A-Level Physics Key Difficulties | A-Level物理重难点梳理

📚 A-Level Physics Key Difficulties | A-Level物理重难点梳理

A-Level Physics is widely regarded as one of the most challenging A-Level subjects. It demands not only mathematical fluency but also a deep conceptual understanding of how the physical world operates. Many students struggle because they memorise formulas without grasping the underlying principles that connect them. This comprehensive guide identifies and clarifies the key difficulty areas, providing structured revision strategies for each topic.

A-Level物理被公认为A-Level课程中最具挑战性的学科之一。它不仅要求学生具备扎实的数学能力，更需要对物理世界的运作方式有深层的概念理解。许多学生之所以感到困难，是因为他们仅死记公式，却不理解公式之间相互关联的底层原理。本篇综合指南将梳理并厘清各个核心难点，并为每个专题提供结构化的复习策略。

1. Newtonian Mechanics and Momentum | 牛顿力学与动量

The first major hurdle in A-Level Physics is Newtonian mechanics. Students often confuse mass with weight, and struggle to apply Newton’s three laws in multi-body systems. The key insight is that Newton’s second law, F = ma, is a special case of the more general momentum principle. When mass is constant, force equals mass times acceleration; when mass changes, such as in a rocket, the full momentum form is required.

A-Level物理的第一个主要难关是牛顿力学。学生常常混淆质量与重量的概念，并且在多体系统中难以应用牛顿三大定律。关键要理解的是，牛顿第二定律F = ma是更普遍的动量原理的一个特例。当质量恒定时，力等于质量乘以加速度；当质量发生变化时（例如在火箭中），则需要使用完整的动量形式。

Conservation of linear momentum is one of the most tested concepts in the exam. For a collision between two objects, the total momentum before impact equals the total momentum after impact, provided no external force acts on the system:

线性动量守恒是考试中考察最多的概念之一。对于两个物体之间的碰撞，只要系统不受外力作用，碰撞前的总动量等于碰撞后的总动量：

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

Students must also distinguish between elastic collisions, where kinetic energy is conserved, and inelastic collisions, where some kinetic energy is transformed into heat, sound, or deformation energy. In a perfectly inelastic collision, the two bodies stick together and move with a common final velocity.

学生还必须区分弹性碰撞与非弹性碰撞：弹性碰撞中动能守恒，而非弹性碰撞中部分动能转化为热能、声能或形变能。在完全非弹性碰撞中，两个物体粘在一起，以相同的末速度运动。

Always draw a clear diagram showing the direction of each velocity vector before writing equations.

在写方程之前，务必绘制清晰的示意图，标示出每个速度矢量的方向。
Choose a positive direction and stick to it consistently throughout the calculation.

选定一个正方向，并在整个计算过程中始终保持一致。
Remember that momentum is a vector quantity — direction matters in every calculation.

切记动量是矢量——在每次计算中都必须考虑方向。

2. Circular Motion and Gravitational Fields | 圆周运动与引力场

Circular motion often confuses students because the velocity is constantly changing direction, even when the speed is constant. The centripetal acceleration is always directed towards the centre of the circle, and it is provided by a real force such as tension, friction, or gravity. The key equations are:

圆周运动之所以令学生困惑，是因为即使速率恒定，速度的方向也在不断变化。向心加速度始终指向圆心，并由一个真实的力（如张力、摩擦力或重力）提供。关键公式为：

a = v²/r = ω²r and F = mv²/r = mω²r

Gravitational fields extend this concept to orbital mechanics. Students must understand that a satellite in orbit is in a state of continuous free fall — gravity provides the centripetal force required to maintain its circular path. The relationship between orbital radius and orbital speed is derived from equating gravitational force with centripetal force:

引力场将这一概念延伸到轨道力学中。学生必须理解，在轨卫星实际上处于持续自由落体状态——万有引力提供了维持其圆周轨道所需的向心力。将万有引力与向心力相等，即可推导出轨道半径与轨道速度之间的关系：

GMm/r² = mv²/r → v = √(GM/r)

The most common exam error is mixing up the gravitational field strength g with the gravitational constant G. The former is the force per unit mass at a specific location, measured in N kg⁻¹, while the latter is a universal constant with a value of 6.67 × 10⁻¹¹ N m² kg⁻².

