Electromagnetism (Edexcel IGCSE Science 6.3)

📚 Electromagnetism (Edexcel IGCSE Science 6.3) | 电磁学

Electromagnetism brings together two fundamental aspects of physics – electricity and magnetism. For IGCSE Edexcel Science, this topic explores how an electric current can create a magnetic field, and how magnetic fields can exert forces on current-carrying conductors. From simple straight wires to the rotating coils of a DC motor, these principles explain countless everyday devices. Mastering the right-hand grip rule and Fleming’s left-hand rule is essential for describing directions of fields and forces, and the force equation F = B I L allows you to calculate the size of the motor effect. This article covers all you need to know for the 6.3 Electromagnetism topic, with clear explanations, key rules and real-world applications.

电磁学将电与磁这两个物理学基本领域联系在一起。在IGCSE Edexcel科学课程中，这一主题探讨电流如何产生磁场，以及磁场如何对载流导体施加力。从简单的直导线到直流电动机中转动的线圈，这些原理解释了无数日常设备的工作原理。掌握右手螺旋定则和弗莱明左手定则对于描述场和力的方向至关重要，而公式 F = B I L 则能让你计算电动机效应产生的力的大小。本文涵盖6.3电磁学主题所需的全部内容，包括清晰的解释、关键定则和实际应用。

1. What is Electromagnetism? | 什么是电磁学？

Electromagnetism is the branch of physics that studies the relationship between electric currents and magnetic fields. Whenever charge flows, a magnetic field appears around the path of the current. This is the fundamental principle behind electromagnets, motors, generators and many sensors. In the IGCSE syllabus, we focus on the shape, direction and strength of magnetic fields produced by currents, and the forces experienced by current-carrying conductors placed inside another magnetic field.

电磁学是研究电流与磁场之间关系的物理学分支。只要有电荷流动，电流的路径周围就会出现磁场。这就是电磁铁、电动机、发电机以及许多传感器背后的基本原理。在IGCSE大纲中，我们重点学习电流产生的磁场的形状、方向和强度，以及放置在另一个磁场中的载流导体所受到的力。

2. Oersted’s Discovery | 奥斯特的发现

In 1820, Hans Christian Oersted noticed that a compass needle placed near a wire deflected when an electric current passed through the wire. Before his experiment, electricity and magnetism were thought to be separate phenomena. Oersted’s observation was the first concrete proof that an electric current produces a magnetic field. When the current was switched off, the compass needle returned to pointing north, showing that the magnetic field existed only while charges were moving.

1820年，汉斯·克里斯蒂安·奥斯特注意到，当导线中有电流通过时，放在导线附近的罗盘指针会发生偏转。在他实验之前，人们认为电与磁是互不相关的现象。奥斯特的观察首次具体证明了电流会产生磁场。当电流断开时，罗盘指针重新指向北方，这表明磁场仅在电荷运动时才存在。

3. Magnetic Field Patterns around Current-Carrying Wires | 载流导线周围的磁场模式

A long, straight wire carrying a steady current is surrounded by a magnetic field consisting of concentric circles centred on the wire. The field lines lie in a plane perpendicular to the wire. The spacing between the circles increases with distance from the wire, indicating that the field strength decreases as you move further away. If the current is increased, the magnetic field becomes stronger everywhere, but the circular pattern remains.

一根通有稳定电流的长直导线周围会形成以导线为圆心的同心圆磁场。磁感线位于与导线垂直的平面内。圆圈间的距离随离导线距离的增大而增大，这表明随着距离的增加，磁场强度逐渐减弱。如果增大电流，各处的磁场都会变强，但依然保持同心圆形状。

To visualise the pattern, you can sprinkle iron filings on a card through which a current-carrying wire passes. The filings align themselves into circles around the wire. A small plotting compass placed at different positions near the wire will also show the direction of the circular field.

为了观察这一模式，可以让一根载流导线垂直穿过一张硬纸板，并在纸板上撒上铁屑；铁屑会沿导线周围的圆圈排列。在导线附近不同位置放置一个小型罗盘，也能显示圆形磁场的方向。

4. The Right-Hand Grip Rule | 右手螺旋定则

To find the direction of the magnetic field lines around a straight wire, we use the right-hand grip rule. Imagine gripping the wire with your right hand so that your thumb points in the direction of conventional current (positive to negative). Your curled fingers then indicate the direction of the magnetic field lines – anticlockwise or clockwise according to the current direction. This rule is simple but vital for predicting the magnetic effect of any straight conductor.

要确定直导线周围磁感线的方向，我们使用右手螺旋定则。想象用右手握住导线，让拇指指向传统电流方向（正极到负极），此时弯曲的四指所指的方向即为磁感线的方向——根据电流方向的不同可能是逆时针或顺时针。这个定则虽然简单，但对于预测任何直导体的磁效应都至关重要。

For a vertical wire with current flowing upwards, the magnetic field when viewed from above will appear anticlockwise. If the current flows downwards, the field is clockwise. Always apply the rule using conventional current, not electron flow.

