什麼是電勢差?

The voltage across each light actually depends on their resistances. If it is given that the first light takes 3 volts, then it is apparent that the second light takes the rest of 6 volts in order to equalize the voltage of the 9 volt battery.
"Potential difference" is simply the difference in potential between any two points in a circuit. The presence of a potential difference would cause current to flow in the circuit.

你個圖叫並聯,,

並聯中任何地方的電壓都係一樣的,,

即係話,,

你個 Battery 9V,,

就連個燈都係9V la,,

so,,你話個燈3V,,

應該吾成立la,,

電源有電勢差自會形成電壓

2009-02-24 19:15:58 補充：
並聯中任何地方的電壓都係一樣的,,

應為,,

並聯中任何負的電壓都係一樣的,,