Magnetism and Electromagnetism | 磁学与电磁学

📚 Magnetism and Electromagnetism | 磁学与电磁学

Magnetism is a fundamental non-contact force that arises from the motion of electric charges. In the IGCSE Edexcel Science specification, you will explore permanent magnets, magnetic fields, electromagnets, the motor effect, electromagnetic induction, and transformers. Understanding these concepts helps explain how everyday devices such as electric motors, generators, and transformers work. This article provides a comprehensive yet accessible revision guide, pairing each key idea in English and Chinese.

磁学是一种基本的非接触力，由电荷的运动产生。在IGCSE爱德思科学大纲中，你将学习永磁体、磁场、电磁铁、电动机效应、电磁感应和变压器等内容。理解这些概念有助于解释电动机、发电机和变压器等日常设备的工作原理。本文提供一份全面而易于理解的复习指南，每个要点均以英文和中文对照呈现。

1. Magnetic Materials and Non-Magnetic Materials | 磁性材料与非磁性材料

Only a few materials are strongly attracted by magnets. These are called ferromagnetic materials and include iron, nickel, cobalt, and alloys like steel. Materials that are not attracted by magnets, such as wood, plastic, copper, and aluminium, are described as non-magnetic.

只有少数材料能被磁铁强烈吸引。这些材料被称为铁磁性材料，包括铁、镍、钴以及钢等合金。不能被磁铁吸引的材料，如木头、塑料、铜和铝，则称为非磁性材料。

When a ferromagnetic material is placed in a magnetic field, it becomes an induced magnet. Its domains align, and it gains the ability to attract other magnetic objects. However, when the external field is removed, soft iron quickly loses most of its magnetism, while steel retains it, making it suitable for permanent magnets.

当铁磁性材料置于磁场中时，它会变成感应磁体。其磁畴排列整齐，获得吸引其他磁性物体的能力。然而，当外部磁场撤去后，软铁会迅速失去大部分磁性，而钢则能保留磁性，因此适合制作永磁体。

2. Magnetic Poles and Forces | 磁极与磁力

Every magnet has two poles: a north-seeking pole (north pole) and a south-seeking pole (south pole). Like poles repel each other, while unlike poles attract. The force between two magnetic poles is an example of a non-contact force.

每个磁体都有两个极：指北极（北极）和指南极（南极）。同名极相互排斥，异名极相互吸引。两个磁极之间的力是非接触力的一种例子。

It is impossible to obtain an isolated north or south pole. If you break a bar magnet into two pieces, each piece will become a smaller magnet with its own north and south pole. This is the principle of magnetic dipoles.

不可能单独获得一个北极或南极。如果将条形磁铁断成两截，每一截都会成为具有自己北极和南极的小磁铁。这就是磁偶极子原理。

3. Magnetic Fields and Field Lines | 磁场与磁场线

A magnetic field is the region around a magnet where a magnetic force can be detected. Field lines, also called lines of force, represent the direction and strength of the field. They always point from the north pole to the south pole outside the magnet, and from south to north inside the magnet, forming closed loops.

磁场是磁体周围能检测到磁力的区域。磁感线（也称为力线）表示磁场的方向和强弱。在磁体外，磁感线总是从北极指向南极；在磁体内，从南极指向北极，形成闭合曲线。

The strength of a magnetic field is indicated by the density of field lines: the closer the lines, the stronger the field. A uniform magnetic field, such as that between two opposite flat poles, is represented by parallel, equally spaced lines.

磁场强弱由磁感线的疏密程度表示：磁感线越密集，磁场越强。均匀磁场（例如两个相对的平面磁极之间的磁场）用平行等距的磁感线表示。

4. Electromagnetism: Magnetic Effect of a Current | 电磁学：电流的磁效应

When an electric current flows through a straight wire, it produces a circular magnetic field around the wire. This phenomenon was discovered by Hans Christian Oersted. The direction of the magnetic field can be predicted using the right-hand grip rule.

