Magnetism and Electromagnetism | 磁学与电磁学

📚 Magnetism and Electromagnetism | 磁学与电磁学

Magnetism and electromagnetism form a core topic in the Edexcel IGCSE Science course. This topic links the familiar behaviour of bar magnets to the modern technologies of electric motors, generators and transformers. Understanding magnetic fields, electromagnetic induction and the motor effect is essential for exam success.

磁学与电磁学是 Edexcel IGCSE 科学课程的核心内容。这一主题将常见的条形磁铁行为与电动机、发电机和变压器等现代技术联系起来。理解磁场、电磁感应和电动机效应，对考试取得好成绩至关重要。

1. Magnetic Materials and Magnets | 磁性材料与磁体

Magnetic materials are substances that are attracted to a magnet. The most common magnetic elements are iron, cobalt and nickel. Their alloys, such as steel, are also magnetic. A permanent magnet produces its own magnetic field and does not easily lose its magnetism.

磁性材料是指能被磁铁吸引的物质。最常见的磁性元素是铁、钴和镍。它们的合金，例如钢，也具有磁性。永磁体能够产生自己的磁场，且不容易失去磁性。

Magnets have two poles: north and south.

磁体有两个极：北极和南极。
Opposite poles attract; like poles repel.

异名磁极相互吸引；同名磁极相互排斥。
Repulsion is the only sure test of a magnet.

排斥作用是判断磁体的唯一可靠测试。

Magnetic materials can be magnetised by stroking them with a magnet or by passing a direct electric current through a coil around them. Some materials, like soft iron, magnetise easily but lose their magnetism quickly when removed from the magnetic field.

磁性材料可以通过用磁铁摩擦，或让直流电通过环绕它们的线圈来被磁化。像软铁这类材料容易被磁化，但当它离开磁场时会迅速失去磁性。

Soft iron is often used for the cores of electromagnets because it can be turned on and off quickly. Steel, on the other hand, is used for permanent magnets because it retains magnetism well.

软铁常被用作电磁铁的磁芯，因为它可以迅速磁化和退磁。与之相反，钢被用来制作永磁体，因为它能很好地保持磁性。

2. Magnetic Fields | 磁场

A magnetic field is a region around a magnet where magnetic forces can be detected. Magnetic field lines are drawn to show the direction and strength of the field. They always point from the north pole to the south pole outside the magnet.

磁场是磁体周围能探测到磁力的区域。磁场线用来表示磁场的方向和强弱。在磁体外部，磁场线总是从北极指向南极。

Magnetic field lines are continuous closed loops | 磁场线是连续的闭合曲线

The closer the field lines are, the stronger the magnetic field. Inside the magnet itself, the field lines point from south to north, which completes the loops. You should be able to draw the magnetic field pattern for a bar magnet using a plotting compass.

磁场线越密，磁场越强。在磁体内部，磁场线从南极指向北极，从而形成闭合回路。你应该能够使用小磁针绘制条形磁体的磁场图。

The Earth also has a magnetic field. The Earth behaves as if there is a giant bar magnet inside it. A freely suspended magnet aligns with the Earth’s magnetic field, which is why a compass works. Remember that the Earth’s geographic north pole is actually a magnetic south pole, so the north pole of a compass needle points toward it.

地球也有磁场。地球表现得就像内部有一根巨大的条形磁铁。自由悬挂的磁铁会顺着地球磁场方向排列，这就是指南针的工作原理。需要记住：地球的地理北极实际上是磁南极，因此指南针的北极会指向那里。

3. The Magnetic Effect of a Current | 电流的磁效应

When an electric current flows through a wire, it produces a magnetic field around the wire. This is called the magnetic effect of a current. The field consists of concentric circles centered on the wire, with the direction given by the right-hand grip rule.

当电流通过导线时，导线的周围会产生磁场。这被称为电流的磁效应。磁场由以导线为圆心的同心圆组成，方向由右手螺旋定则确定。

Right-hand grip rule: thumb points along current, fingers curl in direction of field | 右手螺旋定则：拇指指向电流方向，四指弯曲方向为磁场方向

A solenoid is a coil of wire wound into a cylinder. It behaves like a bar magnet when current flows through it. The magnetic field inside the solenoid is strong and uniform, while outside it is similar to the field around a bar magnet.

螺线管是将导线绕成圆柱形的线圈。当电流通过时，它的行为类似于条形磁铁。螺线管内部的磁场强且均匀，外部磁场则与条形磁体周围的磁场相似。

To increase the strength of the magnetic field produced by a solenoid, you can increase the current, increase the number of turns of wire, place a soft iron core inside the coil, or use the turns closer together.

要增强螺线管产生的磁场，可以增大电流、增加线圈匝数、在螺线管内部插入软铁芯，或者让线圈绕得更紧密。

4. Electromagnets | 电磁铁

An electromagnet is a solenoid with a soft iron core. When current flows through the coil, the iron core becomes magnetised and the magnetic field is greatly increased. When the current stops, the iron core loses most of its magnetism.

电磁铁是带有软铁芯的螺线管。当电流通过线圈时，软铁芯被磁化，磁场被大大增强。当电流停止时，铁芯会失去大部分磁性。

Electromagnets are used in a wide range of devices. A simple school bell uses an electromagnet to strike the gong repeatedly. Scrapyard cranes use a powerful electromagnet to lift and drop scrap metal. Electric relays use a small current in an electromagnet to control a larger current in a separate circuit.

