究竟水在真空管道可以上升多少

E個問題有需要用到物理嘅方程式來計:

P2 = P1 + pgh

首先要做幾個假設:
1)水溫 = 4”C
2)海拔 = 海平面
3) 水係靜止嘅
4)實驗擺設係好似: http://en.wikipedia.org/wiki/File:MercuryBarometer.svg

* P1: 真空管內嘅壓力 = 0 Pa
P2: 水所受到嘅外部壓力 (即大氣壓力) = 101325 Pa
p: 水嘅密度 = 1000kg/m^3
g: 地球表面的重力加速度 = 9.81m/s^2
h: 抽真空後真空管內水面與水底嘅高度相差


解釋:

P2 = P1 +pgh
P2 – P1 = pgh
h = (P2- P1)/ pg
h = (101325Pa – 0Pa)/ [(1000kg/m^3)(9.81m/s^2)]

h = 10.3m

咁即係話, 真空管內嘅水上到去大約10.3m就冇辦法再上,因為管內嘅水重到連大氣壓力都唔夠力再將D水推上去。

我唔係好明白你所講嘅負壓係點解, 不過如果大氣壓力冇增加嘅話, 你點抽真空D水都唔會再上。仲有個方法可能令D水上升多D, 就係去降低水既密度, 即係要去除雜質。

希望解答到你嘅疑問啦!

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It is true that atmospheric pressure can help to raise a column by about 10 metres. Above which the water could not be raised further up even if you increase the negative presure of your pump. In this sense, a vacuum pump would not help. You need to use other types of pump.