220V,100W燈泡,若接於110V電源時,其電功率多少?

如果燈泡燈絲電阻不變，計算法如下：

圖片參考：http://img376.imageshack.us/img376/8234/pvie7.png



不過，燈絲電阻會隨溫度變化而變更，所以，唔會穩定係 25W，因爲溫度低變成電阻低，好可能係 20W ~ 30W之間。

R=燈泡resistance (不變)

P = VI (P=100W, V=220)
(1) P = V^2/R ........ ohm's law R=V/I ---> I=V/R

p=vi (p=?, v=110)
(2) p=v^2/R

(2)/(1)---> p/P = v^2/V^2 .........divide equation (2) by (1), R 除掉

p/100 = (110/220)^2 = 1/4
p = 25

電功率 = 25W

R = V/I;
W = VxI
Rearange W = V x I
I = W/V
Sub into R = V / I
R = V^2 / W
First Calculate the Light Bulb resistance
R = (220)^2 / 100 = 484 ohms
Calculate the current arossing the light bulb with 110V
R = V / I ; I =V / R
I = 110 / 484 = 0.227 amps

2008-11-13 02:16:58 補充：
NB: Resistance across a component are fixed, so you should calaculate the resistance first.