Magnetism for IGCSE AQA Physics | IGCSE AQA 物理：磁场考点精讲

📚 Magnetism for IGCSE AQA Physics | IGCSE AQA 物理：磁场考点精讲

Magnetism is a fundamental non-contact force that shapes everything from simple compasses to powerful electric motors and generators. In the IGCSE AQA Physics specification, you are expected to understand the nature of permanent and induced magnets, map magnetic fields, explain electromagnetism, and apply key rules like the right-hand grip rule and Fleming’s left-hand rule. This revision guide covers all the core concepts, equations, and practical applications you need to know for your exam, with clear explanations and comparisons to help you secure top marks.

磁性是一种基本的非接触力，从简单的指南针到强大的电动机和发电机，都离不开它。在 IGCSE AQA 物理考试大纲中，你需要理解永久磁铁与感应磁铁的本质，会描绘磁场，能解释电磁学原理，并应用右手螺旋定则和弗莱明左手定则等重要规则。本复习指南涵盖了你需要掌握的所有核心概念、方程和实际应用，通过清晰的讲解和对比，帮助你稳拿高分。

1. Magnetic Poles and Fields | 磁极与磁场

All magnets have two poles: a north-seeking (N) pole and a south-seeking (S) pole. Like poles repel each other, while unlike poles attract. The region around a magnet where a magnetic force can be felt is called the magnetic field. Magnetic field lines always point from the north pole to the south pole outside the magnet, and continue inside from south to north, forming closed loops. The field is strongest where the lines are closest together, namely at the poles.

所有磁铁都有两个极：指北极（N 极）和指南极（S 极）。同名磁极相互排斥，异名磁极相互吸引。磁铁周围能够感受到磁力的区域称为磁场。在磁铁外部，磁感线总是从北极指向南极；在磁铁内部，磁感线从南极回到北极，形成闭合回路。磁感线越密集的地方磁场越强，即磁极处的磁场最强。

2. Magnetic Materials and Induced Magnetism | 磁性材料与感应磁性

Only a few materials are ferromagnetic, meaning they can be magnetised strongly. The key ones are iron, nickel, cobalt, and their alloys such as steel. When an unmagnetised piece of iron is brought near a permanent magnet, it becomes an induced magnet. The side closest to the north pole of the permanent magnet becomes a south pole, so attraction always occurs. Induced magnetism is temporary; after removing the permanent magnet, most induced magnets quickly lose their magnetism, although steel can retain it to become a permanent magnet.

只有少数材料是铁磁性的，也就是可以被强烈磁化。主要的铁磁材料有铁、镍、钴及其合金（如钢）。当一块未磁化的铁靠近永久磁铁时，它会变成感应磁铁。靠近永久磁铁北极的那一面会变成南极，因此总是表现为吸引。感应磁性是暂时的；移开永久磁铁后，大多数感应磁铁会迅速失去磁性，但钢可以保留磁性，变成永久磁铁。

3. Electromagnetism: The Right-Hand Grip Rule | 电磁学：右手螺旋定则

When an electric current flows through a conducting wire, a circular magnetic field is produced around the wire. To find the direction of the magnetic field, use the right-hand grip rule: point your right thumb in the direction of conventional current (from positive to negative), and your curled fingers show the direction of the magnetic field lines wrapping around the wire. The field strength increases with current and decreases with distance from the wire. This is the foundation of electromagnets.

当电流通过导线时，导线周围会产生环形磁场。要确定磁场方向，可以使用右手螺旋定则：将右手拇指指向常规电流方向（从正极到负极），弯曲的四指所指的方向就是环绕导线的磁感线方向。磁场强度随电流增大而增大，随离导线的距离增大而减小。这是电磁铁的基础原理。

4. Magnetic Field Around a Solenoid | 螺线管周围的磁场

A solenoid is a long coil of insulated wire. When current passes through a solenoid, the magnetic fields from each turn add up to produce a strong, uniform field inside the coil, very similar to the field of a bar magnet. The right-hand grip rule can again be applied: grip the solenoid with your right hand so that your fingers curl in the direction of the conventional current; your thumb then points toward the north pole of the solenoid. Adding a soft iron core inside the solenoid greatly increases the field strength, forming an electromagnet that can be switched on and off.

