12.1 液体压力

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  液体压力

  

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  Did you ever use a bicycle pump like the one seen in the opening image? When you push down on the handle, it forces air out through the hose, and the air enters the tire through a tiny opening. Like other fluids ( and gases), air can flow and take the shape of its container. The air that enters the tire from the pump quickly spreads out to fill the entire tire evenly. As the tire fills with air, it feels firmer. That’s because the air exerts pressure against the inside surface of the tire.
  ::你曾经使用过像在开口图像中看到的那样的自行车泵吗?当你推倒在把手上时,它迫使通过水管气流出,而空气通过一个很小的开口进入轮胎。像其他液体(和气体)一样,空气可以流动,并形成其容器的形状。从泵中进入轮胎的空气迅速向外平均扩散,以填充整个轮胎。随着轮胎充满空气,它感到更加坚固。这是因为空气对轮胎的内表面施压。

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  Why Fluids Exert Pressure
  ::为何流流体出电压

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  All fluids exert pressure like the air inside a tire. The particles of fluids are constantly moving in all directions at random. As the particles move, they keep bumping into each other and into anything else in their path. These collisions cause pressure, and the pressure is exerted equally in all directions. When particles are crowded together in one part of an enclosed space, such as the air particles first entering a tire, they quickly spread out to fill all the available space. That’s because particles of fluids always move from an area of higher pressure to an area of lower pressure. This explains why air entering a tire through a tiny opening quickly fills the entire tire.
  ::所有的液体都像轮胎中的空气一样施压。 液体的粒子不断随机地向各个方向移动。 随着粒子的移动,它们不断相撞, 并在它们行进的道路上遇到其他任何东西。 这些碰撞造成压力, 压力也在所有方向上同样地施加。 当粒子集中在封闭空间的某一部分时, 比如空气颗粒先进入轮胎, 它们迅速扩散, 以填满所有可用的空间。 这是因为液体的粒子总是从高压区域移动到低压区域。 这解释了为什么进入轮胎的空气会迅速进入一个小孔的轮胎。

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  Pressure, Force, and Area
  ::压力、力和面积

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  Pressure is defined as the amount of force acting on a given area. Therefore, pressure can be represented by this equation:
  ::压力定义为在给定区域上行动的力量量。 因此,压力可以用这个方程式表示:

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  $\begin{array}{r}\mathrm{P}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}=\frac{\mathrm{F}\mathrm{o}\mathrm{r}\mathrm{c}\mathrm{e}}{\mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}}\end{array}$
  ::压力=力量

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  Pressure shows how concentrated the force is on a given area. The smaller the area to which force is applied, the greater the pressure is. Think about pressing a pushpin, like the one in the Figure , into a bulletin board. You apply force with your thumb to the broad head of the pushpin. However, the force that the pushpin applies to the bulletin board acts only over the tiny point of the pin. This is a much smaller area, so the pressure the point applies to the bulletin board is much greater than the pressure you apply with your thumb. As a result, the pin penetrates the bulletin board with ease.
  ::压力显示压力在给定区域中的集中程度。 施用武力的区域越小, 压力就越大。 想象一下用拇指将推杆( 如图中的推杆) 压到公告牌上。 您可以用拇指对推杆的大头施压。 但是, 施压板上施压的力只对电线板的小点发生作用。 这是一个小得多的区域, 因此对公告板的压力会比用拇指施压大得多。 结果, 针会轻松地穿透公告板 。

  

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  SI Unit for Pressure
  ::压力 SI 压力单位

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  In the above equation for pressure, force is expressed in Newtons (N) and area is expressed in square meters (m ² ). Therefore, pressure is expressed in N/m ² , which is the SI unit for pressure. This unit is also called the Pascal (Pa) . It is named for the scientist Blaise Pascal whose discoveries about pressure in fluids led to a law of the same name. Pressure may also be expressed in the kilopascal (kPa), which equals 1000 Pascals. For example, the correct air pressure inside a mountain bike tire is usually about 200 kPa.
  ::在以上压力方程式中,压力以牛顿(N)表示,面积以平方米(m2)表示,因此,压力以N/m2表示,这是SI压力单位,这个单位也称为Pascal(Pa)。这个单位的名称是科学家Blaise Pascal,他发现液体中的压力导致同一名称的定律。压力也可以以千帕(kPa)表示,相当于1000帕。例如,山地自行车轮胎内正确的空气压力通常约为200千帕。

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  Calculating Pressure or Force
  ::计算压力或力

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  When you know how much force is acting on a given area, you can calculate the pressure that is being applied to the area using the equation for pressure given above. For example, assume that a rock weighs 5000 N and is resting on the ground on an area of 0.5 m ² . The pressure exerted on the ground by the rock is:
  ::当您知道在给定区域上有多少力作用时,您可以使用上述压力方程式计算对区域施加的压力。例如,假设一块岩石重5000N,正停留在0.5平方米的地面上。岩石在地面上施加的压力是:

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  $\begin{array}{r}\text{Pressure}=\frac{5000\text{N}}{0.5{\text{m}}^2}=10000{\text{N/m}}^2=10000\text{Pa, or }10\text{kPa}\end{array}$
  ::压力=500N0.5 m2=1000N/m2=1000帕,或10千帕

