Phy power

由於你話個燈泡與可變電阻並聯, 即係話個燈泡已經直接由電源供電, 並冇經過可變電阻, 燈泡係一個電路, 可變電阻又係另一個電路, 改變可變電阻既阻值至可以改變電源電流, 並唔可以改變電源電壓, 所以燈泡亮度保持不變(Brightness of the bulb remain unchange) 對啊！

2010-02-27 01:19:38 補充：
就算voltage沒變,可變電阻Resistance阻值有變,可變電阻Power都會有變
但係燈泡Power冇變，因爲燈泡係另一個電路，與可變電阻Power無關
係你既題目内並冇提到Power功率，只有Brightness亮度

Q: give a reason why the they are connected in parallel.
Why..? It is simply given in the question. Should the variable resistance be connected in series with the bulb, adjusting the variable resistance would change the brightness of the bulb.

Q: 就算voltage沒變,佢resistance有變,power都會有變
點解仲話power沒變,我最想知e樣野
You did not give how the ammeter is connected. From the description, it seems that the ammeter is connected either along with the variable resistor or with the battery.

Because the change in resistance in the variable resistor does not affect the current through the bulb, the bulb should remain at a steady temperature. Hence, there is no change of its resistance. THe power (brightness) of the bulb should therefore remain uncahnged.