IGCSE OCR Physics: Magnetism Key Points | IGCSE OCR 物理：磁场考点精讲

📚 IGCSE OCR Physics: Magnetism Key Points | IGCSE OCR 物理：磁场考点精讲

Magnetism is a fundamental non-contact force that appears in permanent magnets, electromagnets and even in the Earth itself. In the IGCSE OCR Physics specification, you must learn how magnetic fields behave, how electric currents produce magnetic effects, and how these principles are applied in motors, generators and transformers. This article breaks down every essential concept, equation and practical application you need to master for your exam.

磁场是一种基本的非接触力，存在于永磁体、电磁铁甚至地球自身中。在 IGCSE OCR 物理考纲中，你必须掌握磁场的行为、电流如何产生磁效应，以及这些原理如何应用于电动机、发电机和变压器。本文逐一分解你需要掌握的每一个核心概念、方程和实际应用。

1. Magnets and Magnetic Materials | 磁铁与磁性材料

A magnet has two poles: a north-seeking (N) pole and a south-seeking (S) pole. Like poles repel each other, while unlike poles attract. The magnetic force is strongest at the poles.

磁铁有两个极：指北极（N）和指南极（S）。同名极相斥，异名极相吸。磁力在两极最强。

Only certain materials can be magnetised strongly. These are ferromagnetic materials such as iron, steel, cobalt and nickel. Steel retains magnetism well, so it is used for permanent magnets, while soft iron is easy to magnetise and demagnetise, making it ideal for temporary electromagnets.

只有某些材料能被强烈磁化，即铁磁性材料，如铁、钢、钴和镍。钢能很好地保持磁性，因此用于永磁体；而软铁容易磁化和退磁，非常适合制作临时电磁铁。

Non-magnetic materials like wood, plastic and glass are not attracted to magnets at all. Paramagnetic materials, such as aluminium, experience a very weak attraction but are not covered in detail at IGCSE.

非磁性材料如木材、塑料和玻璃完全不被磁铁吸引。顺磁性材料（如铝）受到极微弱的吸引力，但在 IGCSE 阶段不作深入要求。

2. Magnetic Fields and Field Lines | 磁场与磁感线

A magnetic field is the region around a magnet where a magnetic force can be detected. You can map the field using iron filings or a small plotting compass. Field lines always run from the north pole to the south pole outside the magnet, and continue through the magnet from south to north, forming closed loops.

磁场是磁铁周围可以检测到磁力的区域。你可以用铁屑或小磁针绘图。磁感线在磁铁外部总是从北极指向南极，在磁铁内部从南极指向北极，形成闭合回路。

The spacing of field lines indicates the strength of the field: the closer the lines, the stronger the field. At the poles, lines are most concentrated. Two unlike poles facing each other produce a uniform field pattern between them, with equally spaced parallel lines.

磁感线的疏密表示磁场强度：线越密，场越强。在两极处，磁感线最集中。两个异名极相对时，它们之间产生均匀磁场，磁感线等距平行。

Remember that magnetic field lines never cross. The Earth itself behaves like a giant bar magnet, with its magnetic south pole near the geographic North Pole, which is why a compass needle’s north pole points northwards.

切记磁感线永不相交。地球本身就像一块巨大的条形磁铁，其磁南极靠近地理北极，因此罗盘针的北极指向北方。

3. Magnetic Effect of a Current – Straight Wire | 电流的磁效应 – 直导线

Whenever an electric current flows through a conductor, it produces a magnetic field around it. For a long straight wire, the field lines form concentric circles centred on the wire. The direction of the magnetic field can be found using the right-hand grip rule: point your right thumb in the direction of conventional current (positive to negative), and your curled fingers show the direction of the magnetic field.

每当电流通过导体时，导体周围就会产生磁场。对于长直导线，磁感线是以导线为中心的同心圆。磁场方向可用右手握线定则判断：将右手拇指指向常规电流方向（正到负），弯曲的四指所指的方向就是磁场方向。

The strength of this magnetic field increases if you increase the current or bring the compass closer to the wire. Reversing the current reverses the direction of the field lines. This effect is the foundation of electromagnetism.

如果增大电流或让罗盘更靠近导线，磁场会增强。反转电流方向，磁感线方向也随之反转。这是电磁学的基础。

4. Solenoids and Electromagnets | 螺线管与电磁铁

A solenoid is a coil of wire. When current flows through it, the magnetic field pattern is similar to that of a bar magnet: one end acts as a north pole, the other as a south pole. The polarity can be determined by the right-hand grip rule applied to the solenoid: curl your right hand around the coil in the direction of current, and your thumb points towards the north pole.