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  Sometimes pressure but not force is known. To calculate force, the equation for pressure can be rewritten as:
  ::有时压力而不是力是已知的。为了计算力,压力的方程式可以重写为:

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  Force = Pressure × Area
  ::力 = 压力x区域

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  For example, suppose another rock exerts 10 kPa of pressure over an area of 0.4 m ² . How much does the rock weigh? Change 10 kPa (10,000 Pa) to 10,000 N/m ² . Then substitute this value for pressure in the force equation as follows:
  ::例如,假设另一块岩石在0.4平方米的面积上产生10千帕的压力,那么岩石的重量是多少?将10千帕(10,000帕)改为10,000牛顿/平方米。

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  Force (Weight) = 10,000 N/m ² × 0.4 m ² = 4,000 N
  ::功率(重量) = 10,000 N/m2 × 0.4 m2 = 4,000 N

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  Q : The break-dancer in the Figure has a of 800 N. He is balancing on the palm of one hand. If the palm of his hand has an area of 0.02 m ² , how much pressure is he exerting on the ground?
  ::问题:图中的断肢丹赛有一个800牛顿。他正在一只手的掌上平衡。如果他的手掌有0.02平方米的面积,他到底在地面上施压多少?

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  A : Use the equation for pressure:
  ::A: 压力使用方程式 :

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  $\begin{array}{r}\text{Pressure}=\frac{800\text{N}}{0.02{\text{m}}^2}=40000\text{Pa, or }40\text{kPa}\end{array}$
  ::压力=800 N0.02平方米=40000帕,或40千帕

  

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  Q : If the break-dancer lies down on the ground on his back, his weight is spread over an area of 0.75 m ² . How much pressure does he exert on the ground in this position?
  ::问题:如果他的背部倒在地上,他的重量分布在0.75平方米的面积上,那么他在这个位置对地面施加了多少压力?

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  A : On his back, the pressure he exerts is:
  ::A:背部压力是:

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  $\begin{array}{r}\text{Pressure}=\frac{800\text{N}}{0.75{\text{m}}^2}=1100\text{Pa, or }1.1\text{kPa}\end{array}$
  ::压力=800 N0.75平方米=1100帕,或1.1千帕

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  Factors That Affect Fluid Pressure
  ::影响液体压力的因素

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  Two factors influence the pressure of fluids. They are the depth of the fluid and its density.
  ::影响液体压力的两个因素是液体的深度和密度。

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  • A fluid exerts more pressure at greater depths. Deeper in a fluid, all of the fluid above it results in more weight pressing down. This causes greater pressure the deeper you go. Watch this video below to help visualize pressure differences with depth:
    ::液体在更深的深度施加更大的压力。 液体越深, 上面所有的液体都会导致更重的压下。 这会给您越深的距离带来更大的压力。 观看下面的这个视频, 有助于以更深的深度显示压力差异 :

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  • Denser fluids such as water exert more pressure than less dense fluids such as air. The particles of denser fluids are closer together, so there are more collisions of particles in a given area. The difference in density of water and air is illustrated in the Figure .
    ::水等密度液体的压力大于空气等密度较低的液体。密度较大的液体的粒子相距较近,因此在特定地区粒子的碰撞较多。图中显示了水和空气密度的差异。

  

   

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  Summary
  ::摘要

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  • All fluids exert pressure because their particles are constantly moving and bumping into things.
    ::所有流体都施加压力 因为它们的粒子 不断移动和撞上东西
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  • Pressure is defined as the amount of force acting on a given area. It measures how concentrated a force is.
    ::压力定义为在特定区域上行动的力量量,它衡量力量的集中程度。
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  • The SI unit for pressure is the Pascal (Pa), or N/m ² .
    ::压力的SI单位为Pascal(Pa)或N/m2。
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  • To calculate pressure, divide force in Newtons (N) by area in square meters (m ² ).
    ::计算压力时,牛顿(N)的面积按平方米(平方米)计算。

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  Review
  ::回顾

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  1. Why do fluids exert pressure?
     ::为什么流体会施加压力?
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  2. Explain how a pushpin concentrates the force you apply to it.
     ::解释一个推柱如何浓缩你对它应用的力。
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  3. What is the SI unit for pressure?
     ::什么是SI单位的压力?
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  4. A large box weighing 700 N is resting on the ground on an area of 1 m ² . How much pressure is the box exerting on the ground?
     ::一个重700牛顿的大箱子 躺在1米2的地面上 这个箱子在地面上施加了多少压力?
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  5. What factors influence fluid pressure?
     ::哪些因素影响流体压力?

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  Explore More
  ::探索更多

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  W atch the demonstration showing how a siphon works. Then answer the questions below.
  ::然后回答下面的问题

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  1. Why does the liquid move into the siphon when air is removed from the siphon by the pipette?
     ::为什么液态会移到吸水器里 当空气从抽水器的抽水器里 被抽水管抽走的时候?
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  2. How does the liquid flow uphill out of the upper cup?
     ::液怎么从上杯上流到山上?
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  3. Why doesn’t the liquid in the siphon flow back down into the upper cup as well as down into the lower cup?
     ::为什么抽水中的液体不回流到上杯,再回流到下杯?