最常见的考试错误是混淆引力场强度g与万有引力常量G。前者是某特定位置单位质量所受的力，单位为N kg⁻¹；后者是普适常量，数值为6.67 × 10⁻¹¹ N m² kg⁻²。

Geostationary satellites orbit at a fixed height above the Equator and have the same angular speed as the Earth’s rotation.

地球同步卫星在赤道上方固定高度运行，其角速度与地球自转角速度相同。
Kepler’s third law, T² ∝ r³, is a direct consequence of the inverse-square law of gravitation.

开普勒第三定律T² ∝ r³是万有引力平方反比定律的直接推论。

3. Simple Harmonic Motion | 简谐运动

Simple harmonic motion (SHM) is a fundamental concept that appears repeatedly across the A-Level syllabus. The defining condition of SHM is that the acceleration is proportional to the displacement from equilibrium and directed towards it. The mathematical formulation is:

简谐运动（SHM）是A-Level教学大纲中反复出现的基本概念。简谐运动的定义条件是：加速度与偏离平衡位置的位移成正比，且方向始终指向平衡位置。其数学表达式为：

a = −ω²x

Many students lose marks because they cannot sketch or interpret displacement-time, velocity-time, and acceleration-time graphs correctly. In SHM, velocity is maximum at the equilibrium position and zero at the amplitude extremes, while acceleration is exactly the opposite. The phase relationships between these three quantities are critical for solving exam problems.

许多学生因无法正确绘制或解读位移-时间、速度-时间和加速度-时间图像而丢分。在简谐运动中，速度在平衡位置处达到最大值，在振幅端点处为零；而加速度恰好相反。三者之间的相位关系是解决考试问题的关键。

The energy exchanges in SHM are also frequently tested. Total mechanical energy remains constant in ideal SHM, oscillating between kinetic energy and potential energy. Both energy forms vary as functions of displacement, not linearly but quadratically:

简谐运动中的能量转化也是常考内容。在理想简谐运动中，总机械能保持恒定，在动能与势能之间相互转化。两种能量形式随位移的变化均为二次关系而非线性关系：

E_total = ½mω²A² and E_p = ½mω²x²

For a mass-spring system, the period depends only on the mass and the spring constant: T = 2π√(m/k). For a simple pendulum, the period depends on the length of the string and the local gravitational field strength: T = 2π√(L/g). It is a classic exam question to ask which of these remains constant when amplitude changes — the answer is the period, because it is independent of amplitude for small oscillations.

对于弹簧振子系统，周期仅取决于质量与弹簧劲度系数：T = 2π√(m/k)。对于单摆，周期取决于摆长与当地引力场强度：T = 2π√(L/g)。一个经典的考试问题是：当振幅改变时，哪个量保持不变？答案是周期，因为在微小振动中周期与振幅无关。

4. Wave Interference and Stationary Waves | 波的干涉与驻波

Wave phenomena represent a rich source of exam questions, and interference is among the most misunderstood topics. The principle of superposition states that when two waves meet, the resultant displacement is the vector sum of the individual displacements. Constructive interference occurs when crest meets crest, producing a larger amplitude; destructive interference occurs when crest meets trough, producing cancellation.

波的现像是考试题目的丰富来源，而干涉是最容易被误解的专题之一。叠加原理指出，当两列波相遇时，合位移是各列波位移的矢量和。当波峰与波峰相遇时发生相长干涉，产生更大的振幅；当波峰与波谷相遇时发生相消干涉，产生抵消效果。

For interference to be observable, the two sources must be coherent — they must maintain a constant phase difference. In the double-slit experiment, the path difference between light from the two slits determines whether a bright or dark fringe appears:

要观察到干涉现象，两个波源必须相干——即它们必须保持恒定的相位差。在双缝实验中，来自两条狭缝的光的光程差决定了出现的是明条纹还是暗条纹：

d sin θ = nλ (bright fringes, constructive)

d sin θ = (n + ½)λ (dark fringes, destructive)

Stationary waves, also called standing waves, are particularly challenging because students cannot visualise them easily. A stationary wave is formed when two progressive waves of equal amplitude and frequency travel in opposite directions. Key features include nodes, where the amplitude is permanently zero, and antinodes, where the amplitude oscillates at its maximum. The allowed wavelengths on a string fixed at both ends are given by λ = 2L/n, where n is a positive integer.