螺线管是用绝缘导线绕成的长线圈。当电流通过螺线管时，每一匝线圈产生的磁场叠加在一起，在线圈内部形成强度大且均匀的磁场，与条形磁铁的磁场非常相似。这里同样可以应用右手螺旋定则：右手握住螺线管，让四指弯曲方向与常规电流方向一致，拇指所指的一端就是螺线管的北极。在螺线管内放入软铁芯可以极大地增强磁场强度，这就构成了一个可以随时开关的电磁铁。

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

When a current-carrying wire is placed in an external magnetic field, it experiences a force. This is called the motor effect. The direction of the force is always perpendicular to both the current direction and the magnetic field direction. To find which way the wire moves, use Fleming’s left-hand rule: hold your thumb, forefinger, and middle finger of your left hand mutually at right angles. The First finger represents the Field direction (N to S), the seCond finger the Current (conventional), and the ThuMb the Motion (force). This rule is crucial for understanding electric motors.

当通电导线置于外部磁场中时，它会受到力的作用。这叫作电动机效应。力的方向总是同时垂直于电流方向和磁场方向。要判断导线运动的方向，可以使用弗莱明左手定则：将左手的拇指、食指和中指彼此垂直伸出。食指（First finger）代表磁场方向（N 到 S），中指（seCond finger）代表常规电流方向，拇指（ThuMb）则代表导线受力的运动方向。这一规则对于理解电动机至关重要。

6. Factors Affecting the Motor Effect Force | 影响电动机效应力的因素

The size of the force on a current-carrying conductor in a magnetic field is given by the equation:

通电导体在磁场中受到的力的大小可用以下公式表示：

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 conductor within the field in metres (m). This equation applies when the conductor is perpendicular to the magnetic field. To increase the force, you can use a stronger magnet (larger B), pass a larger current (I), or use a longer wire inside the field (L). If the wire is parallel to the field, the force is zero.

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 是导体在磁场中的有效长度，单位为米（m）。该公式适用于导体垂直于磁场的情况。要增大作用力，可以使用更强的磁铁（增大 B）、通入更大的电流（I）或增加在磁场中的导线长度（L）。如果导线与磁场平行，力则为零。

7. Direct Current (DC) Motor | 直流电动机

A simple DC motor consists of a coil of wire placed in a magnetic field, often between the curved poles of a permanent magnet. When current flows, opposite sides of the coil experience forces in opposite directions due to Fleming’s left-hand rule, producing a turning effect or torque. A split-ring commutator reverses the current direction every half turn, ensuring the coil keeps rotating in the same direction. Carbon brushes maintain electrical contact with the rotating commutator. Increasing the current, the number of coil turns, or the magnetic field strength all increase the speed and turning force of the motor.

简易直流电动机由一个置于磁场中的线圈构成，通常位于永久磁铁的弧形极靴之间。通入电流后，根据弗莱明左手定则，线圈的两个对边受到方向相反的力，从而产生转动效果（转矩）。换向器（开口环）每半个周期反转一次电流方向，确保线圈始终朝同一方向旋转。碳刷则与旋转的换向器保持电接触。增大电流、增加线圈匝数或增强磁场强度，都可以提高电动机的转速和转动力量。

8. Electromagnetic Induction | 电磁感应

Electromagnetic induction is the process of generating a potential difference (voltage) in a conductor by changing the magnetic field around it. This can be achieved by moving a magnet into a coil, moving a coil in a magnetic field, or changing the current in a nearby coil. If the conductor is part of a complete circuit, an induced current flows. The direction of the induced current always opposes the change that produced it — this is Lenz’s law. Induction is the principle behind generators and transformers.

电磁感应是通过改变导体周围的磁场，从而在导体中产生电势差（电压）的过程。将磁铁插入线圈、让线圈在磁场中运动，或者改变邻近线圈中的电流，都可以实现电磁感应。如果导体是闭合回路的一部分，就会产生感应电流。感应电流的方向总是阻碍引起它的变化——这就是楞次定律。发电机和变压器都是基于电磁感应原理工作的。

9. Factors Affecting Induced Voltage | 影响感应电压的因素

The size of the induced voltage depends on how quickly the magnetic field changes. In the laboratory, a larger voltage is induced when the magnet moves faster into the coil, when a stronger magnet is used, or when the coil has more turns of wire. Moreover, inserting a soft iron core inside the coil concentrates the magnetic field lines, greatly increasing the induced voltage. In contrast, if there is no relative motion between the magnet and the coil, no voltage is induced at all.