螺线管是一个线圈。当电流通过时，其磁场形状与条形磁铁相似：一端为北极，另一端为南极。极性可用右手螺旋定则判断：右手沿电流方向弯曲握住线圈，拇指所指即为北极。

You can increase the strength of the magnetic field of a solenoid by: increasing the current, increasing the number of turns, inserting a soft iron core inside the coil. The soft iron core becomes magnetised and greatly concentrates the field, producing a powerful electromagnet.

你可以通过以下方法增强螺线管磁场：增大电流、增加线圈匝数、在线圈内插入软铁芯。软铁芯被磁化后能极大集中磁场，形成强大的电磁铁。

Electromagnets are used in relays, electric bells, circuit breakers and lifting magnets in scrap yards. Their advantage is that the magnetism can be switched on and off by controlling the current.

电磁铁常用于继电器、电铃、断路器和废料场的起重磁铁。其优点是可以通过控制电流来开关磁性。

5. The Motor Effect – Fleming’s Left-Hand Rule | 电动机效应 – 弗莱明左手定则

When a current-carrying conductor is placed in an external magnetic field, the two magnetic fields interact, producing a force on the wire. This is called the motor effect. The force is perpendicular to both the current direction and the magnetic field.

当载流导线置于外磁场中时，两个磁场相互作用，对导线产生一个力，这就是电动机效应。该力垂直于电流方向和磁场方向。

The direction of the force is given by Fleming’s left-hand rule: hold your left hand with the thumb, forefinger and second finger mutually at right angles. The First finger represents the magnetic Field direction (N to S), the seCond finger represents the Conventional Current (+ to -), and the ThuMb shows the direction of Motion (force). A common mnemonic is ‘F B C’ – Field, B-current, Motion – but remember the sequence: First finger Field, seCond finger Current, ThuMb Motion.

力的方向由弗莱明左手定则给出：伸出左手，使拇指、食指和中指两两垂直。食指（First finger）表示磁场方向（N 到 S），中指（seCond finger）表示常规电流方向（+ 到 -），拇指（ThuMb）表示运动（力）的方向。一个记忆窍门是 ‘F B C’，但记住顺序：食指磁场，中指电流，拇指运动。

The size of the force can be calculated using the equation:

F = B I L

力的大小可用公式计算：

F = B I L

where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A), and L is the length of the conductor in the magnetic field in metres (m). This equation assumes the wire is perpendicular to the field. If the wire is at an angle θ to the field, the effective component is B I L sinθ.

其中 F 是力（牛顿，N），B 是磁通量密度（特斯拉，T），I 是电流（安培，A），L 是导线在磁场中的长度（米，m）。该公式假设导线与磁场垂直。若导线与磁场成角度 θ，则有效分量为 B I L sinθ。

6. The DC Motor | 直流电动机

A simple DC motor consists of a rectangular coil of wire placed in a uniform magnetic field, with a current passed through it via a split-ring commutator and carbon brushes. The forces on the two sides of the coil produce a couple (a pair of opposite forces acting on different lines of action) that causes the coil to rotate.

简易直流电动机由一个矩形线圈组成，放置在均匀磁场中，通过裂环整流器和碳刷引入电流。线圈两侧所受的力形成一对力偶（作用在不同作用线上的一对反向力），使线圈旋转。

The split-ring commutator is a clever device that reverses the direction of current in the coil every half turn. This ensures that the forces always push the coil in the same rotational direction, maintaining continuous rotation. Without a commutator, the coil would merely oscillate.

裂环整流器是一种巧妙的装置，每半圈就反转线圈中的电流方向。这确保力始终推动线圈朝同一方向旋转，维持持续转动。如果没有整流器，线圈只会来回摆动。

To increase the turning effect of a motor, you can: increase the current, use a stronger magnetic field, increase the number of turns on the coil, or place a soft iron core inside the coil to concentrate the field. The turning effect (torque) is maximised when the plane of the coil is parallel to the magnetic field lines.

要增大电动机的转动效果，可以：增大电流、使用更强的磁场、增加线圈匝数、或在线圈内加入软铁芯以集中磁场。当线圈平面与磁感线平行时，转动效应（力矩）最大。

7. Electromagnetic Induction | 电磁感应

Electromagnetic induction occurs when a conductor cuts through magnetic field lines, or when the magnetic field around a conductor changes. This induces a potential difference (voltage) across the ends of the conductor. If the conductor is part of a complete circuit, a current flows.

