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 are 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?

Answer 1

Hi friend,
Tungsten has highest melting point and lowest resisitivity as a result it is used as bulb filament.
Heat produced is directly proportional to resistance and resistance of alloys is higher than that of pure metals.
Series circuit is not used as in series circuits substances heats up quickly and also if a circuit breaks at some point whole connection becomes null(flow of current will stop)
Resistance=length of wire resistivity/area. therefore R is inversely proportional to area.
HOPE THIS HELPS YOU...

Answer 2

Ans. a) 

Tungsten has high melting point and high resistance. so it is used to prepare filament of electric bulbs. Due to high resistance of tungsten, large quantity of heat is produced due to which the bulb glows and due to its high melting point, it does not melt even at higher temperature.

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Ans. b).

An allow has the following characteristics:

i). It has a high melting point.

ii). It has high resistance.

iii). It has high density.

Due to high resistance, it is used to produce large heat in electric heating devices.

H = I²Rt

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Ans. c).

When appliances are connected in series, the value of equivalent resistance will be very large as 

Req = R1 + R2 + R3 + R4 .............................................

Due to high value of equivalent resistance, large quantity of heat will be produced in domestic circuit, which may cause fire in the house. But the equivalent resistance for parallel combinations is very low. This is way, series arrangement is not used for domestic circuit.

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Ans. d)

We know, 
                              R = p x l/A

Where, 
             R = resistance of a wire,
             p = resistivity of wire,
             l = length of the wire,
             A = area of cross-section.

∴ Resistance of conductor increases as radius of cross sectional area decreases and vice-versa.