Understanding the Water Pump: Gravity and Atmospheric Pressure

[Oomph!]

Hello.
I saw this video: ... it is about water pump.

I want to know how it works. First straw is above the water surface, second is below the water surface. To the straw below the water surface goes watter because force of gravity act on it. This is why air column increases and makes it underpressure. So, the underpressure acts to the water in first bottle and sucks the water to maintaining atmospheric pressure. It is right?

However, there is one think which I really don't understant. Why the first glass must be higher than second glass?

 

[Drakkith]

It's a syphon. See here: http://en.wikipedia.org/wiki/Syphon#Theory

The full glass has to be higher than the empty glass so that it has gravitational potential energy for the syphon to use. In other words, the water needs height to flow down the straw and suck water up the other straw. If the height of the glasses were the same, the water would have to flow the same distance up as it does down and there would be no difference in force.

 

[sophiecentaur]

Water flows down the long tube because there is a difference in hydrostatic pressure between the top and bottom liquid surfaces. The pressure in the bottle is Atmospheric minus the above difference. So there is an overall negative pressure difference between the top and bottom of the short tube. Water goes up into the bottle because of this. The pressure difference is so great in the video that the water emerges at the top in a fountain.