A-Level WJEC Physics: AC Circuits Key Points | A-Level WJEC 物理：交流电考点精讲

📚 A-Level WJEC Physics: AC Circuits Key Points | A-Level WJEC 物理：交流电考点精讲

Alternating current (AC) is the backbone of modern electricity supply and a major topic in WJEC A-Level Physics. Understanding sinusoidal waveforms, RMS values, phase relationships, reactance, impedance, resonance, transformers and rectification is essential for both the written examination and practical assessment. This revision guide summarises the key definitions, formulas and concepts you must master, with clear bilingual explanations.

交流电是现代电力供应的主干，也是 WJEC A-Level 物理的重要课题。掌握正弦波形、有效值、相位关系、电抗、阻抗、谐振、变压器和整流对于笔试和实验评估都至关重要。本复习指南用清晰的双语解释总结了必须掌握的关键定义、公式和概念。

1. Alternating Current and Sinusoidal Waveforms | 交流电与正弦波形

An alternating voltage or current changes direction periodically and, in almost all cases of study, follows a sinusoidal pattern. The instantaneous voltage can be written as V(t) = V_₀ sin(2πf t) or V(t) = V_₀ sin(ωt), where V_₀ is the peak voltage, f is the frequency in hertz, and ω = 2πf is the angular frequency in rad/s. In the UK mains supply, f = 50 Hz and V_₀ ≈ 325 V, which yields a root mean square (RMS) voltage of 230 V.

交流电压或电流周期性地改变方向，在大多数研究情况下遵循正弦模式。瞬时电压可写作 V(t) = V_₀ sin(2πf t) 或 V(t) = V_₀ sin(ωt)，其中 V_₀ 是峰值电压，f 是频率（赫兹），ω = 2πf 是角频率（弧度/秒）。英国市电的频率 f = 50 Hz，峰值电压 V_₀ ≈ 325 V，得出有效值电压 230 V。

An AC generator (alternator) produces such a voltage by rotating a coil in a uniform magnetic field, inducing an emf that varies as the sine of the angle between the coil normal and the field. An oscilloscope connected across the output displays a sine wave whose amplitude and period can be measured directly.

交流发电机（交流发电机）通过在线圈在均匀磁场中旋转产生这种电压，感应电动势随线圈法线与磁场夹角的正弦变化。连接输出的示波器显示一个正弦波，其振幅和周期可直接测量。

2. Peak, Peak-to-Peak, and RMS Values | 峰值、峰峰值与有效值

The peak value V_₀ is the maximum displacement from zero. The peak-to-peak value is 2V_₀. However, for power calculations we use the root mean square (RMS) value. For any sinusoidal quantity, I_rms = I_₀ / √2 and V_rms = V_₀ / √2. The RMS value is the equivalent direct current (or voltage) that would dissipate the same power in a given resistor.

峰值 V_₀ 是偏离零的最大幅度。峰峰值为 2V_₀。但在功率计算中我们使用有效值（RMS）。对于任何正弦量，I_rms = I_₀ / √2，V_rms = V_₀ / √2。有效值是能在给定电阻中耗散相同功率的等效直流电流（或电压）。

From an oscilloscope trace, you can determine V_₀ from the vertical sensitivity (volts/div) and T from the time base (time/div). The frequency f = 1/T. Always divide V_₀ by √2 to obtain the RMS value quoted in data sheets. For the UK mains, V_rms = 230 V, and the peak voltage is 230√2 ≈ 325 V.

通过示波器轨迹，你可以根据垂直灵敏度（伏/格）确定 V_₀，根据时基（时间/格）确定周期 T。频率 f = 1/T。务必将 V_₀ 除以 √2 以获得数据表中给出的有效值。对于英国市电，V_rms = 230 V，峰值电压为 230√2 ≈ 325 V。

3. AC in a Pure Resistor | 纯电阻交流电路

When an AC supply is connected to a pure resistor, the current and voltage are exactly in phase: the phase difference φ = 0. Ohm’s law holds for instantaneous, peak and RMS values: V_rms = I_rms R. The power dissipated in the resistor varies with time, but the average power is P_av = I_rms V_rms = I²_rms R.

当交流电源连接到纯电阻时，电流与电压同相：相位差 φ = 0。欧姆定律适用于瞬时值、峰值和有效值：V_rms = I_rms R。电阻消耗的功率随时间变化，但平均功率为 P_av = I_rms V_rms = I²_rms R。

The power is always positive, meaning the resistor continuously converts electrical energy to heat. This is the simplest AC circuit and often the reference for phasor diagrams.

功率始终为正，意味着电阻不断地将电能转化为热能。这是最简单的交流电路，常作为相量图的基准。

4. AC in a Pure Capacitor: Capacitive Reactance | 纯电容电路：容抗

In a purely capacitive circuit, the current leads the voltage by 90° (π/2 rad). The opposition to current is called capacitive reactance, X_C = 1 / (2πf C), which has units of ohms. The RMS current follows I_rms = V_rms / X_C. Reactance decreases as frequency increases, so a capacitor passes high frequencies more easily.

在纯电容电路中，电流超前电压 90° (π/2 rad)。阻碍电流的作用称为容抗，X_C = 1 / (2πf C)，单位为欧姆。有效值电流满足 I_rms = V_rms / X_C。容抗随频率增加而减小，因此电容器更容易通过高频信号。

No net power is consumed because energy is stored in the electric field during charging and returned to the circuit during discharging. The average power over a full cycle is zero, although there is a temporary energy flow.

不消耗净功率，因为充电时能量存储于电场，放电时又返回电路。一个完整周期内的平均功率为零，尽管存在暂时的能量流动。

5. AC in a Pure Inductor: Inductive Reactance | 纯电感电路：感抗

In a purely inductive circuit, the current lags the voltage by 90° (π/2 rad). The inductive reactance is X_L = 2πf L, and I_rms = V_rms / X_L. X_L increases with frequency, so an inductor blocks high frequencies while passing low frequencies.

在纯电感电路中，电流滞后电压 90° (π/2 rad)。感抗为 X_L = 2πf L，I_rms = V_rms / X_L。感抗随频率增加而增加，因此电感阻断高频而通过低频。

Like the capacitor, a pure inductor dissipates no net power. Energy is stored in the magnetic field and returned to the source, resulting in zero average power.

与电容一样，纯电感不消耗净功率。能量存储于磁场并回馈给电源，平均功率为零。

6. Impedance and Phasor Diagrams | 阻抗与相量图

Impedance Z is the total opposition of a circuit to AC, combining resistance and

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