当电流通过一根直导线时，会在导线周围产生环形磁场。这一现象由汉斯·克里斯蒂安·奥斯特发现。磁场方向可利用右手螺旋定则判断。

The strength of the magnetic field around a current-carrying wire depends on the magnitude of the current and the distance from the wire. A larger current or a smaller distance results in a stronger magnetic field.

载流导线周围磁场的强弱取决于电流大小和与导线的距离。电流越大或距离越近，磁场越强。

5. The Right-Hand Grip Rule | 右手螺旋定则

For a straight current-carrying conductor, if you hold the wire in your right hand with your thumb pointing in the direction of the conventional current, your fingers will curl in the direction of the magnetic field lines around the wire.

对于载流直导体，如果用右手握住导线，拇指指向电流方向（正电荷移动方向），则弯曲的四指指向导线周围磁场线的方向。

For a solenoid (a coil of wire), if you wrap your right hand around the solenoid with your fingers following the current direction, your thumb will point towards the north pole of the electromagnet. This allows you to determine the polarity of the solenoid.

对于螺线管（线圈），如果用右手握住螺线管，四指指向电流方向，则拇指指向电磁铁的北极。这样可以判断螺线管的极性。

6. Electromagnets and Their Uses | 电磁铁及其用途

An electromagnet is a temporary magnet made by placing a soft iron core inside a solenoid. When current flows, the iron core becomes strongly magnetised; when the current stops, the magnetism largely disappears. Electromagnets are very useful because their magnetic strength can be controlled by changing the current or the number of turns in the coil.

电磁铁是一种临时磁体，通过在螺线管中放入软铁芯制成。通电时，铁芯被强烈磁化；断电后，磁性基本消失。电磁铁非常有用，因为可以通过改变电流大小或线圈匝数来控制磁力强弱。

Common applications of electromagnets include electric bells, relays, circuit breakers, magnetic locks, and lifting scrap metal in scrapyards. In all these devices, the ability to switch the magnet on and off is essential.

电磁铁的常见应用包括电铃、继电器、断路器、磁力锁和废料场中起吊废金属。在所有这些设备中，能够接通和断开磁铁是至关重要的。

7. Motor Effect: Force on a Current-Carrying Conductor | 电动机效应：载流导体所受的力

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect. The force is maximum when the conductor is perpendicular to the magnetic field and zero when it is parallel. The size of the force can be calculated using the equation:

当载流导体置于磁场中时，会受到力的作用。这称为电动机效应。当导体与磁场方向垂直时，力最大；平行时，力为零。力的大小可由下式计算：

F = B I L

where F is the force in newtons (N), B is the magnetic flux density in teslas (T), I is the current in amperes (A), and L is the length of the conductor within the field in metres (m). This equation applies when the current is at right angles to the magnetic field.

其中 F 是力，单位为牛顿 (N)；B 是磁通量密度，单位为特斯拉 (T)；I 是电流，单位为安培 (A)；L 是导体在磁场中的长度，单位为米 (m)。此方程适用于电流与磁场方向垂直的情况。

8. Fleming’s Left-Hand Rule | 弗莱明左手定则

To predict the direction of the force on a current-carrying conductor in a magnetic field, use Fleming’s left-hand rule. Hold your thumb, first finger, and second finger mutually at right angles:

First finger: direction of the magnetic Field (N to S)
Second finger: direction of the Current (conventional, + to –)
Thumb: direction of the Thrust (force) on the conductor

要预测载流导体在磁场中所受力的方向，可使用弗莱明左手定则。将拇指、食指和中指互相垂直伸出：

食指：磁场方向（从N到S）
中指：电流方向（正电荷移动方向）
拇指：导体所受推力（力）的方向

This rule is essential for understanding electric motor operation and for solving motor effect problems in the exam. Always remember to use your left hand specifically for motors.

此定则对于理解电动机工作原理和解决考试中电动机效应问题至关重要。请务必记住：判断电动机力方向时要用左手。

9. Electric Motors (DC Motor) | 直流电动机

A simple DC motor consists of a coil of wire placed in a magnetic field, with a split-ring commutator and brushes to supply current. When current flows in the coil, opposite sides experience forces in opposite directions (due to Fleming’s left-hand rule), producing a turning effect or torque. The split-ring commutator reverses the current every half turn, ensuring the coil continues to rotate in the same direction.

