📚 Magnetic Fields | 磁场
Magnetism plays a central role in modern technology, from the simple compass to the powerful electric motors that drive everything from household appliances to electric vehicles. In the GCSE OCR Physics specification, you are expected to understand the nature of magnetic fields, how they are created by permanent magnets and electric currents, and how the forces they produce can be harnessed to do useful work. This revision guide breaks down each key concept, provides clear explanations, and links theory to familiar applications such as electromagnetic relays, electric bells, DC motors and loudspeakers.
磁学在现代科技中扮演着核心角色，从简单的指南针到驱动家用电器和电动汽车的强大电动机，都离不开磁场。在 GCSE OCR 物理考试规范中，你需要理解磁场的本质、永磁体和电流如何产生磁场，以及如何利用磁场产生的力来做功。这篇复习指南逐项解析每个关键概念，提供清晰的解释，并将理论与电磁继电器、电铃、直流电动机和扬声器等常见应用联系起来。
1. Magnets and Magnetic Materials | 磁铁与磁性材料
All magnets have two poles – a north pole and a south pole. Like poles repel each other, while unlike poles attract. This fundamental rule governs every interaction between magnets.
所有磁铁都有两个磁极——北极和南极。同名磁极相互排斥，异名磁极相互吸引。这一基本规律支配着磁铁之间的所有相互作用。
Not all metals are magnetic. The only three pure metals that can be permanently magnetised are iron, nickel and cobalt. These are known as ferromagnetic materials. Alloys such as steel (which contains iron) are also ferromagnetic and can form permanent magnets. Other metals like aluminium, copper and silver are non-magnetic.
并非所有金属都具有磁性。能够被永磁化的纯金属只有铁、镍和钴三种，它们被称为铁磁性材料。合金（如含铁的钢）也具有铁磁性，可以制成永磁体。铝、铜、银等其他金属则没有磁性。
A permanent magnet produces its own magnetic field and retains its magnetism for a long time. An induced magnet, on the other hand, only becomes magnetic when placed within a strong magnetic field. Once removed, it quickly loses most of its magnetism. Soft iron is a typical material that behaves as an induced magnet.
永磁体能够产生自身的磁场，并能长时间保持磁性。而感应磁体只有在置于强磁场中时才会具有磁性，一旦移开，就会迅速失去大部分磁性。软铁就是典型的感应磁体材料。
2. Magnetic Field Lines | 磁感线
A magnetic field is a region around a magnet where another magnet or magnetic material experiences a non-contact force. The field is invisible, but we represent it using magnetic field lines (also called lines of force). These lines have a direction – by convention, they point from the north pole of a magnet to its south pole outside the magnet.
磁场是磁铁周围其他磁铁或磁性材料会受到非接触力的区域。虽然磁场看不见，但我们用磁感线（也叫磁力线）来表示。按照规定，在磁铁外部，磁感线的方向总是从北极指向南极。
The spacing of the field lines tells us about the strength of the field. Where the lines are close together, the magnetic field is strong. Where they are further apart, the field is weaker. A uniform magnetic field has parallel, equally spaced field lines, whereas a non-uniform field shows curved, converging or diverging lines.
磁感线的疏密表示磁场的强弱。磁感线密集的地方磁场强，稀疏的地方磁场弱。均匀磁场的磁感线相互平行且间距相等，而非均匀磁场的磁感线则是弯曲的、会聚的或发散的。
You should be able to sketch the magnetic field pattern around a single bar magnet, two bar magnets placed side by side with like poles facing, and two bar magnets with unlike poles facing. In the like-poles case, the field lines bulge away from each other at the centre, indicating repulsion; for unlike poles, the field lines connect the north to the south pole across the gap, showing attraction.
你应该能画出单根条形磁铁、同名极相对放置的两根条形磁铁以及异名极相对放置的两根条形磁铁周围的磁场分布图。在同名极相对的情况下，磁感线在中间相互向外凸出，表示排斥；异名极相对时，磁感线跨越间隙从北极连接到南极，表示吸引。
3. The Earth’s Magnetic Field | 地磁场
The Earth itself acts like a giant bar magnet. Its magnetic south pole is located near the geographic North Pole, and its magnetic north pole is near the geographic South Pole. This is why the north pole of a compass needle points towards the geographic North – it is attracted to the Earth’s magnetic south pole.
