Question regarding water physics /

[wittyusername]

This will probably be obvious to most of you so my apologies in advance. Some classmates and I got into a discussion about how an automatic feeder like this (http://i.ebayimg.com/t/Auto-Refill-...QCGk~$(KGrHqEH-C0EtrFJ6qq-BLdHF((cVg~~_35.JPG ) would work. When thinking about it I had guessed that it may be similar to hydraulics where the high pressure of the column would be countered by a large surface area in the pool (where a classmate had suggested atm pressure would be part of the opposing force here).

Other classmates suggested that when the water level is low enough air can displace water (essentially creating a vacuum in the bottle). Can anyone shed light on the actual physics at play here?

Your time is much appreciated.

 

[rcgldr]

I can't see the photo, but assume that the column is sealed. The weight of the water would result in the water moving downwards somewhat until the pressure differential of 1 atm outside at the surface of the water and the lower than ambient pressure at the top of the column results in an equal and opposing upwards force on the water, preventing further flow.

 

[wittyusername]

sorry about that, hopefully that works better

maybe I didn't understand the response (and you answered it) but I'm still unsure what is preventing the entire discharge of the bottle. Is it merely pressure such as in a hydraulics equation (F1A2 = F2A1) or is it a vacuum created in the bottle?

 

[rcgldr]

Maybe I didn't understand the response (and you answered it) but I'm still unsure what is preventing the entire discharge of the bottle. Is it merely pressure such as in a hydraulics equation (F1A2 = F2A1) or is it a vacuum created in the bottle?

The vacuum created in the bottle is the source of the pressure difference, since the outside pressure remains at 1 atm. When the water level in the bowl drops below the opening in the bottle, then water will fall out and air will be drawn into the bottle, until the water level in the bowl rises above the opening in the bottle, preventing air from flowing inwards, and then the water moves down slightly resulting in a partial vacuum in the bottle.

 

[PJC01]

Hello,

The physics behind an automatic pet water feeder is actually quite interesting. As you mentioned, hydraulics do play a role in the functioning of this device. When the water level in the bowl is low, the pressure inside the bottle is greater than the pressure in the bowl. This creates a pressure difference, causing the water to flow from the bottle to the bowl until the pressure is equalized.

However, the role of air displacement should not be overlooked either. As the water level drops, air is able to enter the bottle and push the water out, creating a vacuum effect. This helps to maintain the flow of water from the bottle to the bowl.

Overall, both hydraulics and air displacement work together to keep the water flowing in an automatic pet water feeder. It's a great example of how different physical principles can work together to achieve a common goal.

I hope this helps to shed some light on the physics at play in this device. Thank you for your question and interest in understanding the science behind everyday objects. Keep asking questions and exploring the world around you!

Best,