📚 Mastering AS AQA PH02: Electricity & Circuits Exam Walkthrough | AS AQA PH02 电学与电路真题精讲
The January 2023 AS AQA International Physics PH02 paper focuses on the core principles of electricity, circuit analysis, and experimental measurement. This revision guide breaks down every major topic you need, with worked examples and examiner-style tips to maximise your marks.
2023年1月的 AQA 国际版 AS 物理 PH02 试卷集中考查电学基本原理、电路分析与实验测量技能。本备考指南将逐一拆解每个核心考点，配合典型例题与考官级答题技巧，帮助你最大限度拿分。
1. Overview of the PH02 Paper | PH02 试卷概览
The PH02 paper is a written examination, typically lasting 1 hour 15 minutes. It tests your understanding of electricity, including current, resistance, circuits, potential dividers, EMF, and internal resistance, alongside data analysis and practical-based questions.
PH02 为笔试，考试时长通常为1小时15分钟。该卷考查你对电学知识的理解，包括电流、电阻、电路分析、分压器、电动势和内阻等，同时涉及数据处理与实验类题目。
Questions vary in style: multiple-choice items test quick recall and calculation, structured questions require step-by-step workings, and extended response items ask you to evaluate circuits or justify experimental methods.
题型多样：选择题考查快速回忆与计算能力；结构化问题要求分步推导；拓展性题目则要求你评估电路设计方案或论证实验方法的合理性。
Always read the mark allocation before answering. A three-mark calculation expects a method line, a substitution line, and a final answer with a unit.
答题前务必查看分值分配。3分的计算题通常需要写出公式、代入数值，最终给出带单位的答案，缺一不可。
2. Current and Charge | 电流与电荷
Electric current is defined as the rate of flow of charge. The equation is:
电流定义为电荷流动的速率。其表达式为：
I = ΔQ / Δt
where I is the current in amperes (A), ΔQ is the charge passing a point in coulombs (C), and Δt is the time interval in seconds (s).
其中 I 为电流，单位安培（A）；ΔQ 为通过某截面的电荷量，单位库仑（C）；Δt 为时间间隔，单位秒（s）。
In metallic conductors, the charge carriers are free electrons. The conventional current direction is from positive to negative terminal, which is opposite to the direction of electron flow.
在金属导体中，电荷载流子是自由电子。传统电流方向规定为从正极到负极，与电子实际运动方向相反。
Kirchhoff’s first law states that the total current entering a junction equals the total current leaving it, which follows directly from the conservation of charge.
基尔霍夫第一定律指出：流入节点的总电流等于流出节点的总电流，这直接由电荷守恒定律推出。
3. Potential Difference, Resistance and Ohm’s Law | 电势差、电阻与欧姆定律
Potential difference (p.d.) across a component is the energy transferred per unit charge passing through it:
元件两端的电势差（电压）表示每单位电荷通过该元件时转移的能量：
V = W / Q
Resistance is defined by Ohm’s law as the ratio of potential difference to current:
电阻由欧姆定律定义为电势差与电流之比：
R = V / I
For a metal wire at constant temperature, current is directly proportional to potential difference, so the I-V graph is a straight line through the origin.
对于恒温下的金属导线，电流与电势差成正比，其 I-V 图像为过原点的一条直线。
Ohm’s law is an empirical relationship valid only for ohmic conductors under constant physical conditions. Temperature changes complicate the relationship for many components.
欧姆定律是经验规律，仅适用于恒定物理条件下的欧姆导体。温度变化会使许多元件的伏安关系变得复杂。
4. I-V Characteristics of Circuit Components | 电路元件的伏安特性
You must be able to describe and explain the I-V graphs for a fixed resistor, filament lamp, and semiconductor diode. These appear regularly in PH02 questions.
你必须能够描述并解释定值电阻、白炽灯丝和半导体二极管的 I-V 图像。这些都是 PH02 高频考点。
| Component | Graph shape | Physical reason |
| Fixed resistor | Straight line through origin | Resistance stays constant |
| Filament lamp | Curved line, gradient decreases | Temperature rises, resistance increases |
| Diode | Nearly zero in reverse; steep rise after threshold in forward | Charge carriers blocked in reverse; forward bias allows conduction beyond 0.6 V |
For a filament lamp, as current increases, the metal filament heats up. Increased lattice vibration scatters conduction electrons more frequently, thus increasing resistance and reducing the graph gradient.
对白炽灯而言，电流增大时灯丝温度升高。晶格振动加剧，对导电电子的散射增强，电阻增大，曲线梯度减小。
The diode conducts significantly only when the forward bias exceeds roughly 0.6 V for silicon. Below this threshold, current remains essentially zero.
