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Hi there. Came upon another tricky question in my mcat prep book.. this one makes no sense to me.

https://dl-web.getdropbox.com/get/siphon.PNG?w=389b1d3a

"A siphon is used to draw water from a water tower. What is the approximate maximum height d at which the siphon will be capable of draining the water tower nearly completely?"

A. 1m

B. 10m

C. 100m

D. There is no maximum height.

The answer is B--here is the explanation given, which, again, makes absolutely no sense to me.

"B is correct. Atmospheric pressure pushes the water up through the siphon, thus P_atmos = rho*g*y, where y is the height from the surface of the liquid to the top of the siphon. At a greater height than h, the absolute pressure would be lower than zero; an impossibility. (Remember: 10m of water creates 1 atm of pressure)"

Here are the things that don't make sense to me:

OK, atmospheric pressure is pushing on the fluid. Fine; what about the pressure from the rest of the fluid? Isn't that also pushing water into the siphon, which is at atmospheric pressure? Isn't the whole reason the siphon even works the pressure due to the rest of the fluid? Isn't one end of the siphon at atmospheric pressure, and the other end at a higher pressure due to the weight of the fluid surrounding it? And this pressure difference allows fluid to flow? Why is atmospheric pressure the only responsible party?

What is it talking about when it says 'at a greater height than h'? Why would absolute pressure be lower than zero? What does this have anything to do with the height of the siphon?

I would really appreciate it if someone could walk me through their own thought process. Personally, I didn't even understand how a definite numerical height could be calculated since no numbers were even given in the initial problem.

https://dl-web.getdropbox.com/get/siphon.PNG?w=389b1d3a

"A siphon is used to draw water from a water tower. What is the approximate maximum height d at which the siphon will be capable of draining the water tower nearly completely?"

A. 1m

B. 10m

C. 100m

D. There is no maximum height.

The answer is B--here is the explanation given, which, again, makes absolutely no sense to me.

"B is correct. Atmospheric pressure pushes the water up through the siphon, thus P_atmos = rho*g*y, where y is the height from the surface of the liquid to the top of the siphon. At a greater height than h, the absolute pressure would be lower than zero; an impossibility. (Remember: 10m of water creates 1 atm of pressure)"

Here are the things that don't make sense to me:

OK, atmospheric pressure is pushing on the fluid. Fine; what about the pressure from the rest of the fluid? Isn't that also pushing water into the siphon, which is at atmospheric pressure? Isn't the whole reason the siphon even works the pressure due to the rest of the fluid? Isn't one end of the siphon at atmospheric pressure, and the other end at a higher pressure due to the weight of the fluid surrounding it? And this pressure difference allows fluid to flow? Why is atmospheric pressure the only responsible party?

What is it talking about when it says 'at a greater height than h'? Why would absolute pressure be lower than zero? What does this have anything to do with the height of the siphon?

I would really appreciate it if someone could walk me through their own thought process. Personally, I didn't even understand how a definite numerical height could be calculated since no numbers were even given in the initial problem.

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