当导体切割磁感线，或导体周围的磁场发生变化时，就会发生电磁感应。这会在导体两端感应出电势差（电压）。如果导体是闭合回路的一部分，就会有电流流动。

The size of the induced p.d. depends on the rate at which the magnetic field changes or the conductor cuts the field lines. A faster movement, a stronger magnetic field, or a larger number of turns in a coil all increase the induced voltage. Lenz’s law states that the direction of the induced current is such that it opposes the change causing it, which is a direct consequence of the conservation of energy.

感应电动势的大小取决于磁场变化的快慢或导体切割磁感线的速率。运动越快、磁场越强、线圈匝数越多，感应电压越大。楞次定律指出，感应电流的方向总是阻碍引起感应的变化，这是能量守恒的直接结果。

Lenz’s law explains why you must do work to push a magnet into a coil; the induced current creates a magnetic field that repels the magnet. If the induced current aided the motion, you would get energy for free, which is impossible.

楞次定律解释了为什么把磁铁推入线圈时必须做功；感应电流产生的磁场会排斥该磁铁。如果感应电流促进运动，就会凭空获得能量，这是不可能的。

8. The Generator Effect – Fleming’s Right-Hand Rule | 发电机效应 – 弗莱明右手定则

The generator effect is the practical application of electromagnetic induction. When a coil is rotated in a magnetic field, or a magnet is moved near a coil, a changing magnetic flux linkage induces an alternating p.d. and, if connected, an alternating current (a.c.).

发电机效应是电磁感应的实际应用。当线圈在磁场中旋转，或磁铁在线圈附近移动时，变化的磁通链会感应出交变电势差，若接通回路则产生交流电。

For a moving conductor, you can use Fleming’s right-hand rule to find the direction of the induced conventional current. Arrange your right hand with thumb, forefinger and second finger mutually at right angles: First finger Field (N to S), ThuMb Motion of the conductor, and seCond finger gives the direction of induced Conventional Current. Notice the switch from the left hand for motors.

对于运动的导体，可用弗莱明右手定则判断感应常规电流的方向。伸出右手，使拇指、食指和中指两两垂直：食指磁场（N 到 S），拇指导体运动方向，中指为感应常规电流的方向。注意与电动机使用的左手不同。

In a simple alternator (a.c. generator), slip rings and brushes maintain continuous electrical connection without reversing the current, so the output is a.c. In a dynamo, a split-ring commutator is used to produce direct current (d.c.) output. The frequency of the a.c. output equals the number of rotations per second of the coil.

在简易交流发电机中，滑环和电刷保持持续电连接而不反转电流，因此输出为交流电。在直流发电机中，使用裂环整流器产生直流电输出。交流电输出频率等于线圈每秒转动的圈数。

9. Transformers | 变压器

A transformer is a device that changes the size of an alternating voltage. It consists of two coils wound on a complete soft iron core. The primary coil is connected to the input a.c. supply, and the secondary coil provides the output voltage. Transformers only work with alternating current because a changing magnetic field is required for induction.

变压器是一种改变交流电压大小的装置。它由两个绕在完整软铁芯上的线圈组成。初级线圈连接输入交流电源，次级线圈提供输出电压。变压器只能使用交流电，因为感应需要变化的磁场。

The relationship between the voltages and the numbers of turns is given by the transformer equation:

Vp / Vs = Np / Ns

电压与匝数之间的关系由变压器方程给出：

Vp / Vs = Np / Ns

where Vp and Vs are the primary and secondary voltages, and Np and Ns are the numbers of turns on the primary and secondary coils. A step-up transformer has more turns on the secondary coil (Ns > Np) and increases voltage; a step-down transformer has fewer secondary turns and decreases voltage.

其中 Vp 和 Vs 是初、次级电压，Np 和 Ns 是初、次级线圈的匝数。升压变压器次级匝数更多（Ns > Np），电压增大；降压变压器次级匝数较少，电压减小。

If the transformer is 100% efficient (ideal transformer), the input power equals the output power: Vp × Ip = Vs × Is. In practice, some energy is lost as heat due to resistance in the coils and eddy currents in the core. Laminated cores reduce eddy current losses.