感应电压的大小取决于磁场变化的快慢。在实验中，磁铁移入线圈的速度越快、磁铁的磁性越强、线圈的匝数越多，感应电压就越大。此外，在线圈中插入软铁芯可以集中磁感线，从而大大增加感应电压。反之，如果磁铁与线圈之间没有相对运动，则完全不会产生感应电压。

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

A simple AC generator (alternator) uses a coil rotating in a magnetic field. As the coil turns, it cuts through magnetic field lines, inducing a voltage. Slip rings and brushes allow the coil to remain connected to the external circuit without twisting the wires. Since the coil repeatedly changes its orientation relative to the field, the induced voltage alternates in direction, producing an alternating current (AC). A DC generator operates on the same principle but uses a split-ring commutator to produce a direct current in one direction only. In both cases, rotating the coil faster produces a higher frequency and a larger peak voltage.

简易交流发电机（交流发电机）利用在磁场中转动的线圈来工作。线圈旋转时切割磁感线，从而感应出电压。滑环和电刷使线圈与外电路保持连接，同时不会扭断导线。由于线圈相对于磁场的方向不断变化，感应电压的方向随之交替，从而产生交流电（AC）。直流发电机原理相同，但使用开口环换向器使输出电流只朝一个方向流动。无论交流还是直流发电机，线圈旋转得越快，频率越高，峰值电压也越大。

11. Transformers | 变压器

A transformer consists of two coils of insulated wire wound around the same soft iron core. An alternating current in the primary coil produces a changing magnetic field in the core. This changing field passes through the secondary coil and induces an alternating voltage across its ends. The ratio of the voltages is directly related to the ratio of the number of turns on the coils:

变压器由绕在同一软铁芯上的两组绝缘导线线圈组成。初级线圈中的交流电在铁芯中产生变化的磁场，该变化的磁场穿过次级线圈，在其两端感应出交流电压。输入输出电压之比与线圈的匝数比直接相关：

Vₚ / Vₛ = Nₚ / Nₛ

Where Vₚ and Vₛ are the primary and secondary voltages, and Nₚ and Nₛ are the number of turns on the primary and secondary coils. Assuming 100% efficiency, the input power equals the output power, so Vₚ × Iₚ = Vₛ × Iₛ. Step-up transformers increase voltage and reduce current, while step-down transformers do the opposite.

其中 Vₚ 和 Vₛ 分别为初级线圈和次级线圈的电压，Nₚ 和 Nₛ 为对应的匝数。假设变压器的效率为 100%，则输入功率等于输出功率，即 Vₚ × Iₚ = Vₛ × Iₛ。升压变压器升高电压、减小电流，降压变压器则相反。

12. The National Grid and Efficient Transmission | 国家电网与高效输电

The National Grid is the network of high-voltage transmission lines and transformers that distributes electricity across the country from power stations to homes and industries. Electricity is transmitted at very high voltages (typically 132 kV or more) to minimise energy losses. Using P = I² × R, reducing the current for the same power transfer drastically cuts resistive heating in the cables. Step-up transformers at power stations boost the voltage, and step-down transformers near towns and houses safely reduce it to 230 V (in the UK). This system makes the transmission process much more efficient and safer for end users.

国家电网是由高压输电线与变压器组成的网络，将电力从发电站输送到全国各地家庭和工厂。电能在非常高的电压（通常为 132 kV 或更高）下进行传输，以最大限度地减少能量损耗。根据 P = I² × R，对于相同的传输功率，减小电流可大幅降低电缆的电阻发热。发电站的升压变压器提升电压，而在城镇和住宅附近的降压变压器则将其安全降至 230 V（英国标准）。这套系统大大提高了输电效率，也保障了终端用户的安全。

Published by TutorHao | IGCSE AQA Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Magnetism for IGCSE AQA Physics | IGCSE AQA 物理：磁场 考点精讲