简单的直流电动机由一个置于磁场中的线圈、供电用的换向器（开口环）和电刷组成。当线圈中有电流通过时，根据左手定则，线圈对边受到方向相反的力，产生转动效应或转矩。开口环换向器每半圈反转电流方向，确保线圈持续沿同一方向旋转。

Factors that increase the turning effect include: using a stronger magnetic field, increasing the current, increasing the number of turns on the coil, and placing a soft iron core inside the coil. Practical motors use many turns and curved magnets to give a more constant torque.

增大转矩的因素包括：增强磁场、加大电流、增加线圈匝数，以及在线圈中放置软铁芯。实际电动机中采用多匝线圈和弧形磁铁，以获得更均匀的转矩。

10. Electromagnetic Induction | 电磁感应

Electromagnetic induction is the process of generating an electromotive force (e.m.f.) across a conductor when it experiences a changing magnetic field. Michael Faraday discovered that moving a wire through a magnetic field, or changing the magnetic field around a conductor, induces a voltage. This is the basis of generators.

电磁感应是指导体在变化的磁场中产生电动势的过程。迈克尔·法拉第发现，将导线在磁场中移动或改变导体周围的磁场都会感应出电压。这是发电机的工作原理。

The magnitude of the induced e.m.f. depends on the rate of change of the magnetic field (or magnetic flux linkage). Increasing the speed of movement, the strength of the magnet, or the number of turns in the coil all increase the induced e.m.f.

感应电动势的大小取决于磁场（或磁通链）的变化率。加快运动速度、使用更强磁铁或增加线圈匝数都能增大感应电动势。

11. Generators and Alternating Current | 发电机与交流电

A simple AC generator (alternator) consists of a coil rotating in a magnetic field, with slip rings and brushes to transfer the current. As the coil rotates, the magnetic flux linkage changes continuously, producing an alternating e.m.f. and therefore an alternating current. The output voltage varies sinusoidally.

简单的交流发电机（交流发电机）由一个在磁场中转动的线圈、滑环和电刷组成。线圈旋转时，磁通链不断变化，产生交变电动势，从而形成交流电。输出电压呈正弦变化。

In contrast, a DC dynamo uses a split-ring commutator, which rectifies the current so that it flows in one direction only. The output from a DC dynamo is a varying direct current. In the exam, you should be able to sketch voltage‑time graphs for both types of generator.

相比之下，直流发电机使用开口环换向器，将电流整流为一个方向。直流发电机的输出是变化的直流电。在考试中，你应能够绘制两种发电机输出电压随时间变化的图像。

12. Transformers | 变压器

A transformer is a device that changes the voltage of an alternating current. It consists of two coils of insulated wire wound around a common laminated soft iron core. An alternating current in the primary coil produces a changing magnetic field, which induces an alternating e.m.f. in the secondary coil through the iron core.

变压器是一种改变交流电压的装置。它由绕在共用叠片软铁芯上的两个绝缘线圈组成。初级线圈中的交流电产生变化的磁场，该磁场通过铁芯在次级线圈中感应出交变电动势。

The relationship between the primary and secondary voltages (V₁ and V₂) and the number of turns (N₁ and N₂) is given by the transformer equation:

初级和次级电压（V₁ 与 V₂）与线圈匝数（N₁ 与 N₂）之间的关系由变压器方程给出：

V₁ / V₂ = N₁ / N₂

For an ideal transformer, the power input equals the power output (I₁ V₁ = I₂ V₂). Step‑up transformers increase voltage and decrease current, while step‑down transformers do the opposite. Transformers are essential in the national grid for efficient electricity transmission over long distances.

对于理想变压器，输入功率等于输出功率（I₁ V₁ = I₂ V₂）。升压变压器升高电压、降低电流；降压变压器则相反。变压器在国家电网中对远距离高效输电至关重要。

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Magnetism and Electromagnetism | 磁学与电磁学