509. Pressure in Liquids | 509. 液体压强

📚 509. Pressure in Liquids | 509. 液体压强

This article explores the concept of pressure in liquids, a key topic in the Edexcel IGCSE Science specification. We will look at what pressure means, how liquids exert pressure, the factors that affect it, and how to calculate it. Real-world applications such as dams, submarines and hydraulic brakes are also covered to help you prepare for exams.

本文探讨液体中的压强这一概念，它是 Edexcel IGCSE 科学课程中的一个重要考点。我们将了解压强的意义、液体如何产生压强、影响压强的因素以及如何计算压强。同时也会涉及水坝、潜艇和液压制动器等实际应用，帮助你备考。

1. What is Pressure? | 1. 什么是压强？

Pressure is defined as the force acting perpendicularly on a unit area of a surface. In everyday life, pressure tells us how concentrated a force is over a particular area. The same force applied over a smaller area produces a larger pressure, while the same force over a larger area produces a smaller pressure.

压强定义为单位面积表面所受的垂直于该表面的力。日常生活中，压强表示力在某一面积上的集中程度。相同的力作用在较小的面积上会产生较大的压强，而相同的力作用在较大的面积上则会产生较小的压强。

The basic equation for pressure from a direct force is:

p = F ÷ A 或 p = F/A

Here, p is the pressure, F is the perpendicular force in newtons (N), and A is the area in square metres (m²). The unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m².

这里，p 是压强，F 是以牛顿（N）为单位的垂直力，A 是以平方米（m²）为单位的面积。压强的单位是帕斯卡（Pa），其中 1 Pa = 1 N/m²。

In liquids, pressure is special: it is exerted not only on the bottom of the container but also on the walls and on any object immersed in the liquid. This is because the liquid particles can move and collide with every surface they touch.

液体中的压强很特殊：它不仅作用于容器底部，还会作用于器壁以及浸在液体中的任何物体。这是因为液体粒子能够运动，并与它们接触到的每个表面发生碰撞。

2. Why Do Liquids Exert Pressure? | 2. 液体为什么会产生压强？

Liquids have weight. Every small layer of liquid is pulled downwards by gravity, and this layer presses down on the layer below it. The result is a force transmitted throughout the liquid, creating pressure.

液体有重量。每一薄层液体都受重力向下拉动，这一层会压在下面的一层上。结果是一个力在整个液体中传递，从而产生压强。

Liquid particles are close together, but they are not fixed in place. They slide past each other and collide with any surface in contact with the liquid. These collisions transfer momentum to the surface and therefore produce a force on it.

液体粒子之间紧密排列，但并不固定在某一位置。它们会彼此滑动，并与接触液体的一切表面发生碰撞。这些碰撞将动量传递给表面，因而对表面产生一个力。

Pressure in a liquid acts in all directions. This means that a point inside a liquid feels pressure from above, from below, from the sides and from every angle. This is very different from a solid, which usually only presses downwards.

液体内部的压强向所有方向作用。这意味着液体中的某个点会感受到来自上方、下方、侧面以及各个角度的压强。这与固体通常只向下压迫的情况截然不同。

3. Factors Affecting Liquid Pressure | 3. 影响液体压强的因素

Three main factors control the pressure at a point inside a liquid: the depth of the point, the density of the liquid, and the gravitational field strength.

决定液体中某点压强的三个主要因素是：该点的深度、液体的密度和重力场强度。

Depth: The deeper you go below the surface, the more liquid there is above you, so the greater the weight pressing down and the larger the pressure.

深度：离液面越深，上方的液体越多，所以向下压迫的重量越大，压强也越大。
Density: A denser liquid has more mass in the same volume, so at the same depth it produces a greater pressure than a less dense liquid.

密度：密度更大的液体在相同体积内有更多质量，因此在相同深度产生的压强大于密度较小的液体。
Gravitational field strength: On a planet with stronger gravity, the liquid weighs more for the same mass, so the pressure increases.

重力场强度：在重力更强的星球上，相同质量的液体重量更大，因此压强会增大。

It is important to note that liquid pressure does not depend on the shape or width of the container, nor on the total volume of liquid present. Two containers with different shapes can have the same pressure at the same depth if they contain the same liquid.

