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QUESTIONS AND ANSWERS
 
Water Distribution
Page 1 of 3 (30 Questions and Answers)

107. Can air have gravitational potential energy?
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Yes. However, you often don't notice this because as you lower a volume of air downward, you displace a similar volume of air upward. Thus you can't just raise or lower air to observe changes in its gravitational potential energy. You'd have less trouble if you compressed the air tightly together, perhaps turning it into a liquid, and then raised or lowered it. It's gravitational potential energy would then be much more noticeable.

108. How does water move toward your mouth through a straight straw if you don't suck on the straw?
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If the straw is horizontal and the water wasn't moving to begin with, it won't move toward your mouth unless you suck. To make the water accelerate, it must experience net force and the two ways to achieve that net force are (1) to create a pressure imbalance on the water's ends and (2) to have the water's weight accelerate it. In a horizontal force with no pressure imbalance on it, there is no net force on the water and it doesn't accelerate.

109. 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?
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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.

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

111. 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?
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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.

112. Is air a fluid or a gas or both?
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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).

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

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

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

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

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