如何計算出....歐姆

電阻計算法如下係對既

圖片參考：http://img443.imageshack.us/img443/8421/vped0.png



不過，你要留意，燈泡燈絲計法唔同，不可相提並論。
唔信？用表兩下就知道啦。

W = VI = V(V/R) = V^2/R (R=V/I ohms law, I=V/R)
R = V^2/W

R = 220 x 220 / 100 = 484 ohms

Consider the following equation:
1.) Watts(W) = Voltage(V) x Current(I) ;
2.) Resistance (R) = Voltage (V) / Current (I)
Find out the Current pass throw a light bulb:
By Equation 1.) W = V x I ;
100 = 220 x I ;
I = 100 / 220 ;
I = 0.4545 Amps; (Or you can leave it as 5 / 11, this then you won't loss the digit)
Find out the resistance on the light bulb:
By Equation 2.) R = V / I ;
R = 220 / 0.4545 ;
R = 484 Ohms

2008-11-13 02:06:19 補充：
Alternatively you can combine the two equation into one to ease the difficulty on sub in value
Eq 1.) W = VxI ;
Eq 2.) R = V/I ;

Rewrite equation Eq1.) :
I = W / V -- Eq 3.)

Sub in Eq 3.) to Eq 2.):
R = V / (W / V)
R = V^2 / W -- Eq 4.)

Sub in value V = 220 W = 100 to Eq 4.)
R = (220)^2 / 100 = 484 ohms