📚 Standard Atmospheric Pressure and 760 mmHg | 标准大气压与760毫米汞柱
At sea level, the atmosphere pushes on every surface with a force that supports a column of mercury exactly 760 mm high. This value, 760 mmHg, is one of the most important reference points in IGCSE science.
在海平面，大气对每一个表面施加的力恰好可以支撑起 760 毫米高的汞柱。这个数值——760 mmHg——是 IGCSE 科学中最重要的参考点之一。
1. What is Pressure? | 什么是压强？
Pressure is defined as the force acting perpendicularly on a unit area. The equation is pressure = force ÷ area.
压强定义为垂直作用在单位面积上的力。公式为：压强 = 力 ÷ 面积。
P = F ÷ A
For example, a sharp knife produces a large pressure with a small force because the contact area is tiny.
例如，锋利的刀刃用很小的力就能产生很大的压强，因为接触面积非常小。
2. Atmospheric Pressure | 大气压强
The Earth’s atmosphere is a layer of air about 100 km thick. Gravity pulls the air down, so every kilogram of air above us exerts a force on the surfaces below.
地球大气层是大约 100 千米厚的空气层。重力将空气向下拉，因此上方每一千克空气都会对下方的表面施加力。
This weight of air creates atmospheric pressure. At sea level, the average atmospheric pressure is strong enough to support a 760 mm column of mercury.
空气的重量产生了大气压强。在海平面，平均大气压强足以支撑起 760 毫米高的汞柱。
3. The 760 mmHg Standard | 760毫米汞柱标准
In 1643, Evangelista Torricelli inverted a long glass tube filled with mercury into a dish of mercury. The mercury column dropped to about 760 mm above the dish surface.
1643 年，托里拆利将一支装满汞的长玻璃管倒插入汞槽中。汞柱下降到距槽面约 760 毫米处。
This height is independent of the tube’s width or shape. It depends only on atmospheric pressure and the density of mercury.
这一高度与玻璃管的粗细或形状无关。它只取决于大气压强和汞的密度。
Therefore, standard atmospheric pressure at sea level is defined as 760 mmHg. This is also called one atmosphere (1 atm).
因此，海平面标准大气压被定义为 760 mmHg。这也被称为一个大气压（1 atm）。
4. Units of Pressure | 压强的单位
IGCSE science uses several pressure units. You must be comfortable converting between them.
IGCSE 科学中使用多种压强单位。你必须能够熟练地在它们之间进行换算。
| Unit | 单位 | Value | 数值 |
| atmosphere (atm) | 1 atm |
| millimetres of mercury (mmHg) | 760 mmHg |
| torr | 760 torr |
| pascal (Pa) / kilopascal (kPa) | 101,325 Pa = 101.325 kPa |
| bar | 1.01325 bar |
Notice that 1 atm = 760 mmHg = 101,325 Pa ≈ 100 kPa in many IGCSE calculations.
注意：1 atm = 760 mmHg = 101,325 Pa ≈ 100 kPa（在许多 IGCSE 计算中简化使用）。
5. Measuring Pressure: Barometer | 测量压强：气压计
A barometer is an instrument that measures atmospheric pressure. A mercury barometer is a long glass tube closed at one end, filled with mercury, and inverted into a mercury reservoir.
气压计是测量大气压强的仪器。汞气压计是一根一端封闭的长玻璃管，装满汞后倒插入汞槽中。
The space above the mercury column is a vacuum, so the only pressure pushing down in the tube is the weight of the mercury column. Atmospheric pressure balances this weight.
汞柱上方的空间是真空，因此管内唯一向下的压强来自汞柱自身的重量。大气压强与这个重量相平衡。
If atmospheric pressure increases, the mercury level rises. If it decreases, the mercury level falls.
如果大气压强增大，汞柱液面上升；如果大气压强减小，汞柱液面下降。
6. Boyle’s Law | 波义耳定律
Boyle’s law describes the relationship between pressure and volume for a fixed mass of gas at constant temperature.
波义耳定律描述了在温度恒定时，一定质量气体的压强与体积之间的关系。
It states that pressure is inversely proportional to volume, meaning as volume increases, pressure decreases.
它指出压强与体积成反比，也就是说体积增大时压强减小。
P₁V₁ = P₂V₂
For example, if a gas is compressed to half its original volume, its pressure doubles, provided temperature remains constant.
例如，如果气体被压缩到原来体积的一半，其压强就会加倍，前提是温度保持不变。
7. Pressure and Altitude | 压强与海拔
Atmospheric pressure decreases as altitude increases. At higher altitudes, there is less air above, so the weight of air pushing down is smaller.
大气压强随海拔升高而降低。在更高的海拔，上方空气更少，因此向下压的空气重量更小。
Mountain climbers often need oxygen tanks because the lower pressure means fewer oxygen molecules in each breath.
