Toilet physics question!!

[dannybht]

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Can you guys help me with this question with explaining the statements? I'm especially confused with Roman numeral 2. I thought hydrostatic pressure is greater at the input since it flows downward??

 

[sps27]

I remember this question and I had to struggle with it too. And I hated it because the input end and output end was not labeled and so I had no clue what they meant. If you see the ans choices, each one of them has II) in it except choice b. And II) is basically false and so you can eliminate all 3 of them, remaining with b, so I think I went for b with this question. Correct me if I am wrong there. The reason I eliminated 2 is because water cannot simply flow from input end to output end if output end is at higher pressure. Goes against logic. Water will flow from higher pressure to lower pressure, not the other way around.

The way I understood this, the Erkel tube is connected to the tank through aspirator tube, which empties into the Tank, kept at a higher level than the potty so that water can flow from greater height to lower height. Now when the tank is full of water, there is some volume of air in that tank. Also there is some volume of air in the Erkel tube as well. It is not completely full of water because if it was, no flushing would ever take place. When we flush, the water gushes out of the tank, the volume of air in the tank increases, which decreases the pressure in there, as a result the air in the Erkel tube at a higher atm pressure gushes in the tank due to decreased pressure which in turn creates a lower pressure in the Erkel tube (output end) and because of that the water along with its composites is flushed out of the Erkel tube (input end) which is at a higher pressure. So that is why no II) was wrong I thought because flushing happens due to water gushing out from higher pressure to a momentary lower pressure created in Erkel tube. Let me know if I am wrong.

 

[dannybht]

I remember this question and I had to struggle with it too. And I hated it because the input end and output end was not labeled and so I had no clue what they meant. If you see the ans choices, each one of them has II) in it except choice b. And II) is basically false and so you can eliminate all 3 of them, remaining with b, so I think I went for b with this question. Correct me if I am wrong there. The reason I eliminated 2 is because water cannot simply flow from input end to output end if output end is at higher pressure. Goes against logic. Water will flow from higher pressure to lower pressure, not the other way around.

The way I understood this, the Erkel tube is connected to the tank through aspirator tube, which empties into the Tank, kept at a higher level than the potty so that water can flow from greater height to lower height. Now when the tank is full of water, there is some volume of air in that tank. Also there is some volume of air in the Erkel tube as well. It is not completely full of water because if it was, no flushing would ever take place. When we flush, the water gushes out of the tank, the volume of air in the tank increases, which decreases the pressure in there, as a result the air in the Erkel tube at a higher atm pressure gushes in the tank due to decreased pressure which in turn creates a lower pressure in the Erkel tube (output end) and because of that the water along with its composites is flushed out of the Erkel tube (input end) which is at a higher pressure. So that is why no II) was wrong I thought because flushing happens due to water gushing out from higher pressure to a momentary lower pressure created in Erkel tube. Let me know if I am wrong.

The correct statements are 2 and 3 according to the book. I still don't get it. Anyone can clarify?

 

[BerkReviewTeach]

The correct statements are 2 and 3 according to the book. I still don't get it. Anyone can clarify?

An easy way to visualize the siphon (essentially want the Erkel tube is) is to consider a vertical straw filled with a liquid. It is open to the atmosphere at the top and the bottom, so each end has the same atmospheric pressure pushing on it. We all know that the liquid is going to flow out of the bottom of the straw (from experience). This is due to the hydrostatic pressure of the liquid in the straw that is pushing down. At the top of the straw, there is only the atmospheric gas exerting a downward pressure on the fluid in the straw. At the bottom of the straw, there is both the atmospheric pressure pushing down and the weight of the fluid in the straw pushing down over the unit area opposing the atmospheric pressure at the bottom that is pushing back up. The sum of the hydrostatic pressure and atmospheric pressure pushing down (P_atm + P_hydrostatic) exceeds the atmoshperic pressure pushing up, so the higher pressure pushing down at the bottom of the straw than the pressure pushing up forces the fluid to flow down and out of the straw.

If you take that straw and curve it, then you have an Erkel tube. Fluid will flow down the Erkel tube (curved straw) as long as (a) there is a weight of fluid pushing down (which happens when you flush) or b) a reduced atmospheric pressure below the straw (caused by the apiratior tube). This is what Statement II is telling you.

 

[sps27]

An easy way to visualize the siphon (essentially want the Erkel tube is) is to consider a vertical straw filled with a liquid. It is open to the atmosphere at the top and the bottom, so each end has the same atmospheric pressure pushing on it. We all know that the liquid is going to flow out of the bottom of the straw (from experience). This is due to the hydrostatic pressure of the liquid in the straw that is pushing down. At the top of the straw, there is only the atmospheric gas exerting a downward pressure on the fluid in the straw. At the bottom of the straw, there is both the atmospheric pressure pushing down and the weight of the fluid in the straw pushing down over the unit area opposing the atmospheric pressure at the bottom that is pushing back up. The sum of the hydrostatic pressure and atmospheric pressure pushing down (P_atm + P_hydrostatic) exceeds the atmoshperic pressure pushing up, so the higher pressure pushing down at the bottom of the straw than the pressure pushing up forces the fluid to flow down and out of the straw.

If you take that straw and curve it, then you have an Erkel tube. Fluid will flow down the Erkel tube (curved straw) as long as (a) there is a weight of fluid pushing down (which happens when you flush) or b) a reduced atmospheric pressure below the straw (caused by the apiratior tube). This is what Statement II is telling you.

I guess I didn't really know what a siphon is, otherwise it seems like a pretty straight forward question. One clarification though - what is the work of an aspirator tube? I thought the aspirator tube created a low pressure in the Erkel tube, a suction if you will when the flush was pressed, which pushes the potty and its contents along in the Erkel tube. Is that right?

 

[dannybht]

An easy way to visualize the siphon (essentially want the Erkel tube is) is to consider a vertical straw filled with a liquid. It is open to the atmosphere at the top and the bottom, so each end has the same atmospheric pressure pushing on it. We all know that the liquid is going to flow out of the bottom of the straw (from experience). This is due to the hydrostatic pressure of the liquid in the straw that is pushing down. At the top of the straw, there is only the atmospheric gas exerting a downward pressure on the fluid in the straw. At the bottom of the straw, there is both the atmospheric pressure pushing down and the weight of the fluid in the straw pushing down over the unit area opposing the atmospheric pressure at the bottom that is pushing back up. The sum of the hydrostatic pressure and atmospheric pressure pushing down (P_atm + P_hydrostatic) exceeds the atmoshperic pressure pushing up, so the higher pressure pushing down at the bottom of the straw than the pressure pushing up forces the fluid to flow down and out of the straw.

If you take that straw and curve it, then you have an Erkel tube. Fluid will flow down the Erkel tube (curved straw) as long as (a) there is a weight of fluid pushing down (which happens when you flush) or b) a reduced atmospheric pressure below the straw (caused by the apiratior tube). This is what Statement II is telling you.

Thank you so much! I get it now.

 

[dannybht]

I guess I didn't really know what a siphon is, otherwise it seems like a pretty straight forward question. One clarification though - what is the work of an aspirator tube? I thought the aspirator tube created a low pressure in the Erkel tube, a suction if you will when the flush was pressed, which pushes the potty and its contents along in the Erkel tube. Is that right?

That's correct. The water from the tank gives the extra potential energy to force it down I think.