对于电流竖直向上的导线，从上方俯视时磁场呈逆时针方向；如果电流向下，则磁场呈顺时针方向。请始终使用传统电流方向（正电荷流动方向）来应用该定则，而非电子流动方向。

5. The Solenoid’s Magnetic Field | 螺线管的磁场

A solenoid is a long coil of insulated wire, often wound around a cylindrical former. When a current passes through the solenoid, the magnetic field inside becomes strong and remarkably uniform, while the external field closely resembles that of a bar magnet. The solenoid has a distinct north and south pole, and its field lines run from north to south outside the coil, and from south to north inside it, forming closed loops.

螺线管是一个由绝缘导线绕成的长线圈，通常缠绕在圆柱形骨架上。当电流通过螺线管时，管内的磁场变得很强且非常均匀，而外部的磁场则与条形磁铁的磁场非常相似。螺线管具有明确的N极和S极，磁感线在管外从N极指向S极，在管内则从S极指向N极，形成闭合回线。

The polarity of a solenoid can be determined using a variation of the right-hand grip rule. Grip the solenoid with your right hand so that your fingers follow the direction of current around the coil; your thumb will then point towards the north pole of the solenoid. Adding a soft iron core intensifies the field dramatically, a principle used in electromagnets.

螺线管的极性可以用右手螺旋定则的一个变体来判断。用右手握住螺线管，让弯曲的四指沿着线圈中电流的方向，此时伸直的拇指所指的方向就是螺线管的N极。在线圈中加入软铁芯可以大幅增强磁场，这一原理被应用在电磁铁中。

6. Electromagnets | 电磁铁

An electromagnet consists of a solenoid wound around a core of soft magnetic material, typically soft iron. When current flows, the iron becomes strongly magnetised, producing a field far stronger than the coil alone. When the current stops, the iron quickly loses most of its magnetism. This temporary, controllable magnetism makes electromagnets ideal for applications where the magnetic force needs to be switched on and off or varied in strength.

电磁铁由一个绕在软磁材料（通常是软铁）芯上的螺线管组成。当有电流通过时，铁芯被强烈磁化，产生的磁场远强于单独使用线圈时。电流断开后，铁芯会迅速失去大部分磁性。这种可瞬时产生、可控制的磁性使电磁铁非常适合那些需要开关磁力或调节磁场强度的应用场景。

Common uses include scrapyard cranes that lift heavy steel objects, electric bells that repeatedly interrupt a circuit to strike a gong, and relays that allow a low-current circuit to switch a high-current circuit safely. The strength of an electromagnet can be increased by increasing the current, adding more turns to the coil, or inserting a higher-permeability core.

常见的用途包括用于吊起沉重钢制品的废料场起重机、通过反复通断电路敲击铃碗的电铃，以及用低电流电路安全控制高电流电路的继电器。增强电磁铁磁性的方法包括增大电流、增加线圈匝数或者使用磁导率更高的铁芯。

7. The Motor Effect | 电动机效应

When a current-carrying conductor is placed in an external magnetic field, it experiences a force. This phenomenon is called the motor effect. The force arises because the current’s own magnetic field interacts with the external field: on one side of the wire the two fields reinforce each other, producing a stronger region, while on the opposite side they partially cancel, producing a weaker region. The imbalance in magnetic pressure results in a net force that tries to push the conductor from the stronger field region to the weaker one.

当一根载流导体被置入外部磁场时，它会受到一个力，这个现象称为电动机效应。产生该力的原因是电流自身的磁场与外部磁场之间发生了相互作用：在导线的一侧，两个磁场互相增强，形成一个较强的区域；而在另一侧，两者相互削弱，形成一个较弱的区域。磁场压力的不平衡产生一个净力，试图将导体从强场区推向弱场区。

The direction of the force depends on the direction of both the current and the magnetic field. If either is reversed, the force reverses. If both are reversed, the force remains in the same original direction. This directional dependence is summarised by Fleming’s left-hand rule.

力的方向取决于电流方向和磁场方向。若其中任意一个方向反转，力的方向也会反转；若两者同时反转，则力的方向保持不变。这种方向依赖性可以用弗莱明左手定则来总结。

8. Fleming’s Left-Hand Rule | 弗莱明左手定则

Fleming’s left-hand rule is a mnemonic for the relative orientation of force, magnetic field and current in the motor effect. Hold your left hand with the thumb, first finger and second finger all mutually at right angles. The First finger points in the direction of the magnetic Field (N to S), the seCond finger points in the direction of the Current (conventional, positive to negative), and the ThuMb then indicates the direction of the Thrust (force) on the conductor.

弗莱明左手定则是用来记忆电动机效应中力、磁场和电流三者方向关系的助记方法。伸出左手，让拇指、食指和中指两两垂直。食指（First finger）指向磁场（Field）的方向（N极到S极），中指（seCond finger）指向电流（Current）的方向（传统方向，正极到负极），那么拇指（ThuMb）所指的方向就是导体所受推力（Thrust）的方向。

It is crucial to use the left hand for the motor effect. When working with generators and electromagnetic induction, a different rule – Fleming’s right-hand rule – applies. Always check which effect you are dealing with before choosing the hand. Practise with simple diagrams: draw the field lines, mark the current, and then use the rule to find the thrust.