登山者常常需要氧气瓶，因为较低的气压意味着每一口呼吸中氧气分子更少。
This is why food also boils at lower temperatures on mountains, because lower pressure reduces the boiling point.
这也是为什么在高山上食物会在较低温度下沸腾，因为较低的气压降低了沸点。
8. Standard Temperature and Pressure (STP) | 标准温度和压力（STP）
Standard temperature and pressure is a set of reference conditions used for gas calculations. The standard pressure is exactly 760 mmHg.
标准温度和压力是一组用于气体计算的标准参考条件。标准压力正好是 760 mmHg。
- Standard temperature: 0 °C (273 K) | 标准温度：0 °C（273 K）
- Standard pressure: 760 mmHg = 101,325 Pa = 1 atm | 标准压力：760 mmHg = 101,325 Pa = 1 atm
- Molar volume of an ideal gas at STP: 22.4 dm³/mol | 理想气体在 STP 下的摩尔体积：22.4 立方分米/摩尔
In IGCSE chemistry, STP is often used to convert moles of gas to volume and vice versa.
在 IGCSE 化学中，STP 常用于气体摩尔数与体积之间的转换。
9. Manometer: Measuring Gas Pressure | 压力计：测量气体压强
A manometer is a U-shaped tube containing liquid, usually mercury or water. It compares the pressure of a gas with atmospheric pressure.
压力计是一种装有液体（通常是汞或水）的 U 形管。它将气体压强与大气压强进行比较。
If the gas pressure equals atmospheric pressure, the liquid levels in both arms are equal.
如果气体压强等于大气压强，两臂中的液面高度相等。
If the gas pressure is higher than atmospheric pressure, the liquid is pushed lower on the gas side and higher on the open side. The height difference h gives the excess pressure.
如果气体压强高于大气压强，液体会在气体一侧被压得更低，而在开口一侧升得更高。高度差 h 表示超出大气压的压强值。
P_gas = P_atmospheric + h (in mmHg or cmH₂O)
Always state the unit of h when answering exam questions.
在回答考试题目时，务必写出 h 的单位。
10. Everyday Applications | 日常应用
Pressure differences explain many everyday phenomena.
压强差解释了许多日常现象。
- Drinking through a straw works because reducing the air pressure in your mouth allows atmospheric pressure to push the liquid upward.
- 用吸管喝水是因为降低口腔内气压后，大气压强将液体向上推。
- Plane cabins are pressurised so passengers can breathe comfortably at high altitudes.
- 飞机舱内会加压，使乘客在高海拔下能舒适呼吸。
- Suction cups stick to walls because atmospheric pressure pushes them against the surface after air is removed.
- 吸盘能吸附在墙上是由于空气被挤出后，大气压强将其推向表面。
- Medical syringes withdraw medicine because the low pressure inside the barrel draws liquid in.
- 注射器抽取药液是因为筒内较低的气压将液体吸入。
Remember that atmospheric pressure is not a single force but a result of countless air molecules colliding with surfaces.
请记住，大气压强不是一个单独的力，而是无数空气分子与表面碰撞的结果。
11. Common Misconceptions | 常见误区
Many students confuse ‘760 mmHg’ with the physical height of a mercury barometer in every situation. In fact, 760 mmHg is only the standard value at sea level under defined conditions.
许多学生将 760 mmHg 与任何情况下汞气压计的实际高度混为一谈。实际上，760 mmHg 只是在规定的海平面标准条件下的值。
Another misconception is that pressure depends on area. Pressure is force per unit area, but for a given force, the pressure on a surface changes if the area changes.
另一个误区是认为压强取决于面积。压强是单位面积上的力，但对于给定的力，当面积改变时，表面上的压强会改变。
Also, Boyle’s law only applies when temperature is constant. If temperature changes, pressure and volume are not simply inversely proportional.
此外，波义耳定律只在温度恒定时适用。如果温度改变，压强和体积并不简单地成反比。
12. Exam Tips | 考试提示
In IGCSE exams, show all working when converting between pressure units. Write the equation first, then substitute numbers.
在 IGCSE 考试中，换算压强单位时要写出完整步骤。先写公式，再代入数值。
For barometer and manometer questions, draw a clear diagram and label the heights and open ends.
对于气压计和压力计题目，画出清晰示意图并标注高度和开口端。
Memorise the key value: 760 mmHg = 1 atm = 101,325 Pa ≈ 100 kPa.
牢记关键数值：760 mmHg = 1 atm = 101,325 Pa ≈ 100 kPa。
Finally, always check whether a question asks for pressure in mmHg, Pa, kPa, or atm. A missing unit loses marks.
最后，务必检查题目要求的是 mmHg、Pa、kPa 还是 atm。漏写单位会丢分。
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