A potential difference is applied across the filament of a bulb at `t = 0` and it is maintained at a constant value while the filament gets heated to its equilibrium temperature. We find that final current in the filament is one fifth the current drawn at `t = 0`. If the temperature of filament at `t = 0` is `20^(@)C` and the temperature coefficient of resistivity at `20^(@)C` is `0.004(.^(@)C)^-1`, find the final temperature of the filament.

A potential difference is applied across the filament of a bulb at `t

1.	A potential difference is applied across the filament of a bulb at `t = 0` and it is maintained at a constant value while the filament gets heated to its equilibrium temperature. We find that final current in the filament is one fifth the current drawn at `t = 0`. If the temperature of filament at `t = 0` is `20^(@)C` and the temperature coefficient of resistivity at `20^(@)C` is `0.004(.^(@)C)^-1`, find the final temperature of the filament.
Answer» At constant potential different, current becomes `1//5^th` i.e., resistance is five times of initial resistance. `R_theta = 5 R_20` `R_20[1 + alpha(theta - 20)]= 5 R_20` `alpha(theta - 20) = 4` `theta - 20 = (4)/(alpha) = (4)/(0.004) = 1000` `theta = 1020.^@C`.

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