硅二极管只有在正向偏压超过约0.6V时才显著导通。低于该阈值时，电流几乎为零。
5. Resistivity and its Applications | 电阻率及其应用
Resistivity is an intrinsic property of a material, defined by:
电阻率是材料的固有属性，其定义式为：
ρ = RA / L
where R is resistance (Ω), A is the cross-sectional area (m²), L is the length (m), and ρ has units of Ω·m.
式中 R 为电阻（Ω），A 为横截面积（m²），L 为长度（m），ρ 的单位为Ω·m。
Typical questions ask you to find the resistance of a wire given its dimensions, or to determine resistivity from experimental measurements. Remember to convert all lengths to metres.
常见题型包括：已知导线尺寸求电阻，或由实验测量值求电阻率。注意所有长度必须统一换算为米。
Resistivity also links to temperature: for metals, resistivity increases with temperature due to increased lattice vibrations; for thermistors, resistance decreases sharply as temperature rises, making them useful temperature sensors.
电阻率与温度紧密相关：金属的电阻率随温度升高而增大，源于晶格振动加剧；热敏电阻的阻值则随温度升高急剧下降，因而常用作温度传感器。
6. Resistors in Series and Parallel | 串并联电路分析
For resistors connected in series, the total resistance is the sum of individual resistances:
电阻串联时，总电阻等于各分电阻之和：
R_total = R₁ + R₂ + R₃ + …
For resistors in parallel, the reciprocal of the total resistance equals the sum of reciprocals:
电阻并联时，总电阻的倒数等于各分电阻倒数之和：
1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + …
In a series circuit, the same current flows through every component and the supply p.d. is shared across them. In a parallel circuit, each branch receives the full supply p.d., while the current splits between branches.
串联电路中，通过每个元件的电流相同，电源电压分配在各元件之间。并联电路中，每条支路承受完整的电源电压，而总电流在各支路间分配。
When only two resistors are in parallel, the total resistance can be written as:
当只有两个电阻并联时，总电阻可写成：
R_total = R₁R₂ / (R₁ + R₂)
This shortcut saves time in calculations but only works for exactly two resistors.
这个简便公式能节省计算时间，但仅适用于恰好两个电阻的情况。
7. Potential Divider Circuits | 分压电路
A potential divider uses two resistors in series to produce a desired output voltage from a larger supply. The output potential difference across R₂ is:
分压器利用串联的两个电阻从较大的电源电压中得到所需输出电压。R₂ 两端的输出电压为：
V_out = V_in × R₂ / (R₁ + R₂)
If a variable resistor replaces one of the fixed resistors, the output voltage becomes adjustable. This is the basis of a dimmer switch and volume control.
若将其中一个定值电阻换成变阻器，输出电压即可连续调节。这正是调光开关和音量调节器的基本原理。
In sensing applications, an LDR or thermistor replaces one resistor. When light intensity or temperature changes, the resistance changes, altering V_out. This converted signal can then be fed to a monitoring circuit.
在传感应用中，光敏电阻或热敏电阻替代其中一个电阻。当光照强度或温度变化时，其阻值改变，从而导致 V_out 变化。转换后的信号可输送至后续监测电路。
When analysing potential divider questions, first identify the component whose resistance changes, then predict whether V_out increases or decreases. A common mistake is inverting this logic, costing easy marks.
分析分压器问题时，先确定哪个元件的阻值发生变化，然后判断 V_out 增大还是减小。常见错误是判断反了方向，白白丢分。
8. EMF and Internal Resistance | 电动势与内阻
Every real battery has internal resistance r. The electromotive force (EMF) ε is the total energy supplied per unit charge by the cell, while the terminal p.d. is always less than ε when current flows.
每节真实电池都存在内阻 r。电动势 ε 表示电池每单位电荷提供的总能量，而端电压在有电流输出时总是小于 ε。
ε = I(R + r)
Equivalently, the terminal potential difference is: V = ε − Ir.
等价地，端电压可写为：V = ε − Ir。
A classic PH02 experiment involves changing the external resistor R, measuring the terminal p.d. V and current I, then plotting a graph of V against I. The y-intercept equals ε, and the gradient equals −r.
PH02 经典实验是：改变外电阻 R，测量对应的端电压 V 和电流 I，绘制 V-I 图像。纵截距等于电动势 ε，斜率等于 −r。
The maximum power is transferred to the external load when R = r. At this point, half of the cell’s power is dissipated internally, and efficiency is only 50%.