驻波尤为具有挑战性，因为学生难以直观想象。驻波是由两列振幅和频率相同、传播方向相反的行波叠加而成的。其关键特征包括波节点（振幅永久为零）和波腹点（振幅以其最大值振荡）。两端固定的弦上允许存在的波长为λ = 2L/n，其中n为正整数。

In a stationary wave, energy is not transferred along the medium — it is stored in the oscillation.

在驻波中，能量并不沿介质传播——能量储存在振荡之中。
The fundamental frequency and harmonics form an arithmetic sequence: f, 2f, 3f, 4f…

基频和谐波构成等差数列：f, 2f, 3f, 4f……
Diffraction is most pronounced when the gap or obstacle size is comparable to the wavelength.

当狭缝或障碍物的尺寸与波长相近时，衍射现象最为明显。

5. Electric Circuits and Capacitance | 电路分析与电容

Circuit analysis trips up many A-Level students because it requires systematic method rather than intuition. Kirchhoff’s laws are the foundation: the junction rule states that the total current entering a junction equals the total current leaving it (conservation of charge), and the loop rule states that the algebraic sum of potential differences around a closed loop is zero (conservation of energy).

电路分析让许多A-Level学生栽跟头，这是因为电路分析需要系统化的方法而非直觉。基尔霍夫定律是根本基础：节点定律指出，流入节点的总电流等于流出节点的总电流（电荷守恒）；回路定律指出，沿闭合回路的电势差代数和为零（能量守恒）。

Potential dividers are a heavily examined application of circuit theory. When two resistors are connected in series across a supply voltage, the voltage across each resistor is proportional to its resistance:

分压器是电路理论中一个考试频率极高的应用。当两个电阻串联跨接在电源电压上时，每个电阻两端的电压与其阻值成正比：

V_out = V_in × R₂ / (R₁ + R₂)

Capacitance is a topic where many students lose conceptual clarity. A capacitor stores energy in an electric field, not in the charge itself as is often mistakenly assumed. The time constant τ = RC determines how quickly a capacitor charges or discharges through a resistor. After one time constant, the capacitor has charged to 63% of its full value; after five time constants, it is effectively fully charged. The exponential decay equation for discharge is:

电容是许多学生概念模糊的专题。电容器将能量储存在电场中，而非如人们常误认为的那样储存在电荷本身。时间常数τ = RC决定了电容器通过电阻充放电的快慢。经过一个时间常数，电容器充电至满值的63%；经过五个时间常数后，可视为完全充满。放电指数衰减方程为：

Q = Q₀ e⁻ᵗ/ᴿᶜ or equivalently V = V₀ e⁻ᵗ/ᴿᶜ

When capacitors are in parallel, the total capacitance is the sum: C₁ + C₂ + C₃…

电容器并联时，总电容为各电容之和：C₁ + C₂ + C₃……
When capacitors are in series, the reciprocal rule applies: 1/C_total = 1/C₁ + 1/C₂ + …

电容器串联时，采用倒数相加规则：1/C总 = 1/C₁ + 1/C₂ + ……
Energy stored in a capacitor: E = ½CV² = ½QV.

电容器储存的能量：E = ½CV² = ½QV。

6. Magnetic Fields and Electromagnetic Induction | 磁场与电磁感应

Electromagnetism is often described by students as the hardest module in A-Level Physics. The challenge lies in three-dimensional visualisation: a charged particle moving through a magnetic field experiences a force that is perpendicular to both its velocity and the field direction. For a charge q moving with velocity v perpendicular to a magnetic field B, the force is:

电磁学常被学生描述为A-Level物理中最难的模块。难点在于三维空间想象能力：带电粒子在磁场中运动时，所受力同时垂直于其速度方向和磁场方向。对于以速度v垂直于磁场B运动的电荷q，受力为：

F = Bqv (when v ⊥ B)

This force acts as a centripetal force, causing the particle to move in a circular path. The radius of this path is r = mv/Bq. This principle underlies the operation of cyclotrons and mass spectrometers.

