燈泡光度取決於什麼???

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You asked:
Which one represents the brightness of the bulb? Circuit or voltage?
Both of them. The brightness of the bulb is take the WATT (W), equal to Voltage (V) X circuit (A).
For example:
A bulb that is 4.8V and 0.5A, this equal to how many watts?
Watts=4.8X0.5
=2.4W
In daily life, we can find the brightness of the bulbs by checking the watt. By using 220V, the smallest watts is 25W, and the biggest can be up to 200W. The bulb is brighter by incrasing the watts. And the save-energy bulbs is calculated in other format, example: The save-energy bulb 11W equals to a normal bulb 60W.

2007-01-31 00:18:21 補充：
有些字不明白？名詞解釋：WWatt，單位為瓦特，即是電功率。VVolt，單位為伏特，即電壓。AAmp，單位為安培，即電流。

不同火(watt)數的燈泡有不同的光度, 因為不同功率的燈泡, 光管, 慳電燈泡等, 它的內阻值都是不同的.
功率的計算除了=VxI 外, 亦可以是 IxIxR
因此不同火數的燈泡同樣接於 220V 的市電下, 但它們的內阻值都是不同的, 其功率消耗亦不同.
例: 一枚 100W 的燈泡它的內阻約是 50歐姆, 耗用之電流約是 0.455A. 因此電費較多.
一枚 10W 的燈泡它的內阻約是 5000歐姆, 耗用之電流約是 0.0455A. 因此電費較少.

即 燈泡的光度取決於 功率數(內阻值).

Both!

Watt is determination of maximum brightness it can reach.

And Watt = Current(電流) x Voltage(電壓)

So both are part of bulb’s brightness determination.