First slide

Resistivity of various materials

Question

Two filament bulbs A and B are connected in series combination. Filament of A is made of carbon and that of B is made of platinum. At t=0, the combination is connected to a d.c. source. Then after a long time

Easy

A

brightness of A will be more than the brightness of B
B

brightness of A will be less than the brightness of B
C

brightness of A will be same as that of B
D

Nothing can be said about the brightness of A as information given is not sufficient.

Solution

The same current flowing through the filaments of A and B with the rise of temperature, resistivity of carbon decreases and that of platinum increases since rate of heat generation is $i^{2} R$ ,and brightness is proportional to rate of heat generation. Energy loss or heat is more where resistance is more.

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