该力作为向心力，使粒子做圆周运动。圆周运动的半径为r = mv/Bq。这一原理是回旋加速器和质谱仪工作的基础。

Faraday’s law of electromagnetic induction states that the induced electromotive force (EMF) is equal to the negative rate of change of magnetic flux linkage. Lenz’s law determines the direction of the induced current: it always opposes the change that produces it. These laws are combined into the standard equation:

法拉第电磁感应定律指出，感应电动势等于磁通链变化率的负值。楞次定律决定了感应电流的方向：感应电流总是阻碍产生它的变化。这两条定律合并为标准方程：

E = −N ΔΦ/Δt where Φ = BA cos θ

Students frequently fail to calculate magnetic flux correctly because they forget the angle dependence. The flux Φ is maximum when the field is perpendicular to the area (θ = 0°), and zero when the field is parallel to the area (θ = 90°).

学生常常在计算磁通量时出错，因为他们忽略了角度依赖性。当磁场垂直于面积时（θ = 0°），磁通量Φ最大；当磁场平行于面积时（θ = 90°），磁通量为零。

Situation	Flux Φ = BA cos θ	Induced EMF
Coil face perpendicular to field	Maximum (BA)	Zero (flux is not changing)
Coil face parallel to field	Zero	Maximum (flux changing fastest)

7. Quantum Physics: Photoelectric Effect | 量子物理：光电效应

The photoelectric effect is the topic that forces students to abandon classical physics and embrace quantum ideas. Classical wave theory predicts that the kinetic energy of emitted electrons should increase with the intensity of the incident light, and that electron emission should occur at any frequency provided enough time passes. Both predictions are wrong.

光电效应是迫使学生放弃经典物理、接受量子观念的重要专题。经典波动理论预言：发射电子的动能应随入射光强度的增强而增大，且只要光照时间足够长，任何频率的光都应能引发电子发射。然而这两个预言都是错误的。

Einstein’s explanation introduces the concept of the photon — a discrete package of energy given by E = hf, where h = 6.63 × 10⁻³⁴ J s. Each photon can be absorbed by at most one electron. If the photon’s energy exceeds the work function φ (the minimum energy needed to liberate an electron from the metal surface), the excess energy becomes kinetic energy:

爱因斯坦的解释引入了光子的概念——光子的能量是一个离散的值，表达式为E = hf，其中h = 6.63 × 10⁻³⁴ J s。每个光子最多只能被一个电子吸收。如果光子的能量超过逸出功φ（将电子从金属表面释放所需的最小能量），则多余的能量转化为动能：

hf = φ + K_max or equivalently K_max = hf − φ

This equation is known as Einstein’s photoelectric equation, and it is essential for interpreting all photoelectric effect problems. The threshold frequency f₀ is given by f₀ = φ/h — below this frequency, no electrons are emitted regardless of how intense the light is. The maximum kinetic energy of the photoelectrons is independent of intensity, but the number of photoelectrons emitted per second is directly proportional to intensity.

该方程被称为爱因斯坦光电效应方程，对于解答所有光电效应问题至关重要。截至频率f₀由f₀ = φ/h给出——低于这个频率，无论光多强都不会发射电子。光电子的最大动能与光强度无关，但每秒发射的光电子数量与光强度成正比。

Wave-particle duality extends this idea across all of quantum physics. Electrons, traditionally considered particles, exhibit diffraction patterns when passed through a crystal lattice. The de Broglie wavelength connects momentum with wavelength: λ = h/p = h/mv. This relationship is tested both numerically and conceptually.

波粒二象性将这一思想延伸到整个量子物理学中。传统上被视为粒子的电子在通过晶体点阵时也能产生衍射图样。德布罗意波长将动量与波长联系起来：λ = h/p = h/mv。这一关系在考试中既考数值计算，也考概念理解。

8. Atomic and Nuclear Physics | 原子物理与核物理

Nuclear physics is a favourite examination topic because it combines well-defined equations with conceptual significance. Students first encounter the atomic model, beginning with Rutherford’s scattering experiment, which demonstrated that most of the atom is empty space and that the positive charge is concentrated in a tiny nucleus.

核物理是考试的热门专题，因为它将定义明确的方程与概念意义完美结合。学生首先学习原子模型——从卢瑟福散射实验开始，该实验表明原子内部大部分是空的，正电荷集中在微小原子核内。

Radioactive decay follows first-order kinetics, giving rise to the exponential decay law. The activity of a radioactive sample decreases according to:

放射性衰变遵循一级动力学规律，由此产生指数衰变定律。放射性样品的活度按以下规律衰减：

A = A₀ e⁻λᵗ and T½ = ln 2 / λ = 0.693/λ

where λ is the decay constant and T½ is the half-life. Students must be comfortable converting between decay constant and half-life, and applying these formulas to both numerical calculations and graphical analysis.

