题型概述：电磁学长题Question types: electricity and magnetism
Round 2 常把陌生器件拆成连续的小问，先建模，再推导，最后讨论应用。本分类表中 2016–2025 年仅 2017 Q2 属于电磁学，因此本模块另选 2013 涡流制动与 2012 霍尔效应。原卷未标具体题目分值；顶部统计数值核对完成度。常见失分点是串并联对象混淆、几何面积选错，以及省略近似条件。页面分数只按数值核验完成度计算，文字与作图部分仍需自行对照完整解答。Round 2 builds unfamiliar devices into a sequence of modelling, derivation and application. In this classification, 2017 Q2 is the only electricity-and-magnetism long question from 2016–2025, so we also use the 2013 retarder and 2012 Hall effect. The papers print no numerical question totals; the header counts numerical checks. Typical losses come from incorrect series/parallel models, wrong areas and unstated approximations. The page score measures numerical checks only; compare explanations and sketches with the full solutions.
1电路：先找独立通道Circuits: identify independent channels
相同电流依次经过的元件串联；分流后具有相同端电压的通道并联。磁性材料中也可以用两条自旋通道建立等效电路。Elements carrying the same current in succession are in series; branches sharing a terminal voltage are in parallel. Two spin channels provide an equivalent circuit for magnetic materials.
不要把空间上并排误当成电学并联。Physical placement alone does not determine electrical topology.
2磁力：方向与平衡Magnetic force: direction and equilibrium
磁力使运动载流子偏转，电荷分离又建立电场。稳定霍尔效应中两种横向力抵消，磁力本身不对单个载流子做功。Magnetic force deflects moving carriers, and separated charge creates an electric field. These transverse forces cancel in the steady Hall effect; magnetic force itself does no work on an individual carrier.
电压反号可来自磁场、常规电流或载流子符号反转。Reversing field, conventional current or carrier sign can reverse the voltage.
3能量：制动功最终变成热Energy: braking work becomes heat
涡流阻碍相对运动，外界需要提供机械功以维持转动。温度先按热容增加，随后散热增强并可能达到平衡。Eddy currents oppose relative motion, so mechanical work is needed to sustain rotation. Temperature initially rises according to heat capacity; increasing heat loss may lead to equilibrium.
必须说明表面积、环境回辐射和材料状态的假设。State assumptions about emitting area, ambient radiation and material phase.
| 子题型Sub-type | 识别特征Recognition | 核心步骤Core step | 真题Question |
|---|---|---|---|
| 磁阻Magnetoresistance | 两层、两种自旋Layers and spin channels | 等效电路与极值Equivalent circuits and optimisation | 2017 Q2 |
| 涡流制动Eddy-current braking | 稳速下坡、铜盘Steady descent and copper disc | 机械功率→热平衡Mechanical power → thermal balance | 2013 Q3 |
| 霍尔效应Hall effect | 横向电压、薄箔Transverse voltage and thin foil | 数载流子→横向平衡Count carriers → transverse balance | 2012 Q5 |