InterviewSolution
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InterviewSolution
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How much (in per cent) has a filament diameter decreased due to evaporation if the maintances of the previous temperature due to evaporation if the maintenance of the previous temperatur required an increases of the voltage by `eta = 1.0 %` ? The amount of heat transfereed fromt the filament into surrounding space is assumed to be propotional to the filament surface area. |
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Answer» If the wire diameter decreases by `sigma` then by the information given `P` = Power input `= (V^(2))/(R)` = beat lostg throgh the surface, `H`. Now, `H prop (1 - sigma)` like the surface area and `R prop (1 - sigma)^(2)` So, `(V^(2))/(R_(0)) (1 - sigma)^(2) = A (1 - sigma)` or, `V^(2) (1 - sigma)` = constant But `V prop 1 + eta` so `(1 + eta)^(2) (1 - sigma)` = constant = 1 Thus `sigma = 2 eta = 2%` |
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