Tag: water distribution

I was wondering about the change in pipe sizes within a house. In many cases, wa…

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I was wondering about the change in pipe sizes within a house. In many cases, water pipes coming to a house are very large, only to drop to small pipes when they reach the house. Does this mean that the water from the water company is slow velocity, high pressure, and houses turn this water into fast velocity, low pressure?

Yes, but the effect is not so extreme. As the water from the water company enters the narrower pipes in your house, it does have to speed up slightly and its pressure does drop slightly. But its pressure is still well above atmospheric pressure. However, the fact that the water must move faster through the narrower pipes in your house means that this water loses energy relatively quickly in your house. And the more water you draw through your house’s plumbing, the larger the fraction of its energy it loses. That’s why drawing a huge amount of water out of one faucet will diminish the flow through another faucet—increasing the flow by opening that first faucet wastes the energy of the water reaching the second faucet and it flows out more slowly.

In a siphon, what makes water flow from one container to the other without a pum…

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In a siphon, what makes water flow from one container to the other without a pump?

The water is propelled by a pressure imbalance. When the water level in one container is higher than that in the other container, the pressures at the two ends of the siphon aren’t equal. There is more pressure on the high water side than on the low water side. As a result, the water accelerates toward the low water side and the water levels gradually become equal.

In the book section on Water Distribution, there was a question (exercise 5) abo…

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In the book section on Water Distribution, there was a question (exercise 5) about a novelty straw. The answer says that the straw can’t be taller than 0.5 meters. I thought you could suck liquid up a straw 10 meters tall? Why can this straw only be 0.5 meters tall?

The question itself said that the straw was only 0.5 meters tall. In the answer I was intending to point out that you can have as much tubing as you like in that straw, because it’s only 0.5 meters tall overall. I didn’t intend to mean that straws taller than 0.5 meters but shorter than 10 meters wouldn’t work. Just that a short straw will work no matter how much tubing it contains. Sorry for an imperfect answer in the book. I’ll change it in future editions.

Please define the 3 types of energy that flowing water has?

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Please define the 3 types of energy that flowing water has?

Whenever water (or any incompressible fluid) passes fixed obstacles in a laminar flow, its total energy is conserved (we’re neglecting friction effects—viscous drag). That total energy consists of (1) the water’s gravitational potential energy (how high up it is), (2) the water’s pressure potential energy (how hard it pushes on surfaces), and (3) the water’s kinetic energy (how fast it’s moving). Since the water’s total energy doesn’t change, a change in one of these forms of energy necessitates a change in one or both of the other forms. For example, if water speeds up during its flow, the water’s pressure or height or both must decrease.

Water seeks areas of lowest pressure. Is this the concept behind low-pressure we…

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Water seeks areas of lowest pressure. Is this the concept behind low-pressure weather systems bringing precipitation and high pressure bringing clear, dry conditions?

Not really. Fluids do accelerate toward lower pressures, so a low-pressure weather system does attract surface winds (the air near the surface of the earth accelerates toward regions of lower pressure). But the precipitation issues are generally related to temperature changes. Hot air can hold more moisture than cold air, so if a low-pressure system attracts air and causes hot and cold airs to mix, the new air temperature and moisture may be incompatible. When that happens, the moisture emerges from the air as water droplets and it rains.

When kinetic energy goes down (like in the Bernoulli tube), does potential energ…

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When kinetic energy goes down (like in the Bernoulli tube), does potential energy go up?

Yes. When a fluid that’s in steady state flow (moving smoothly and continuously past stationary obstacles) loses kinetic energy, its potential energy goes up—either its pressure rises or it moves upward against gravity. That assumes that the kinetic energy isn’t being lost to thermal energy because of some terrible friction problem.

Why are water towers larger on top than on the bottom?

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Why are water towers larger on top than on the bottom?

The goal of the water tower is to store water high in the air, where it has lots of gravitational potential energy. This stored energy can be converted to pressure potential energy or kinetic energy for delivery to homes. Since height is everything, building a cylindrical water tower is inefficient. Most of the water is then near the ground. By making the tower wider near the top, it puts most of its water high up.

Why can’t you pull the water up above a certain point without a pump?

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Why can’t you pull the water up above a certain point without a pump?

When you draw water up through a pipe (or straw) by removing the air inside that pipe, you are allowing the atmospheric pressure around the water to push the water up the pipe. The water experiences a pressure imbalance between the pressure around it (atmospheric pressure) and the pressure in the pipe (less than atmospheric pressure), so it accelerates into the pipe. But as the water column inside the pipe grows taller, a new problem appears: gravity. The water’s weight pushes downward and begins to oppose the pressure imbalance. At a certain height, the two effects balance and the water stops accelerating upward. When the water’s height reaches 10 m, atmospheric pressure can’t overcome this weight problem, even if all the air has been removed from the pipe.

Why does water stay in the straw when a finger is pressed over one end? How does…

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Why does water stay in the straw when a finger is pressed over one end? How does sealing off the one end make the pressure less?

When you fill a straw with water and then seal one end with your finger, you can then hold the straw vertically without any water falling out of the straw. That’s because the air pressure above the column of water decreases until the upward force caused by the unbalanced pressure at the top and bottom of the water column is exactly equal to the weight of the water column. The drop in pressure above the water column occurs because the water initial does fall downward. When you first tip the straw from horizontal to vertical, the air pressures above and below the water column are equal and there is no pressure force to opposite the weight of the water. The water begins to fall. As it does, it creates a relatively empty region above the water column and below your finger. The air molecules in that region become sparser and their pressure decreases as a result. The water descends just far enough to lower the pressure inside that trapped air region until the pressure force balances the water’s weight. Actually, the water column bounces up and down briefly, just like a weight at the end of a spring or a person at the end of a bungee cord. But after a second or so, the water column just hangs there motionless in the straw, supported against gravity by the pressure imbalance. If air could work its way through the water column and enter the trapped region between the water column and your finger, the water column would be able to descend further. But the straw is so narrow and the water sticks to tightly to itself (a phenomenon called surface tension) that it prevents air bubbles from working their way up the straw.