水壓同氣壓

When the pressure of water in the bottle (Pw) plus the pressure of air above the water in the bottle (Pa) is euqal or greater than the external atmospheric pressure (P(atm)), then the water can come out.
i.e. Pw + Pa >= P(atm), then water can come out
When the lid of the bottle is taken off, Pw = P(atm), hence,
Pw + Pa = Pw + P(atm), which is apparently greater the the external atmospheric pressure P(atm) by an amount Pw, this pressure (Pw) will then forces the water out.
When the bottle is closed, as the water comes out, the volume of air above water in the bottle increases in capacity. By Boyle's Law, the pressure will decrease and is now lower than atmospheric pressure, i.e. Pa < P(atm). The pressure decreases to such an extent when
Pa + Pw = P(atm), an equilibrium will be reached. The pressure inside the bottle is no longer strong enough to force the water out.
另外想問一問如何計算氣壓, 如果不用儀器,又能不能找到?
Pressure in a gas can be calculated by physics laws and principles, such as gas laws or kinetic theory, in an indeal situation. Unless very complicated theories are invloved, results from simple gas laws and kinetic theory would deviate much from reality for gases under high pressure.

簡單d黎講係因為有水樽個殼支持住，令到水樽唔會亂咁改變樽內的氣壓(就算加左水都係)，除非個水樽唔係地球或者有外來的施力。所以當樽蓋關起來﹑穿一小洞，水都唔會流出黎(當水流出黎個陣水樽的氣壓會少左，但係我講過水樽唔會亂咁改變樽內的氣壓，所以水唔會從水樽流出黎。

或者你會問我點解當水樽d水流出黎就減少水樽的氣壓。首先你要明白氣壓就係空氣係物件的裏面同外面支撐住，令到物件不會變形。而家我再解答問題，咁係因為水流出黎水樽空氣的位置就會多左，但係空氣的含量就無增加到，所以就只可以用同一空氣的含量黎支撐更多位置的裏面，就啫話氣壓少左。我舉一個例子，一個班房裏面本生有好多人(代表氣壓)係到支撐住課室(一樣物件的內部)，但係而家有幾個人走左出課室，咁就少左幾個人的力量去支撐課室。如果有咩問題可以隨時黎問我。

2009-07-28 11:14:29 補充：
我唔係好清楚點樣量度氣壓

將水注入水樽, 在水樽較低地方穿一小洞,打開樽蓋,小洞便會因水壓的關係而噴出水.
把樽蓋關起來, 水便開始停止噴出來, 原因外面氣壓壓著,
把樽蓋關起來,水樽變為獨立系統,氣壓變小,
把樽蓋打開內外氣壓一樣,
example -1 將水注入氣球tight up氣球,穿一小洞(球內有空氣或沒有空氣也相同的)水便開始停止噴出來,原因自身收縮將水壓出,
example -2 將水注入杯子,用卡紙蓋上.倒轉杯子,外面氣壓壓著卡紙.水也不會漏出,