🔄 电磁感应：磁也能生电🔄 Electromagnetic Induction: When Magnetism Makes Electricity
上一片，我们看着电流造出了磁场——电生磁。小火箭马上想反过来试试：磁场能不能造出电流？法拉第用一块磁铁和一卷线圈回答了这个问题——答案是：能，但有一个关键条件。Last leaf we watched a current create a magnetic field. Naturally, the rocket asked the reverse: can a magnet create a current? Faraday answered with one magnet and one coil — yes, but with one key condition.
第 1 步：法拉第的实验——磁铁动，才有电流Step 1: Faraday's experiment — the magnet must move
把磁铁静静放进线圈里，什么也没发生——问题到底出在哪儿？Placing a magnet quietly inside the coil does nothing at all. So what's the trick?
磁铁插进线圈的一瞬间，检流计指针向右摆动；把磁铁抽出来，指针向左摆；磁铁停在线圈里不动，指针就回到零。法拉第明白了：不是磁本身能生电，而是“磁的变化”能生电——磁铁动起来，才有感应电流。The instant the magnet slides in, the galvanometer needle kicks one way; pull it out and the needle kicks the other way; leave it still and the needle settles at zero. Faraday saw it: it's not the magnet itself that makes electricity — it's the change. Move the magnet, and you get an induced current.
没有变化就没有电流——可变的到底是什么？No change, no current — but what exactly is changing?
第 2 步：为什么“变化”才是关键Step 2: Why "change" is the key
穿过线圈的磁场在变化，线圈里才生电，这就是电磁感应。变得越快，电流越大：磁铁动得快、换更强的磁铁、线圈多绕几圈，都能让指针摆得更大。磁铁一停下来，变化的脚步一停，电流也就没了。The magnetic field through the coil has to change for a current to appear — that's electromagnetic induction. Change it faster and the current grows: move the magnet quickly, use a stronger magnet, or wind more turns. Once the magnet stops, the change stops, and so does the current.
一个“磁铁动一动就生电”的发现，怎么点亮了整个世界？A discovery about moving magnets — how did it light up the whole world?
第 3 步：从实验室到全世界——发电机与无线充电Step 3: From the lab to the world — generators and wireless charging
发电机的核心就是让线圈在磁场里不停转动：变化不停，感应电流就不停，千家万户的灯因此亮起。无线充电板用的是同一招：板里的电流不停变化，在手机线圈里感应出电流，不用插线就能充电。Inside a generator, coils spin non-stop in a magnetic field: change after change, current after current, lighting up entire cities. A wireless charging pad pulls the same trick — the changing current in the pad induces a current in your phone's coil, no plug required.
🎮 你来当法拉第（30 秒）🎮 Your turn: be Faraday (30 seconds)
抓住磁铁，快慢进出线圈，盯着检流计指针——只有“正在变化”时它才摆动。4 个挑战，找出能生电的操作。Grab the magnet and move it in and out of the coil, watching the galvanometer — the needle only swings while things are changing. Four challenges: find the moves that make electricity.
一句话记住它：磁铁“动”（变化）才生电——插进、拔出、动得越快，感应电流越大；静止不动就没有电流。发电机和无线充电都靠它。Remember it in one line: only a changing magnet makes current — in, out, and faster means bigger; held still, nothing happens. Generators and wireless chargers both run on it.
磁铁进出线圈才有感应电流，静止在线圈里没有Current appears only while the magnet moves in or out — not when it sits still变化越快、磁性越强、匝数越多 → 电流越大Faster change, stronger magnet, more turns → bigger current靠近与远离，电流方向相反；发电机、无线充电都靠它In and out give opposite directions; generators and wireless charging rely on it
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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