📚 AS Physics Unit 2 Concepts Explained | AS物理第二单元概念解析
This article provides a detailed breakdown of the key concepts found in the AQA AS Physics Unit 2 specification, as highlighted in the January 2020 exam insert. Understanding these principles is crucial for mastering mechanics, materials and waves. Each section pairs an English explanation with its Chinese equivalent to support bilingual learners.
本文详细解析了2020年1月AS物理第二单元考试资料页中强调的关键概念，这些概念是掌握力学、材料与波的核心。每个小节都提供中英双语对照讲解，帮助双语学习者深入理解。
1. Scalars and Vectors | 标量与矢量
Physical quantities are divided into scalars and vectors. A scalar has magnitude only, such as distance, speed, mass, and energy. A vector possesses both magnitude and direction, for example displacement, velocity, force, and momentum.
物理量分为标量和矢量。标量只有大小，如距离、速率、质量和能量。矢量则同时具有大小和方向，例如位移、速度、力和动量。
. For perpendicular vectors, the resultant magnitude is found via Pythagoras’ theorem: R = √F₁² + F₂².
标量用普通算术相加，而矢量相加必须考虑方向。对于互相垂直的矢量，合矢量大小由勾股定理求得：R = √F₁² + F₂²。
| Scalars / 标量 | Vectors / 矢量 |
|---|---|
| distance 距离, speed 速率, mass 质量, time 时间, energy 能量 | displacement 位移, velocity 速度, acceleration 加速度, force 力, momentum 动量 |
2. Moments and Equilibrium | 力矩与平衡
inciple of moments states that for an object in equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments.
力对支点的力矩等于力的大小乘以支点到力作用线的垂直距离：力矩 = F × d。力矩原理指出，当物体处于平衡状态时，顺时针力矩之和等于逆时针力矩之和。
An object's centre of mass is the point where its entire weight appears to act. For symmetrical objects with uniform density, the centre of mass lies at the geometric centre.
物体的质心是整个重力看起来作用的点。对于密度均匀的对称物体，质心位于几何中心。
Σ clockwise moments = Σ anticlockwise moments
3. Motion Graphs and Equations | 运动图像与方程
Displacement–time and velocity–time graphs are powerful tools. The gradient of a displacement–time graph gives velocity; the gradient of a velocity–time graph gives acceleration, and the area under it gives displacement.
位移-时间图和速度-时间图是重要工具。位移-时间图的斜率表示速度；速度-时间图的斜率表示加速度，其图线下面积表示位移。
For uniform acceleration, the SUVAT equations link displacement (s), initial velocity (u), final velocity (v), acceleration (a), and time (t).
对于匀加速运动，SUVAT 方程将位移 s、初速度 u、末速度 v、加速度 a 和时间 t 联系起来。
v = u + at
s = ut + ½at²
v² = u² + 2as
Remember to assign a positive direction; quantities in the opposite direction become negative.
注意需规定正方向，相反方向的量取负值。
4. Newton’s Laws and Momentum | 牛顿定律与动量
e causes acceleration proportional to mass. The third law: every action has an equal and opposite reaction on different bodies.
牛顿第一定律：物体将保持静止或匀速直线运动状态，除非受到合外力作用。第二定律：F = ma，合外力引起加速度，加速度与质量成反比。第三定律：每一个作用力都有一个大小相等、方向相反的反作用力作用在不同物体上。
⁻¹. Impulse Ft equals the change in momentum Δp. In a closed system, total momentum before collision equals total momentum after collision (conservation of momentum).
线动量 p = mv，单位为 kg m s⁻¹。冲量 Ft 等于动量的变化量 Δp。在封闭系统中，碰撞前总动量等于碰撞后总动量（动量守恒）。
F = Δp / Δt
5. Work, Energy and Power | 功、能与功率
ty to do work. The principle of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed.
功是力与在力方向上位移的乘积：W = F s cosθ。能量是做功的本领。能量守恒定律指出能量既不会凭空产生也不会凭空消失，只会转移或转化。
surface). Power is the rate of doing work: P = W / t = Fv for motion at constant velocity.
动能 KE = ½mv²，重力势能 GPE = mgh（近地面）。功率是做功的速率：P = W / t = Fv（恒速运动时适用）。
Efficiency = (useful energy output / total energy input) × 100%
6. Materials: Stress and Strain | 材料：应力与应变
per unit original length: ε = ΔL / L₀, a dimensionless ratio or expressed as a percentage.
