题型概述：电学与磁学Question types: electricity & magnetism
SPC 电磁学题既有快速判断，也有把电路和实际情境连接起来的多步计算。本组选取三道各 1 练习分的 Part I 题，以及 7 练习分和 4 练习分的 Part II 题，共 14 练习分。最容易失分的地方是漏掉支路电流、单位前缀换错，以及混淆单次事件与长期平均。SPC electricity questions range from quick judgements to multi-step circuit and real-world calculations. This set has three Part I questions worth 1 practice point each and Part II questions worth 7 and 4 practice points, totalling 14 practice points. Common losses come from missing branch currents, incorrect unit prefixes and mixing single events with long-term averages.
所提供源文件未标注每题官方分值。页面按可判分输入均分练习分；这些不是官方分值。The supplied sources omit per-question marks. Practice points are shared equally among graded inputs; these are not official marks.
1电荷与能量要分别守恒Keep charge and energy distinct
电流是电荷经过截面的速率；电势差是每单位电荷转移的能量。环形束流中，同一批电荷可以反复经过同一截面。Current is charge crossing a section per second; voltage is energy transferred per unit charge. In a circulating beam, the same charge repeatedly crosses the same section.
电荷量用 C，能量用 J，功率用 W。Charge in C, energy in J, power in W.
2先辨认节点，再使用欧姆定律Identify nodes before using Ohm’s law
并联支路电压相同，串联部分通过总电流。电压表只有在电阻足够大时才近似不扰动电路。Parallel branches share a voltage; series sections carry the total current. A voltmeter is approximately non-invasive only when its resistance is sufficiently large.
先画出电流分流，再选择方便的功率公式。Mark current splits first, then choose the useful power formula.
3相互作用与场强Interactions and field strength
两个电荷间的相互作用力等大反向。另一个常见模型是近似均匀电场：固定电压下，间距越小，平均场强越大。Two charges exert equal and opposite forces on each other. In a separate common model, the mean field between two potentials grows as their separation decreases.
不要混淆点电荷的反平方定律和均匀场模型。Do not confuse the inverse-square point-charge field with a uniform-field model.
| 子题型Sub-type | 识别特征How to spot it | 核心步骤Core step | 真题Question |
|---|---|---|---|
| 作用力对Interaction pair | 电荷量不相等Unequal charges | 交换两电荷检验对称性Exchange the two charges | 2013 I-1 |
| 电阻功率比Resistor power ratios | 混联、已知一支路功率Mixed network; one known power | 先求各支路电流比Find branch-current ratios | 2014 I-7 |
| 循环电流Circulating current | 周长、每团粒子数Circumference and bunch population | 整束电荷除以周期Beam charge divided by period | 2017 I-4 |
| 测量扰动Measurement loading | 非理想电源或电压表Non-ideal source or meter | 重新列节点方程Rebuild the node equation | 2023 II-9 |
| 宏观电学估算Macroscopic estimates | 雷暴、电流、能量Storms, current, energy | 统一时间和体积基准Use common time and volume bases | 2024 II-11 |