题型概述：电磁学长题Question type: electricity & magnetism
BPhO Round 1 Section 2 电磁学题通常以多个递进小问组成一道 25 分长题。本模块三题共 75 分，覆盖非线性电路、静电场和电容；常见失分点是漏判二极管状态、把合场用于计算自身受力、混淆固定电荷与固定电压，以及没有给出图像的边界条件。BPhO Round 1 Section 2 electricity questions usually form a 25-mark sequence of linked parts. These three questions total 75 marks and cover nonlinear circuits, electrostatics and capacitors. Common losses come from unchecked diode states, self-fields in force calculations, confusing fixed charge with fixed voltage, and missing graph boundary conditions.
1先写约束，再用能量State the constraint before using energy
电池接通时电压固定；断开后电荷固定。外力做功必须把电池的能量交换一起算入。A connected battery fixes voltage; disconnection fixes charge. Mechanical work must include energy exchanged with the battery.
试着把间距加倍，比较两种约束下的储能比。Double the separation and compare stored-energy ratios.
2叠加电势与电场Superpose potential and field
电势是标量，电场是矢量。求物体受力时只使用其他电荷产生的场，表面自身的场不计入。Potential is a scalar and field is a vector. To calculate force, use only fields from other charges, excluding the body’s self-field.
镜像法必须满足导体表面的电势条件。An image-charge construction must satisfy the conductor potential boundary.
3分段模型必须自洽Check each piecewise model
理想二极管导通时电压为零，截止时电流为零。求出节点电压后，检查导通电流方向及截止支路的偏置方向。An ideal conducting diode has zero voltage; an off diode has zero current. Check current direction for on diodes and bias polarity for off diodes.
每个阈值都要用阈值前的电路状态计算。Calculate each threshold in the circuit state immediately before it.
| 子题型Sub-type | 识别特征Recognise | 核心步骤Core step | 真题Question |
|---|---|---|---|
| 电阻积分与钳位电路Resistance integral & clamps | 截面积变化、理想二极管Varying cross-section; ideal diodes | 切片积分、节点分析、分区间Slice, analyse nodes, split into regions | 2025 Q3 |
| 导体电场Conductor fields | 极板、尖端、接地面Plates, tips, earthed plane | 场叠加、能量法、镜像法Superposition, energy, images | 2021 Q4 |
| 电容与能量Capacitance & energy | 断电、重连、恒流Disconnection, reconnection, constant current | 守恒量、功与能、图像边界Constraints, energy, graph boundaries | 2018 Q6 |
原题英文保留。数值检验会标注新增参数；自动评分把每题 25 分按输入项分配，仅作为练习进度，不替代正式评分标准。Original question text is retained in English. Numerical checks label added parameters. Automatic scoring distributes each question’s 25 marks across its inputs as a practice indicator, not an official mark scheme.