若变压器效率为 100%（理想变压器），输入功率等于输出功率：Vp × Ip = Vs × Is。实际中，由于线圈电阻和铁芯中的涡流，会有部分能量以热的形式损失。分层铁芯可减少涡流损失。

Transformers are essential in the National Grid: step-up transformers raise the voltage to hundreds of kilovolts for transmission, reducing current and thus I²R power loss in the cables; step-down transformers then lower the voltage to safe levels for homes and industries.

变压器在国家电网中至关重要：升压变压器将电压升至数十万伏进行传输，以降低电流，从而减少导线中的 I²R 功率损耗；降压变压器再将电压降至家庭和工业安全使用的水平。

10. Magnetising and Demagnetising Methods | 磁化与退磁方法

A ferromagnetic material like steel can be magnetised by: stroking it repeatedly in one direction with a strong permanent magnet, placing it inside a solenoid connected to a direct current (d.c.) supply, or aligning it in the Earth’s magnetic field and hammering it. The stroking and d.c. solenoid methods produce a permanent magnet.

像钢这样的铁磁性材料可通过以下方法磁化：用强永磁体沿一个方向反复摩擦，将其放入通以直流电的螺线管内，或将其对准地磁场方向并锤击。摩擦法和直流螺线管法可制造永磁体。

To demagnetise a magnet, you can: heat it strongly and let it cool in an east-west direction to avoid the Earth’s field, hammer it vigorously while pointing east-west, or place it inside a solenoid connected to an alternating current (a.c.) and slowly withdraw the magnet. The a.c. method repeatedly reverses and reduces the magnetic domains’ alignment, leaving the material unmagnetised.

要退磁，可以：对磁铁强烈加热并沿东西方向冷却以避免地磁场的影响，指着东西方向用力锤击，或将其放入通有交流电的螺线管中并缓慢抽出。交流电法不断反转并削弱磁畴的取向，使材料恢复未磁化状态。

11. Magnetic Materials and Domain Theory | 磁性材料与磁畴理论

Domain theory explains how ferromagnetic materials become magnetised. Inside a ferromagnetic material, there are tiny regions called domains, each acting as a tiny magnet with a north and south pole. In an unmagnetised material, the domains point in random directions, so their magnetic fields cancel out.

磁畴理论解释了铁磁性材料如何被磁化。在铁磁性材料内部，有称为磁畴的微小区域，每个区域就像一个带有南北极的小磁铁。未磁化时，磁畴指向随机，其磁场相互抵消。

When the material is placed in a strong external magnetic field, the domains align in the direction of the field. The more domains that align, the stronger the overall magnetism. In a permanent magnet, most domains remain aligned even after the external field is removed. In a temporary magnet like soft iron, the domains easily fall back into random orientations once the field is gone.

当材料置于强外磁场中时，磁畴沿磁场方向排列。排列的磁畴越多，整体磁性越强。在永磁体中，撤去外磁场后，大多数磁畴仍保持取向。而像软铁这样的临时磁铁，一旦磁场消失，磁畴很容易恢复为随机方向。

12. Key Formulas and Exam Tips | 核心公式与备考提示

Memorise the following equations exactly and know when to apply them: F = B I L for the motor effect when the conductor is perpendicular to the field. Vp / Vs = Np / Ns for ideal transformers. Power input = Power output for ideal transformers, so Vp Ip = Vs Is. Pay attention to units: B in tesla (T), I in amperes (A), L in metres (m), F in newtons (N).

准确记忆以下公式并知道何时使用：F = B I L 适用于导线垂直于磁场时的电动机效应。Vp / Vs = Np / Ns 适用于理想变压器。理想变压器输入功率 = 输出功率，即 Vp Ip = Vs Is。注意单位：B 为特斯拉（T），I 为安培（A），L 为米（m），F 为牛顿（N）。

In exams, always state the rule you are using – for example, ‘by Fleming’s left-hand rule, the force on the wire is upwards’. Clearly explain the direction of field, current and force/motion. When describing electromagnetic induction, use the phrase ‘cutting magnetic field lines’ and mention the rate of change. For transformers, stress that they only work with a.c. because a changing flux is needed for induction. Practise drawing the field patterns and labelling poles.

考试中，务必说明你使用的定则——例如，“根据弗莱明左手定则，导线所受的力向上”。清楚说明场、电流和力/运动的方向。在描述电磁感应时，使用“切割磁感线”这一说法并提及变化率。对于变压器，强调只能使用交流电，因为感应需要变化的磁通。练习绘制磁场图形并标注磁极。

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IGCSE OCR Physics: Magnetism Key Points | IGCSE OCR 物理：磁场 考点精讲