在分析电动机效应时必须使用左手。当研究发电机和电磁感应现象时，适用的则是另一条法则——弗莱明右手定则。在选择使用哪只手之前，务必明确所涉及的是哪一种效应。通过简单的示意图进行练习：画出磁感线，标出电流方向，然后用左手定则找出推力方向。

9. Calculating the Force on a Conductor | 计算导体所受的力

For a straight conductor carrying a current I, placed perpendicular to a uniform magnetic field of flux density B, the magnitude of the force F experienced by a length L of the conductor is given by:

对于一根载有电流 I 的直导体，当其长度 L 垂直于磁通量密度为 B 的均匀磁场放置时，导体所受的力的大小由下式给出：

F = B I L

Here F is measured in newtons (N), B in teslas (T), I in amperes (A) and L in metres (m). The tesla can be understood by rearranging the equation: one tesla equals one newton per ampere per metre, written as N A⁻¹ m⁻¹. If the conductor is not perpendicular to the field, only the perpendicular component of the field contributes to the force; the force is zero when the conductor is parallel to the field.

其中 F 的单位为牛顿（N），B 的单位为特斯拉（T），I 的单位为安培（A），L 的单位为米（m）。通过公式变形可以理解特斯拉的定义：1特斯拉等于每安培每米1牛顿，记作 N A⁻¹ m⁻¹。如果导体不垂直于磁场，则只有磁场的垂直分量才对力有贡献；当导体平行于磁场放置时，所受的力为零。

This equation is extremely useful in explaining how factors affect the turning effect in a motor. Increasing the current, using stronger magnets (higher B) or lengthening the coil inside the field all increase the force and therefore the torque produced. In IGCSE problems, you will often be asked to calculate the force or predict the effect of changing one variable while keeping the others constant.

该方程在解释电动机中影响转动效应的各种因素时非常有用。增大电流、使用更强的磁铁（提高 B 值）或增加处于磁场中的线圈长度，都会增大作用力，进而增大产生的扭矩。在IGCSE考题中，常会要求计算力的大小，或者预测在保持其他变量不变的情况下改变某一变量所产生的效果。

10. The DC Electric Motor | 直流电动机

A simple DC motor uses the motor effect to produce continuous rotation. It consists of a rectangular coil of wire placed between the poles of a permanent magnet. When a direct current flows through the coil, opposite sides of the coil experience forces in opposite directions according to Fleming’s left-hand rule, creating a turning effect (a couple). The split-ring commutator and carbon brushes play a critical role by reversing the direction of the current every half turn.

简单的直流电动机利用电动机效应来产生持续转动。它由一个放置在永磁体两极之间的矩形线圈构成。当直流电通过线圈时，根据弗莱明左手定则，线圈相对的两条边会受到方向相反的力，从而形成一个力偶（转动力矩）。裂环换向器和碳刷发挥着关键作用：每转过半圈就反转电流的方向。

As the coil passes the vertical position, the split ring commutator swaps the connections to the brushes, so the current in the coil reverses. This ensures that the side of the coil which was previously pushed downwards is now pushed upwards, maintaining rotation in the same direction. Without a commutator, the coil would simply oscillate back and forth. Practical motors often use multiple coils (armature) and a curved permanent magnet to make the rotation smoother and more powerful.

当线圈转过竖直位置时，裂环换向器会交换与电刷的连接，从而使线圈中的电流反向。这样，原来被向下推的线圈边现在被向上推，从而维持同一方向的旋转。如果没有换向器，线圈只会来回摆荡。实际使用的电动机常常采用多个线圈（电枢）和弧形永磁体，以使转动更加平稳有力。

11. Applications: Relays and Loudspeakers | 应用：继电器与扬声器

A relay consists of an electromagnet that operates a switch. When a small current flows through the electromagnet’s coil, it produces a magnetic field that attracts an iron armature, pulling the switch contacts together and completing a separate high-current circuit. Because the control circuit and the switched circuit are electrically isolated, relays allow safe, remote switching of high-power devices using a low-power signal.

继电器由一个操纵开关的电磁铁组成。当一个小电流流过电磁铁的线圈时，它产生的磁场会吸引铁制衔铁，将开关触点吸合，从而接通另一个独立的高电流电路。由于控制电路与被开关的电路之间电气隔离，继电器允许利用低功率信号安全地远程控制高功率设备。

Loudspeakers work on the same motor-effect principle. A coil of wire is attached to a lightweight paper cone and placed inside the field of a permanent magnet. An alternating current, representing a sound signal, flows through the coil. The coil experiences a rapidly varying force, which makes it move back and forth, vibrating the cone and producing sound waves. The frequency and amplitude of the electrical signal determine the pitch and loudness of the sound.

扬声器基于同样的电动机效应原理工作。一个线圈连接在轻质纸盆上，并置于永磁体的磁场内。代表声音信号的交流电流过线圈，线圈受到迅速变化的力，从而前后运动，带动纸盆振动并产生声波。电信号的频率和振幅决定了声音的音调和响度。

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Electromagnetism (Edexcel IGCSE Science 6.3) | 电磁学