其中λ是衰变常数，T½是半衰期。学生必须熟练地在衰变常数与半衰期之间进行换算，并能够将上述公式应用于数值计算和图解分析。

Mass defect and binding energy are conceptually demanding. The mass of a nucleus is always less than the sum of the masses of its constituent protons and neutrons. This missing mass, called the mass defect Δm, is converted into the binding energy that holds the nucleus together. Einstein’s mass-energy equivalence gives:

质量亏损与结合能是概念上具有挑战性的内容。原子核的质量总是小于其组成质子和中子的质量之和。这个缺失的质量称为质量亏损Δm，它转化为将核子结合在一起所需的结合能。爱因斯坦的质能方程给出：

E = mc² where c = 3.00 × 10⁸ m s⁻¹

Nuclear fission and fusion both release energy because the products have greater binding energy per nucleon than the reactants. The iron nucleus, Fe-56, has the highest binding energy per nucleon, which explains why energy is released by both fission of heavy nuclei and fusion of light nuclei.

核裂变和核聚变都会释放能量，因为产物比反应物的每个核子结合能更高。铁核（Fe-56）具有最高的比结合能，这解释了为什么重核裂变和轻核聚变都会释放能量。

9. Thermodynamics and Ideal Gases | 热力学与理想气体

The kinetic theory of gases is a logical junction between mechanics and statistics. The ideal gas law combines Boyle’s law, Charles’s law, and Gay-Lussac’s law into a single equation:

气体分子动理论是连接力学与统计学的逻辑枢纽。理想气体状态方程将玻意耳定律、查理定律和盖-吕萨克定律合并为一个方程：

pV = nRT or pV = NkT

where n is the number of moles, R = 8.31 J mol⁻¹ K⁻¹ is the molar gas constant, N is the number of molecules, and k = 1.38 × 10⁻²³ J K⁻¹ is the Boltzmann constant. Students must remember that temperature in gas laws is always measured in kelvin, never in degrees Celsius.

其中n为物质的量（摩尔数），R = 8.31 J mol⁻¹ K⁻¹为摩尔气体常量，N为分子数，k = 1.38 × 10⁻²³ J K⁻¹为玻尔兹曼常量。学生必须记住，气体定律中的温度始终以开尔文为单位，绝不能用摄氏度。

The root-mean-square speed of gas molecules is a concept that appears in quantitative problems. Its derivation connects macroscopic pressure with microscopic molecular motion:

气体分子的均方根速率是定量计算中常考的概念。它的推导将宏观压强与微观分子运动联系起来：

p = ⅓ ρ⟨c²⟩ and ⟨c²⟩ = 3kT/m

The first law of thermodynamics, ΔU = Q + W, is frequently tested with signed quantities. Careful sign conventions are critical: Q is positive when heat is added to the system, and W is positive when work is done on the system. In an isothermal expansion of an ideal gas, the internal energy does not change because temperature is constant, so all the heat absorbed is converted into work done by the gas.

热力学第一定律ΔU = Q + W经常以带符号的量进行考察。仔细处理符号约定至关重要：Q为正值表示系统吸热，W为正值表示外界对系统做功。在理想气体的等温膨胀过程中，由于温度恒定，内能不变，因此所有吸收的热量都转化为气体对外所做的功。

At constant volume, all heat supplied increases internal energy and raises temperature.

在定容条件下，所有供给的热量都增加内能并升高温度。
At constant pressure, some heat is used to do expansion work, so the temperature rise is smaller than at constant volume.

在定压条件下，部分热量用于做功膨胀，因此温度升高幅度比定容情况下小。

10. Materials: Stress, Strain and Young Modulus | 材料性质：应力、应变与杨氏模量

The properties of materials form a short but conceptually rich section of the A-Level syllabus. Stress is defined as force per unit cross-sectional area, measured in N m⁻² or Pa. Strain is the fractional change in length, defined as extension divided by original length, making it dimensionless. Young modulus is the ratio of stress to strain in the elastic region:

材料性质是A-Level教学大纲中内容精炼但概念丰富的部分。应力定义为单位横截面积所受的力，单位为N m⁻²或Pa。应变是长度的相对变化量，定义为伸长量除以原始长度，因此是无量纲的量。杨氏模量是弹性区域内应力与应变的比值：

E = σ/ε = (F/A) / (ΔL/L₀) = FL₀ / AΔL

Students must understand the distinction between elastic deformation, where the material returns to its original shape when the load is removed, and plastic deformation, where permanent structural change occurs. The limit of proportionality marks the end of the linear stress-strain relationship, while the elastic limit marks the point beyond which permanent deformation begins.

