📚 Electric Motors and EM Induction | 电动机与电磁感应
Electric motors and electromagnetic induction are fundamental concepts in Edexcel IGCSE Physics. The motor effect allows us to convert electrical energy into mechanical rotation, while electromagnetic induction forms the basis of generators and transformers. This article covers the essential principles, rules and applications you need to master.
电动机与电磁感应是 Edexcel IGCSE 物理的核心内容。电动机效应使我们能够将电能转化为机械转动，电磁感应则是发电机和变压器的基础。本文将梳理必须掌握的关键原理、定则与应用。
1. Introduction to Electric Motors and EM Induction | 电动机与电磁感应简介
An electric motor uses the motor effect: a force acts on a current-carrying conductor in a magnetic field, producing rotation. Electromagnetic induction, on the other hand, generates an electromotive force (e.m.f.) when a conductor cuts magnetic field lines or experiences a changing magnetic flux.
电动机利用电动机效应——载流导体在磁场中受到力的作用而产生旋转。而电磁感应是指当导体切割磁感线或磁通量发生变化时产生电动势的过程。
2. Magnetic Force on a Current-Carrying Conductor | 磁场对载流导体的力
A wire carrying an electric current placed in a magnetic field experiences a force, provided the current is not parallel to the field lines. The force is maximum when the current is perpendicular to the field. This force can be calculated using the equation:
当通电导线置于磁场中且电流方向不与磁感线平行时，导线会受到力的作用。当电流方向与磁场垂直时，力最大。该力的大小可用下式计算：
F = B I L
where F is the force (N), B is the magnetic flux density (T), I is the current (A), and L is the length of the conductor within the field (m). The direction of the force is always perpendicular to both the current and the magnetic field.
式中 F 为力（牛顿），B 为磁通量密度（特斯拉），I 为电流（安培），L 为处于磁场中的导线长度（米）。力的方向始终与电流方向和磁场方向垂直。
3. Fleming's Left-Hand Rule | 弗莱明左手定则
Fleming's left-hand rule helps determine the direction of the force on a current-carrying conductor in a magnetic field. Hold the thumb, forefinger and second finger of your left hand mutually at right angles:
弗莱明左手定则用于判断载流导体在磁场中受力的方向。将左手的拇指、食指和中指互相垂直展开：
- ThuMb — Motion (Force)
- First finger — Field (from N to S)
- SeCond finger — Current (from + to −)
If you point your forefinger in the direction of the magnetic field and your second finger in the direction of the current, your thumb will point in the direction of the force (movement).
若食指指向磁场方向（N 到 S），中指指向电流方向（正到负），则拇指所指即为导体受力（运动）方向。
4. The Simple DC Motor | 简单直流电动机
A simple DC motor consists of a rectangular coil of wire placed between two permanent magnets, a split-ring commutator and two carbon brushes. When a direct current flows through the coil, each side experiences a force according to Fleming's left-hand rule. One side is pushed up, the other pushed down, creating a turning effect (torque).
简单直流电动机由矩形线圈、永磁体、一个换向器和两个碳刷组成。当直流电通过线圈时，线圈两边根据左手定则受到方向相反的力，一边向上、一边向下，产生转动力矩。
As the coil rotates past the vertical position, the split-ring commutator reverses the direction of the current through the coil. This ensures the forces continue to act in the same rotational direction, keeping the coil spinning smoothly.
当线圈转过竖直位置时，换向器切换电流方向，使线圈两边受力方向不变，从而维持持续转动。
5. Factors Affecting Motor Speed and Torque | 影响电动机转速和转矩的因素
The turning force (torque) and speed of a DC motor can be increased by:
直流电动机的转动力矩和转速可通过以下方法提高：
- increasing the current in the coil
- using stronger magnets to provide a greater magnetic flux density
- increasing the number of turns on the coil
- winding the coil on a soft iron core, which concentrates the magnetic field
这些方法包括：增大线圈中的电流、使用更强的磁铁以增大磁通量密度、增加线圈匝数、以及在线圈中加入软铁芯来增强磁场。
These changes increase the magnitude of the force on each side and therefore the total torque, leading to faster rotation when the motor is under load.
