devices, such as bread-toasters and electric irons

(c) Series arrangement is not used for domestic circuits

In series circuits voltage is divided. Each component of a series circuit receives a small voltage so the amount of current decreases and the device becomes hot and does not work properly. Hence, the series arrangement is not used in domestic circuits.

(d) The resistance of a wire vary with its area of cross-section

Resistance (R) of a wire is inversely proportional to its area of cross-section (A), i.e. when the area of cross-section increases the resistance decreases or vice versa.

The resistance and length relation can be mathematically expressed as follows-

R=ρLA
Where,

R is the resistance measured using ohms.
L is the length of the material measured using the meter.
A is the cross-sectional area measured in m2.
𝜌 is the resistivity of the material measured using ohm/meter

(e) Copper and aluminium wires usually employed for electricity transmission

Copper and aluminium are good conductors of electricity also they have low resistivity. So they are usually used for electricity transmission. They have very low resistance and allow the current to flow through them easily. Hence, they are used to make wires for electrical transmissions.

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