(1) 这盏白炽灯正常工作时的电流:I = W/V= 100W/220V = 0.4545(A).
这盏白炽灯正常工作时的电阻:R = V/I= 220V/0.4545A= 484(欧).
(2) 设:长导线的电阻为:R.
则:接在长导线上后,白炽灯的工作时的电流:I2 = 220V/(484+R).
这时白炽灯的实际电功率:81W = I2^2*484
= [220V/(484+R)]^2*484.
220*220*484/81 = 484^484+2*484*R+R^2.
R^2 + 968*R - 54949 = 0.
R = 53.78(欧).I2 = 根号(81/484) = 0.409(A).
导线上损耗的电功率:W2 = I2^2*R = 0.409*0.409*53.78
= 8.99637(W)