4 ms·

I think that it's actually the hydrostatic pressure at the bottom of the intake that pushes the water up, not the atmospheric pressure. Hydrostatic pressure is

by RK 16y ago

I think that it's actually the hydrostatic pressure at the bottom of the intake that pushes the water up, not the atmospheric pressure. Hydrostatic pressure is essentially only a function of depth: P = rho.g.h. Think about the design of a damn, it needs to be much thicker at the bottom due to higher pressures. You can experiment (or thought experiment) with two cups of water and a tube on the same level. Fill one cup and fill the tube. As long as the depth of one side is greater, and thus the hydrostatic pressure is greater on that side, the siphon will continue until the pressures equal. The difference in atmospheric pressures is almost negligible.

ars 16y ago
The hydrostatic pressure can not push the water higher than the level of the top of the water. The cup experiment is showing gravity, not hydrostatic pressure. There is more water in the lower tube, so it's heavier than the water on the other side.
- RK 16y ago
  I will amend my claim. In a typical siphon setup, arms of uneven length, hydrostatic pressure (a result of gravity, as explained above) is what pulls the water down. There will be greater hydrostatic pressure in the longer arm of the siphon. This is the same as what you said above. In a siphon with arms of equal lengths, the hydrostatic pressure (due to the weight of the water, if you want) in each arm is equal, but the arm in the full container has higher hydrostatic pressure at the bottom of the tube than the arm in the empty container. It is the hydrostatic pressure from the container water that pushes the water up the arm. Otherwise, the water would sit motionless in the tube, since the water pressures in the arms balance each other out. (I did this experiment in my kitchen just to make sure I wasn't crazy.) For others: If it were due to atmospheric pressure, the water should actually run the other way, since the atmospheric pressure is slightly higher at the lower end.
  - ars 16y ago
    You don't measure the length of the physical arm. What you measure is the length of the arm from the highest point to the top of the water. So if one container is less full, then the arms are NOT the same length - the arm in that container is longer. And yes, the length of the arms changes as the water moves. The hydrostatic pressure does NOT push the water up the arm! The hydrostatic pressure can not move the water any higher than the top of the water. Basically, for a siphon any water below the surface essentially does not exist. You could have a mile of water - it doesn't make any difference. The only thing that matters is the relative heights of the TOP of the water. Try it. Get a very tall container, and a short one. Position the containers so that the top of the water level is the same for both, and see what happens when you move one up or down.

by RK 16y ago

ars 16y ago

The hydrostatic pressure can not push the water higher than the level of the top of the water. The cup experiment is showing gravity, not hydrostatic pressure. There is more water in the lower tube, so it's heavier than the water on the other side.

RK 16y ago
I will amend my claim. In a typical siphon setup, arms of uneven length, hydrostatic pressure (a result of gravity, as explained above) is what pulls the water down. There will be greater hydrostatic pressure in the longer arm of the siphon. This is the same as what you said above. In a siphon with arms of equal lengths, the hydrostatic pressure (due to the weight of the water, if you want) in each arm is equal, but the arm in the full container has higher hydrostatic pressure at the bottom of the tube than the arm in the empty container. It is the hydrostatic pressure from the container water that pushes the water up the arm. Otherwise, the water would sit motionless in the tube, since the water pressures in the arms balance each other out. (I did this experiment in my kitchen just to make sure I wasn't crazy.) For others: If it were due to atmospheric pressure, the water should actually run the other way, since the atmospheric pressure is slightly higher at the lower end.
- ars 16y ago
  You don't measure the length of the physical arm. What you measure is the length of the arm from the highest point to the top of the water. So if one container is less full, then the arms are NOT the same length - the arm in that container is longer. And yes, the length of the arms changes as the water moves. The hydrostatic pressure does NOT push the water up the arm! The hydrostatic pressure can not move the water any higher than the top of the water. Basically, for a siphon any water below the surface essentially does not exist. You could have a mile of water - it doesn't make any difference. The only thing that matters is the relative heights of the TOP of the water. Try it. Get a very tall container, and a short one. Position the containers so that the top of the water level is the same for both, and see what happens when you move one up or down.