📚 Physical Constants | 物理常数
Physical constants are the unchanging numbers that lie at the heart of physics. From the speed of light to Planck's constant, these universal values define the fundamental scales and relationships of nature. In IB Physics, understanding physical constants is essential both for theoretical insight and for solving quantitative problems. The 2019 redefinition of SI units further highlighted their central role by fixing several constants exactly.
物理常数是位于物理学核心的不变数字。从光速到普朗克常数，这些普适值定义了自然界的基本尺度和关系。在 IB 物理中，理解物理常数对于理论洞见和解决定量问题都至关重要。2019 年国际单位制的重新定义通过精确固定若干常数，进一步凸显了它们的核心地位。
1. What are Physical Constants? | 什么是物理常数？
Physical constants are quantities that have a fixed numerical value and do not depend on the physical conditions or the measurement system—once properly defined. They emerge from the deep structure of the universe and appear in the most fundamental equations of physics.
物理常数是具有固定数值、不依赖于物理条件或测量系统（一旦正确定义）的量。它们源于宇宙的深层结构，并出现在物理学最基本的方程中。
Constants can be dimensionless (like the fine‑structure constant) or dimensionful (like the speed of light). Dimensionful constants set the scales of space, time, mass, and charge.
常数可以是无量纲的（如精细结构常数）或有量纲的（如光速）。有量纲常数设定了空间、时间、质量和电荷的尺度。
2. The Speed of Light c | 光速 c
The speed of light in vacuum, c, is the ultimate speed limit in the universe. It is exactly 299,792,458 metres per second because the metre is defined by the distance light travels in 1/c seconds.
真空中的光速 c 是宇宙中的终极速度极限。它精确等于 299 792 458 米每秒，因为米的定义是光在 1/c 秒内传播的距离。
c = 299 792 458 m s⁻¹ (exact)
length contraction all involve c. It also links space and time in the spacetime interval.
c 在狭义相对论中起着核心作用：E = mc²、时间膨胀和长度收缩都涉及 c。它还在时空间隔中连接了空间和时间。
3. Planck's Constant h | 普朗克常数 h
Planck' action. It appears in the energy of a photon E = hf and the de Broglie wavelength λ = h/p.
普朗克常数 h 约为 6.626 × 10⁻³⁴ J s，是作用量量子。它出现在光子能量 E = hf 和德布罗意波长 λ = h/p 中。
The reduced Planck constant ℏ π) appears even more widely in quantum mechanics, for example in the commutator [x, p] = iℏ.
约化普朗克常数 ℏ = h/(2π) 在量子力学中出现得更广泛，例如对易关系 [x, p] = iℏ。
e exact value 6.626 070 15 × 10⁻³⁴ J s, used to define the kilogram together with c and ΔνCs.
自 2019 年起，h 的精确值为 6.626 070 15 × 10⁻³⁴ J s，与 c 和 ΔνCs 一起用于定义千克。
4. The Elementary Charge e | 基本电荷 e
e proton, and −e is the charge of an electron. Its exact value is 1.602 176 634 × 10⁻¹⁹ C.
基本电荷 e 是单个质子所带电荷的大小，−e 是电子的电荷。其精确值为 1.602 176 634 × 10⁻¹⁹ C。
nt I = Q/t, the Lorentz force F = q(E + v × B), and the quantisation of charge in Millikan's experiment all rely on e.
e 在电磁学中至关重要：电流 I = Q/t、洛伦兹力 F = q(E + v × B)，以及密立根实验中的电荷量子化都依赖于 e。
The 2019 SI redefinition fixed e exactly, which redefined the ampere. Now the ampere is defined in terms of the elementary charge per second.
2019 年国际单位制重新定义精确固定了 e，重新定义了安培。现在安培的定义基于每秒转移的基本电荷。
5. The Gravitational Constant G | 引力常数 G
Newton' universal gravitation: F = G M m / r². Its value is approximately 6.674 × 10⁻¹¹ N m² kg⁻².
牛顿引力常数 G 出现在万有引力定律中：F = G M m / r²。其数值约为 6.674 × 10⁻¹¹ N m² kg⁻²。
Unlike c, h, and e, G is not exactly fixed— uncertainty of about 2.2 × 10⁻⁵, making it the least precisely known fundamental constant.
与 c、h 和 e 不同，G 没有被精确固定——它的测量相对不确定度约为 2.2 × 10⁻⁵，是已知精度最低的基本常数。
ounter G in gravitational fields: g = G M / r², orbital mechanics, and escape velocity v = √2 G M / r.
在 IB 物理中，你会在引力场中遇到 G：g = G M / r²、轨道力学和逃逸速度 v = √2 G M / r。
6. Avogadro's Number NA | 阿伏伽德罗常数 NA
Avogadro's number NA number of entities in one mole of a substance. It links the microscopic world to macroscopic quantities.
阿伏伽德罗常数 NA = 6.022 140 76 × 10²³ mol⁻¹ 是一摩尔物质中实体的精确数目。它将微观世界与宏观量联系起来。
In thermal physics, the molar gas constant R = NA k, and the molar mass of a substance is given by M = NA mparticle. NA also defines the mole in the SI.
在热物理中，摩尔气体常数 R = NA k，物质的摩尔质量 M = NA m粒子。NA 还在国际单位制中定义了摩尔。
When doing calculations involving ideal gas laws or stoichiometry, NA is essential for converting between number of particles and number of moles.
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