📚 Electromagnetic Induction | 电磁感应
Electromagnetic induction is the process by which a changing magnetic field produces an electromotive force (emf) in a conductor. This phenomenon, discovered by Michael Faraday, forms the foundation of modern electrical power generation and numerous technologies. Understanding the underlying laws — Faraday’s law and Lenz’s law — along with concepts such as magnetic flux, transformers and eddy currents, enables us to analyse generators, motors and wireless charging systems.
电磁感应是变化的磁场在导体中产生电动势（emf）的过程。这一由迈克尔·法拉第发现的现象构成了现代发电和众多技术的基础。理解背后的法拉第定律和楞次定律，以及磁通量、变压器和涡流等概念，使我们能够分析发电机、电动机和无线充电系统。
1. Magnetic Flux and Flux Linkage | 磁通量与磁链
e θ with the normal to an area A, the flux is Φ = BA cosθ. The SI unit is the weber (Wb), where 1 Wb = 1 T m². When a coil has N tightly wound turns and the same flux threads each turn, the flux linkage is defined as NΦ, measured in weber-turns. Maximum flux linkage occurs when the plane of the coil is perpendicular to B (θ = 0°).
磁通量（Φ）衡量穿过某个表面的磁场的总量。对于磁感应强度为 B 的匀强磁场，其方向与面积 A 的法线夹角为 θ，则磁通量 Φ = BA cosθ。国际单位是韦伯（Wb），1 Wb = 1 T·m²。当线圈有 N 匝紧密绕制且每匝穿过相同的磁通量时，磁链定义为 NΦ，单位为韦伯·匝。当线圈平面与 B 垂直（θ = 0°）时磁链最大。
Φ = BA cosθ | Flux linkage = NΦ
2. Faraday's Law of Electromagnetic Induction | 法拉第电磁感应定律
of change of magnetic flux linkage. For a coil of N turns, the average induced emf is ε = – ined by Lenz’s law). When the change in flux is instantaneous, the emf is expressed as ε = – n of voltage in any device where a time-varying magnetic field interacts with a conductor.
法拉第定律指出，回路中感应电动势的大小与磁链的变化率成正比。对于匝数为 N 的线圈，平均感应电动势 ε = – N ΔΦ/Δt。负号表示电动势的方向（由楞次定律解释）。当磁通量瞬时变化时，电动势可写为 ε = – N dΦ/dt。任何存在随时间变化的磁场与导体相互作用的装置，其电压产生都可由这一定律解释。
ε = – N (ΔΦ/Δt) (average) | ε = – N (dΦ/dt) (instantaneous)
3. Lenz's Law and Energy Conservation | 楞次定律与能量守恒
Lenz’s law gives the direction of the induced current: the current always flows in such a direction that it opposes the change in magnetic flux that caused it. If a bar magnet’s north pole is pushed into a coil, the coil produces an induced current that creates a north pole at the facing end, repelling the magnet. This opposition requires mechanical work to be done, converting kinetic energy into electrical energy. Lenz’s law is a direct consequence of the conservation of energy.
楞次定律给出了感应电流的方向：感应电流的方向总是使其磁场阻碍引起感应电流的磁通量变化。若将条形磁铁的 N 极推入线圈，线圈会产生一个感应电流，使其靠近磁铁的一端形成 N 极，从而排斥磁铁。这种反抗需要做功，将动能转化为电能。楞次定律是能量守恒定律的直接体现。
| Magnet motion | Flux through coil | Induced pole facing magnet | Magnetic force |
|---|---|---|---|
| N pole moving in | Increasing | N pole | Repulsion |
| N pole moving out | Decreasing | S pole | Attraction |
| S pole moving in | Increasing | S pole | Repulsion |
| S pole moving out | Decreasing | N pole | Attraction |
4. Motional Electromotive Force | 动生电动势
When a straight conductor of length l moves with
Published by TutorHao | A-Level Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图