当外电阻 R 等于内阻 r 时，外电路获得最大功率。此时电源一半功率被内部消耗，效率仅为50%。
9. Electrical Power and Energy | 电功率与电能
Electrical power is the rate at which energy is transferred. The fundamental equation is:
电功率是能量转移的速率。基本表达式为：
P = VI
Using Ohm’s law, this can be rewritten in two additional useful forms:
结合欧姆定律，可得到另外两种实用形式：
P = I²R and P = V² / R
Electrical energy transferred is then E = Pt = VIt. In practical questions, you must convert time to seconds unless an appliance’s energy consumption is given in kilowatt-hours.
电能转移量为 E = Pt = VIt。实际题目中，除非能耗以千瓦时为单位给出，否则必须将时间换算为秒。
Heating effect problems often ask you to compare energy dissipated across components in series or parallel. Remember that in series, the larger resistor dissipates more power; in parallel, the smaller resistor dissipates more power.
电热效应题目常要求比较串并联中元件消耗的电能。记住：串联时电阻越大功率越大；并联时电阻越小功率越大。
10. Worked Examples from the January 2023 Paper Style | 2023年1月真题风格演练
Example 1: A battery of EMF 9.0 V and internal resistance 0.50 Ω is connected to a 5.5 Ω resistor. Calculate the terminal p.d.
例题1：电动势为9.0V、内阻为0.50Ω的电池连接一个5.5Ω的电阻。求端电压。
Step 1: Total resistance = 5.5 + 0.50 = 6.0 Ω.
第一步：总电阻 = 5.5 + 0.50 = 6.0Ω。
Step 2: Current I = ε / (R + r) = 9.0 / 6.0 = 1.5 A.
第二步：电流 I = ε / (R + r) = 9.0 / 6.0 = 1.5A。
Step 3: Terminal p.d. V = IR = 1.5 × 5.5 = 8.25 V ≈ 8.3 V.
第三步：端电压 V = IR = 1.5 × 5.5 = 8.25V ≈ 8.3V。
Example 2: A potential divider uses a 12 V supply, R₁ = 2.0 kΩ and R₂ = 4.0 kΩ. Find V_out across R₂.
例题2：分压器电源为12V，R₁ = 2.0kΩ，R₂ = 4.0kΩ。求 R₂ 两端的输出电压。
V_out = 12 × 4.0 / (2.0 + 4.0) = 12 × 4/6 = 8.0 V
Example 3: A wire has length 2.5 m, diameter 0.40 mm, and resistance 1.2 Ω. Determine the resistivity.
例题3：某导线长2.5m，直径0.40mm，电阻1.2Ω。求电阻率。
Convert diameter to radius in metres: r = 0.20 × 10⁻³ m. Area A = πr² = π × (0.20 × 10⁻³)² = 1.26 × 10⁻⁷ m².
直径换算为半径：r = 0.20 × 10⁻³m。截面积 A = πr² = π × (0.20 × 10⁻³)² = 1.26 × 10⁻⁷m²。
ρ = RA / L = 1.2 × 1.26 × 10⁻⁷ / 2.5 = 6.0 × 10⁻⁸ Ω·m
11. Common Mistakes and Exam Tips | 常见错误与答题技巧
By far the most common error in PH02 calculations is failing to convert units. Diameters in millimetres must become metres; kilohms become ohms; minutes become seconds. Losing one mark on every calculation for this reason is entirely avoidable.
PH02 计算题中最常见的错误就是单位换算缺失。毫米直径必须化为米；千欧化为欧姆；分钟化为秒。这一分本不该丢，务必警惕。
When drawing circuit diagrams, use sharp pencil and a ruler. Straight lines only, right-angle bends, and standard symbols are essential. The examiner cannot award marks for an illegible or ambiguous diagram.
绘制电路图时，要用削尖的铅笔和直尺。只使用直线、直角转弯和标准符号。凌乱或含糊的电路图无法获得判定分。
For “show that” questions, do not simply write the target answer. Present the full reasoning and working; otherwise you may score zero even if your final number matches.
对于“证明”类题目，不能只写出目标答案。必须展示完整推理和计算过程，否则即使最终数字正确，也可能得零分。
State units in every numerical final answer, and use the appropriate number of significant figures. If the data in the question is given to two significant figures, your answer should be reported to two as well.
每个数值答案都要带单位，并取适当有效数字。若题目数据为两位有效数字，答案也应保留两位有效数字。
Finally, read the question stem carefully for phrases like “the battery is ideal” — this tells you internal resistance is zero — or “assume constant temperature,” which tells you Ohm’s law applies.
最后，仔细阅读题干关键词：如“电池是理想的”表示内阻为零；“假设温度恒定”则表示欧姆定律适用。
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