学生必须理解弹性形变与塑性形变的区别：弹性形变是指撤去载荷后材料恢复原状，而塑性形变则发生永久性的结构变化。比例极限标志着应力-应变线性关系的终结，而弹性极限则标志着永久形变开始的临界点。

The stress-strain graph contains a wealth of information. The gradient of the linear region is the Young modulus. The area under the graph represents the energy per unit volume stored in the material. For a ductile material such as copper, the graph shows a long plastic region before fracture; for a brittle material such as glass, there is almost no plastic region before sudden fracture.

应力-应变图蕴含了丰富的信息。线性区域的斜率即为杨氏模量。曲线下方的面积表示材料每单位体积储存的能量。对于铜等延性材料，曲线在断裂前出现较长的塑性区域；而对于玻璃等脆性材料，在突然断裂前几乎没有塑性区域。

Ultimate tensile strength (UTS) is the maximum stress the material can withstand.

极限抗拉强度（UTS）是材料能承受的最大应力。
The Young modulus is a property of the material itself, not of the particular sample being tested.

杨氏模量是材料本身的属性，与所测试样品的具体尺寸无关。
When a spring is stretched, the energy stored in it is given by E = ½Fx = ½kx².

当弹簧被拉伸时，储存的能量为E = ½Fx = ½kx²。

11. Particle Physics: Standard Model | 粒子物理：标准模型

Particle physics is the most modern component of the A-Level syllabus and is often the source of distinguishing questions. The Standard Model classifies all known elementary particles into two categories: fermions, which make up matter, and bosons, which mediate forces. Fermions include quarks and leptons; bosons include photons, gluons, W and Z bosons, and the Higgs boson.

粒子物理是A-Level教学大纲中最现代的部分，常作为区分高分考生的考题来源。标准模型将所有已知基本粒子分为两类：组成物质的费米子和传递力的玻色子。费米子包括夸克和轻子；玻色子包括光子、胶子、W与Z玻色子以及希格斯玻色子。

Quarks come in six flavours, the most relevant for A-Level being up (u), down (d) and strange (s). Protons consist of two up quarks and one down quark (uud), while neutrons consist of one up quark and two down quarks (udd). The quark charges are +⅔e for up-type quarks and −⅓e for down-type quarks.

夸克有六种味，A-Level最常涉及的是上夸克（u）、下夸克（d）和奇夸克（s）。质子由两个上夸克和一个下夸克组成（uud），中子由一个上夸克和两个下夸克组成（udd）。上型夸克电荷为+⅔e，下型夸克电荷为−⅓e。

The conservation laws in particle interactions are strictly tested. Baryon number, lepton number, charge, energy and momentum must all be conserved. Weak nuclear interactions can change quark flavour — this is how beta decay occurs when a neutron converts into a proton with the emission of an electron and an antineutrino:

粒子相互作用中的守恒律是严格考察的内容。重子数、轻子数、电荷、能量和动量都必须守恒。弱核力相互作用可以改变夸克的风味——这正是β衰变的过程：一个中子转化为质子，同时发射一个电子和一个反中微子：

n → p + e⁻ + ṽₑ (neutron decay)

Understanding that exchange particles (gauge bosons) mediate fundamental forces provides a unified picture of nature. The photon mediates the electromagnetic force, gluons mediate the strong force between quarks, and W⁻/W⁺ bosons mediate the weak force responsible for beta decay.

理解交换粒子（规范玻色子）如何传递基本力，为我们提供了一幅统一的自然图景。光子传递电磁力，胶子在夸克之间传递强力，W⁻/W⁺玻色子传递引发β衰变的弱力。

12. AC Circuits and Transformers | 交流电与变压器

Alternating current (AC) introduces concepts that students often find abstract: root-mean-square values,

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A-Level Physics Key Difficulties | A-Level物理重难点梳理