Tag: water distribution

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.

Is air a fluid or a gas or both?

Air is both a gas (a material composed of many independent particles that normally expands to fill its container) and a fluid (a material that can be reshaped easily to take on the shape of its container).

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 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 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?

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?

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 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.

Why is high pressure air/fluids slow moving, while low-pressure fluids/air are fast moving?

First, I should point out that high pressure air/fluids can move either fast or slow, depending on the situation. The same holds for low-pressure air/fluids. What Bernoulli’s equation tells us is that when air/fluids slows down, its pressure rises (assuming that it isn’t moving up or down so that gravity is out of the picture) and when air/fluids speed up, its pressure drops. Here are two common examples.

First, when you spray water from a garden hose against your hand, the water goes from moving quickly through the air at atmospheric pressure to moving slowly on your hand at more than atmospheric pressure. You know that this pressure increase has occurred because you feel the water pushing hard on your hand. The water is exchanging kinetic energy for pressure potential energy and its pressure is rising.

Second, when you put your thumb over the end of the garden hose and allow only a fine spray to emerge, the water goes from slow moving water at high pressure inside the hose to fast moving water at atmospheric pressure in the air. You know that this pressure drop has occurred because you feel the water in the hose pushing hard against your thumb. The water is exchanging pressure potential energy for kinetic energy and its pressure is dropping.

Why is it that when I am in my dorm room with my window open and the door closed, there isn’t a change in temperature and no wind comes in or blows around. But if I open the door, the room becomes cold and wind is felt throughout the room?

When the wind blows into your room, it comes to a stop and experiences a rise in pressure. This is an consequence of Bernoulli’s equation, which recognizes that energy is conserved and that in a fluid, energy can exist either as kinetic energy (energy of motion), pressure energy, or gravitational potential energy. In this case, the wind’s kinetic energy becomes pressure energy as it slow down in your room. As the pressure in your room rises, it prevents more air from entering, so you have high pressure but no movement inside your room. As soon as you open the door, the high-pressure air in your room accelerates toward the relatively low-pressure air in your hall. The pressure in your room drops and the wind can get in now. Soon the wind is blowing right through your room, as though you were part of a wind tunnel. If the wind is cold, you will be too.