需要注意的是，液体压强不取决于容器的形状或宽度，也不取决于液体的总体积。如果两种不同形状的容器装有同种液体，在相同深度处它们的压强可以相同。

4. The Pressure-Depth Relationship | 4. 压强与深度的关系

As depth increases, the weight of the overlying liquid becomes greater. Imagine diving into a pool: your ears feel more pressure as you go deeper down. This happens because a taller column of liquid means a larger force from gravity acting on the area.

随着深度增加，上方液体的重量变得更大。想象跳入泳池：潜得越深，耳朵感觉到的压强越大。这是因为液体柱越高，重力对面积产生的力就越大。

For a liquid of constant density, pressure is directly proportional to depth. If you double the depth, the pressure doubles, assuming all other factors are constant.

对于密度恒定的液体，压强与深度成正比。如果深度加倍，在其他条件不变的情况下，压强也加倍。

The vertical depth, not the distance travelled through a slanted pipe, must be used when comparing pressures. If a tube is angled, the pressure at a point still depends only on the vertical height of liquid above it.

在比较压强时，必须使用垂直深度，而不是沿倾斜管道移动的距离。如果管道倾斜，某点的压强仍然只取决于它上方液体的垂直高度。

5. The Pressure-Density Relationship | 5. 压强与密度的关系

At a fixed depth, a denser liquid produces a higher pressure. For example, sea water has a density of about 1025 kg/m³, while fresh water has a density of about 1000 kg/m³. A diver at the same depth in sea water experiences slightly more pressure than in fresh water.

在固定深度处，密度更大的液体产生更高的压强。例如，海水的密度约为 1025 kg/m³，而淡水的密度约为 1000 kg/m³。同一深度的潜水员在海水中受到的压强比在淡水中稍大。

Density is the mass per unit volume of a substance. It is usually measured in kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³). A substance with a high density has its particles packed closely together.

密度是物质单位体积的质量。它通常以千克每立方米（kg/m³）或克每立方厘米（g/cm³）为单位。密度高的物质，其粒子排列得很紧密。

This relationship helps to explain why pressure increases in many natural situations, such as moving from shallow fresh water to deep salt water. Both depth and density combine to produce the final pressure.

这种关系有助于解释为什么在许多自然情况下压强会增大，例如从浅层淡水移动到深层咸水。深度和密度共同作用，产生最终的压强。

6. Calculating Liquid Pressure | 6. 计算液体压强

The pressure at a point in a liquid can be calculated using the following equation:

p = h × ρ × g

In this formula, p is the pressure in pascals (Pa), h is the vertical depth in metres (m), ρ is the density of the liquid in kilograms per cubic metre (kg/m³), and g is the gravitational field strength in newtons per kilogram (N/kg).

在这个公式中，p 是以帕斯卡（Pa）为单位的压强，h 是以米（m）为单位的垂直深度，ρ 是以千克每立方米（kg/m³）为单位的液体密度，g 是以牛顿每千克（N/kg）为单位的重力场强度。

Worked example: Calculate the pressure exerted by water at a depth of 5 m. Take the density of water as 1000 kg/m³ and g as 10 N/kg.

示例：计算水在 5 m 深度处产生的压强。取水的密度为 1000 kg/m³，g 取 10 N/kg。

p = 5 × 1000 × 10 = 50,000 Pa

So the water pressure at that depth is 50,000 Pa. Remember that this is the pressure caused by the liquid alone, not including atmospheric pressure above the liquid.

因此该深度处的液体压强为 50,000 Pa。请记住，这只是液体本身产生的压强，并不包括液面上方的大气压强。

When solving problems, always check that the units are consistent. If depth is in cm, convert it to metres first. If density is in g/cm³, convert it to kg/m³ by multiplying by 1000.

解题时，务必检查单位是否一致。如果深度以厘米为单位，要先把它们换算成米。如果密度以 g/cm³ 为单位，要乘以 1000 换算成 kg/m³。

7. Hydraulic Machines | 7. 液压机械

Liquids are almost incompressible, which means their volume does not change when pressure is applied. This property is used in hydraulic machines to transfer force from one place to another.

液体几乎不可压缩，这意味着施加压强时其体积不会改变。这一特性被用于液压机械中，把力从一个地方传递到另一个地方。

In a simple hydraulic system, a force is applied to a small piston. The pressure created is transmitted equally throughout the liquid, reaching a larger piston. Because the larger piston has a bigger area, the force produced is much larger than the applied force.

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509. Pressure in Liquids | 509. 液体压强