应力 σ 是单位截面积上的作用力：σ = F / A（单位 Pa）。应变 ε 是单位原长的伸长量：ε = ΔL / L₀，是一个无量纲比或以百分比表示。
Beyond the elastic limit, a material behaves plastically and does not return to its original shape. The yield point and ultimate tensile stress are key points on a stress–strain curve. Brittle materials fracture with little plastic deformation, while ductile materials undergo significant plastic strain before breaking.
超过弹性极限后，材料发生塑性形变，不能恢复原状。屈服点和极限抗拉应力是应力-应变曲线上的关键点。脆性材料断裂前塑性形变极小，而延性材料在断裂前会经历较大的塑性应变。
7. Young Modulus | 杨氏模量
strain within the elastic limit: E = σ / ε. The unit is Pa. A steeper gradient on a stress–strain graph indicates a larger Young modulus.
杨氏模量 E 是刚度的量度，定义为在弹性极限内拉伸应力与拉伸应变的比值：E = σ / ε，单位为 Pa。在应力-应变图上，更加陡峭的斜率代表更大的杨氏模量。
Experimentally, E can be determined by applying known masses to a long wire, measuring extension with a Vernier scale, and plotting stress against strain. The wire must be long and thin to produce measurable extension without exceeding the elastic limit.
实验中，可使用已知质量的砝码施加于一根长金属丝，用游标尺测量伸长量，然后绘制应力-应变图求得 E。金属丝需要细长以产生明显的伸长量且不超过弹性极限。
8. Waves: Progressive and Stationary | 波：行波与驻波
related by v = fλ. Transverse waves (e.g., light) have oscillations perpendicular to energy transfer; longitudinal waves (e.g., sound) oscillate parallel to the direction of energy transfer.
行波传递能量不传递物质。关键特征包括振幅、波长 λ、频率 f 和波速 v，满足 v = fλ。横波（如光）的振动方向与能量传递方向垂直；纵波（如声波）的振动方向与能量传递方向平行。
m displacement). This can be demonstrated on a stretched string or in an air column. Phase difference between two points on a wave is given by 2π × (path difference / λ).
当两列相同的波相向传播时，会形成驻波，出现波节（位移为零）和波腹（位移最大）。这一现象可在拉紧的弦或空气柱上演示。波上两点的相位差由 2π × (路径差 / λ) 给出。
9. Refraction and Interference | 折射与干涉
/ v. The angle of incidence gives total internal reflection when the angle exceeds the critical angle (sin C = 1 / n).
折射发生于波进入不同媒质并改变速度时。斯涅尔定律：n₁ sin θ₁ = n₂ sin θ₂，其中折射率 n = c / v。当入射角大于临界角（sin C = 1 / n）时发生全内反射。
D / a, where D is slit-to-screen distance and a is slit separation. Bright fringes occur at path difference = nλ, dark fringes at (n + ½)λ.
来自双缝的相干波产生明暗相间的干涉条纹。对于杨氏双缝实验，条纹间距 x = λ D / a，其中 D 为缝到屏的距离，a 为双缝间距。明纹出现在光程差为 nλ 处，暗纹出现在 (n + ½)λ 处。
x = λD / a
10. Diffraction and the Diffraction Grating | 衍射与衍射光栅
ction grating consists of many equally spaced slits, giving sharp, intense maxima at angles given by d sin θ = nλ, where d = 1 / N (grating spacing).
衍射是波绕过障碍物或通过狭缝时展宽的现象，当狭缝尺寸与波长相近时最为明显。衍射光栅由许多等间距的狭缝组成，能在满足 d sin θ = nλ 的角度产生锐利而明亮的极大值，其中 d = 1 / N（光栅常数）。
Because the grating produces maxima at specific angles, it can be used to measure the wavelength of light precisely. The number of orders n is limited, as sin θ cannot exceed 1.
由于光栅在特定角度产生极大值，可以精确地测量光的波长。由于 sin θ 不能大于 1，级数 n 是有限的。
White light viewed through a grating yields a continuous spectrum for each order except the zero order, which stays white.
复色光通过光栅后，零级保持白色，而其他各级会展开为连续光谱。
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