这些改变增大了每边导线所受的力，从而增大了总转矩，使电机在负载下转速更快。
6. Introduction to Electromagnetic Induction | 电磁感应简介
Electromagnetic induction occurs when there is a change in the magnetic field around a conductor. If a magnet is moved into a coil, the changing magnetic flux induces an e.m.f. across the coil. If the coil forms a complete circuit, an induced current flows. Similarly, moving a coil in a stationary magnetic field also induces an e.m.f.
电磁感应发生在导体周围的磁场发生变化时。将磁铁插入线圈，变化的磁通量会使线圈两端产生感应电动势。若线圈构成闭合回路，便会产生感应电流。同样，在固定磁场中移动线圈也会感应出电动势。
The magnitude of the induced e.m.f. increases if the relative motion is faster, the magnetic field is stronger, or the coil has more turns.
移动越快、磁场越强或线圈匝数越多，感应电动势越大。
7. Faraday's Law of Electromagnetic Induction | 法拉第电磁感应定律
Faraday’s law states that the magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage. For a coil with N turns, this is expressed as:
法拉第电磁感应定律指出：感应电动势的大小与磁通量变化率成正比。对于匝数为 N 的线圈，可表达为：
ε = N (ΔΦ / Δt)
where ε is the induced e.m.f. (V), N is the number of turns, ΔΦ is the change in magnetic flux (Wb), and Δt is the time taken (s). An e.m.f. is generated only while the flux is changing.
式中 ε 为感应电动势（伏特），N 为线圈匝数，ΔΦ 为磁通量变化量（韦伯），Δt 为时间变化量（秒）。仅在磁通量变化期间才会产生电动势。
8. Lenz's Law | 楞次定律
Lenz’s law gives the direction of the induced current: the induced current flows in such a direction as to oppose the change in magnetic flux that produced it. This is a consequence of the conservation of energy. The complete expression for Faraday’s law includes a minus sign:
楞次定律给出了感应电流的方向：感应电流的方向总是阻碍引起它的磁通量变化。这是能量守恒的结果。完整的法拉第定律表达式包含负号：
ε = − N (ΔΦ / Δt)
For example, when the north pole of a bar magnet approaches a coil, the induced current creates a north pole at the coil's nearest end, repelling the magnet and opposing the motion.
例如，当条形磁铁的 N 极靠近线圈时，感应电流使线圈靠近磁铁的一端呈现 N 极，产生排斥力以阻碍磁铁靠近。
9. The Simple AC Generator (Alternator) | 简单交流发电机
An AC generator converts mechanical energy into electrical energy. It consists of a coil rotating between permanent magnets, with its ends connected to two slip rings. Carbon brushes press against the slip rings to capture the induced current. As the coil rotates, its sides cut magnetic field lines, and an alternating e.m.f. is induced.
交流发电机将机械能转化为电能。它由在永磁体之间旋转的线圈和两个滑环组成，碳刷压在滑环上输出感应电流。线圈转动时，其边切割磁感线，产生交变电动势。
Fleming's right-hand rule predicts the direction of the induced current: point the First finger in the direction of the magnetic Field, the thuMb in the direction of Motion, then the SeCond finger shows the direction of induced Current. The output voltage varies sinusoidally, changing direction every half-turn, producing alternating current (AC).
弗莱明右手定则可判断感应电流方向：食指指向磁场方向，拇指指向导线运动方向，则中指指向感应电流方向。输出电压按正弦变化，每半周改变方向，形成交流电。
10. Comparing DC Motors and AC Generators | 直流电动机与交流发电机的比较
DC Motor: Converts electrical energy into mechanical rotation. It uses a split-ring commutator to reverse the current every half-turn, enabling continuous rotation when supplied with DC. The output is rotary motion, used in fans, drills and electric vehicles.
直流电动机：将电能转化为机械转动。利用换向器每半周切换电流方向，使线圈在直流电源下持续旋转。输出为旋转运动，用于风扇、电钻和电动车。
AC Generator
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