电磁铁广泛应用于各类装置。学校的电铃利用电磁铁反复敲击铃钟。废料场起重机用强力电磁铁来起吊和放下废金属。继电器利用小电流控制的电磁铁，去控制另一个电路中的大电流。

In the exam, you may be asked to explain why an electromagnet is used instead of a permanent magnet. The key reason is that its magnetism can be switched on and off by controlling the current.

在考试中，你可能会被要求解释为什么使用电磁铁而不是永磁体。关键原因在于电磁铁可以通过控制电流来开启和关闭磁性。

5. The Motor Effect | 电动机效应

The motor effect is the force experienced by a current-carrying conductor placed in a magnetic field. This force arises because the magnetic field due to the current interacts with the external magnetic field. The force can cause the conductor to move.

电动机效应是指通电导体在磁场中受到的力。这个力是由于电流产生的磁场和外磁场相互作用而产生的。该力可以使导体运动。

F = BIL

The equation above gives the force on a conductor carrying current I, placed perpendicular to a magnetic field of flux density B, with length L inside the field. The force is maximised when the wire is perpendicular to the magnetic field and zero when it is parallel.

上面的公式给出导体在磁场中所受的力：其中 I 是电流，B 是磁通密度，L 是导体在磁场中的有效长度，且导体与磁场垂直。当导线与磁场垂直时力最大，平行时力为零。

The direction of the force can be found using Fleming’s left-hand rule. The thumb points in the direction of the force (motion), the first finger points in the direction of the magnetic field, and the second finger points in the direction of the current.

力的方向可以用弗莱明左手定则确定：拇指指向力（运动）的方向，食指指向磁场方向，中指指向电流方向。

This effect is used in electric motors. A rectangular coil of wire rotates in a magnetic field because the two opposite sides experience forces in opposite directions. A commutator reverses the current every half turn so that the coil continues to rotate in the same direction.

这一效应被应用于电动机。矩形线圈在磁场中转动，因为线圈两侧受到的力方向相反。换向器每半圈改变一次电流方向，从而让线圈持续朝同一方向转动。

6. Electromagnetic Induction | 电磁感应

Electromagnetic induction is the production of a potential difference in a conductor when there is a change in the magnetic field around it. This happens when a conductor cuts through magnetic field lines, or when the magnetic flux through a coil changes.

电磁感应是指当导体周围的磁场发生变化时，在导体中产生电势差的现象。当导体切割磁感线，或通过线圈的磁通量发生变化时，就会发生电磁感应。

There are two important ways to induce an e.m.f. in a coil:

在线圈中产生感应电动势有两种重要方式：

Move a magnet into or out of a coil.

将磁铁移入或移出线圈。
Move the coil relative to a stationary magnet.

让线圈相对静止磁铁运动。

The size of the induced potential difference increases when: the magnet is moved faster; the coil has more turns; the magnet is stronger; or the coil has a larger area. Reversing the direction of movement or changing the magnetic pole reverses the direction of the induced current.

感应电动势的大小会随着以下情况而增大：磁铁移动更快；线圈匝数更多；磁铁更强；或者线圈面积更大。改变运动方向或改变磁极，会使感应电流方向反向。

Lenz’s law states that the direction of an induced current always opposes the change that produced it. This is a consequence of conservation of energy. If the positive and negative peaks are opposite, the current is alternating.

楞次定律指出，感应电流的方向总是阻碍引起它的变化。这是能量守恒的必然结果。若感应电流正负峰值交替出现，则电流为交流电。

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

A generator converts kinetic energy into electrical energy by electromagnetic induction. In its simplest form, a coil of wire rotates in a magnetic field. The output is an alternating potential difference because the conductors in the coil continually change direction relative to the magnetic field.

发电机通过电磁感应将动能转化为电能。最简单的形式是，一个线圈在磁场中旋转。由于线圈中的导体相对于磁场的方向不断变化，因此输出的是交变电势差。

A slip-ring commutator is used to connect the rotating coil to an external circuit. Unlike a motor, the generator uses slip rings to maintain contact, allowing the current to change direction naturally. The output current of a simple generator is sinusoidal alternating current.

发电机使用滑环将旋转线圈连接到外电路。与电动机不同，发电机使用滑环保持接触，使电流能够自然换向。简单发电机的输出电流是正弦交流电。

To increase the output from a generator, you can rotate the coil faster, use a stronger magnet, increase the number of turns on the coil, or increase the area of the coil. In a direct current generator, a split-ring commutator is used to give a unidirectional output.

要增大发电机的输出电压，可以加快线圈旋转速度、使用更强的磁铁、增加线圈匝数，或增大线圈面积。在直流发电机中，使用换向器以获得单向输出。

8. Transformers | 变压器

A transformer is a device that changes the size of an alternating potential difference. It consists of a soft iron core with a primary coil and a secondary coil wound around it. In national grids, step-up transformers increase the voltage for transmission, and step-down transformers reduce it for safe domestic use.

变压器是用来改变交流电压大小的装置。它由一个软铁芯以及绕在铁芯上的初级线圈和次级线圈组成。在电网中，升压变压器提高输电电压，降压变压器则把电压降到适合家庭使用的安全值。

An alternating current in the primary coil produces a changing magnetic flux in the iron core. This changing flux links the secondary coil and induces an alternating current in it. Transformers only work with alternating current, not direct current.

初级线圈中的交流电在铁芯中产生变化的磁通。这个变化的磁通穿过次级线圈，并在其中感应出交流电。变压器只适用于交流电，不适用于直流电。

Vₚ / Vₛ = Nₚ / Nₛ

This is the transformer equation, where Vₚ and Vₛ are the primary and secondary voltages, and Nₚ and Nₛ are

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Magnetism and Electromagnetism | 磁学与电磁学