地球本身就像一个巨大的条形磁铁。它的磁南极位于地理北极附近，而磁北极位于地理南极附近。这就是指南针的北极指向地理北方的原因——它被地磁南极所吸引。
The Earth’s magnetic field is not perfectly aligned with its axis of rotation. The magnetic poles slowly move over time, and the field can even reverse polarity at irregular intervals over geological timescales. The field protects the Earth from charged particles in the solar wind by deflecting them, which is why compasses work and why aurorae form near the poles.
地球的磁场并非与其自转轴完全对齐。磁极会随时间缓慢移动，在地质时间尺度上，磁场甚至会发生极性反转。地磁场通过偏转太阳风中的带电粒子来保护地球，这也是指南针能够工作和极光出现在两极附近的原因。
4. Magnetism from Electric Current | 电流的磁效应
In 1820, Hans Christian Oersted discovered that an electric current flowing through a wire produces a magnetic field around the wire. This magnetic field is circular and can be demonstrated by placing a plotting compass near a current-carrying conductor. When the current flows, the compass needle aligns with the circular field lines.
1820 年，汉斯·克里斯蒂安·奥斯特发现，流过导线的电流会在导线周围产生磁场。这个磁场是圆形的，可以通过在载流导体附近放置一个磁针罗盘来演示。当有电流流动时，罗盘指针会沿着圆形磁感线方向偏转。
The direction of the circular magnetic field depends on the direction of the conventional current (from positive to negative). Reversing the current reverses the direction of the field. The field is stronger closer to the wire and weaker further away.
圆形磁场的方向取决于常规电流（从正到负）的方向。改变电流方向会使磁场方向反转。离导线越近磁场越强，越远则越弱。
5. The Right-Hand Grip Rule for a Straight Wire | 直线电流的右手定则
To find the direction of the magnetic field around a straight current-carrying wire, use the right-hand grip rule. Point your right thumb in the direction of the conventional current; your curled fingers will then indicate the direction of the circular magnetic field lines.
要判断载流直导线周围磁场的方向，可以使用右手握线定则。将右手拇指指向常规电流方向，弯曲的四指就表示圆形磁感线的环绕方向。
This rule is essential for predicting the magnetic field patterns in circuits and should be practised with diagrams showing current flowing vertically up or down through a card, with plotting compasses placed on the card to show concentric circles.
该定则对于预测电路中的磁场分布至关重要。你应该练习使用画有垂直向上或向下电流通过纸板的示意图，在纸板上放置小磁针来显示同心圆形的磁场。
6. The Magnetic Field of a Solenoid | 螺线管的磁场
A solenoid is a long coil of insulated wire. When current passes through it, the solenoid produces a strong and uniform magnetic field inside the coil, similar to the field of a bar magnet. The field lines inside are nearly parallel and evenly spaced, while outside the field resembles that of a bar magnet.
螺线管是由绝缘导线绕成的长线圈。当电流通过时，螺线管内部会产生强而均匀的磁场，类似于条形磁铁的磁场。内部的磁感线几乎是平行且等距的，而外部的磁场形状则类似于条形磁铁。
To determine the poles of a solenoid, use the right-hand grip rule again: curl the fingers of your right hand around the solenoid in the direction of the conventional current; your thumb will then point towards the north pole of the solenoid. This rule helps you quickly identify which end acts as north and which as south.
要确定螺线管的磁极，可以再次使用右手握线定则：将右手四指沿常规电流方向握住螺线管，拇指所指的一端就是螺线管的北极。这个规则可以帮助你快速判断哪一端是北极，哪一端是南极。
You can increase the strength of a solenoid’s magnetic field by increasing the current, increasing the number of turns of wire per unit length, or by inserting a soft iron core inside the coil. The iron core becomes an induced magnet and greatly concentrates the magnetic flux, making the electromagnet much stronger.
你可以通过增大电流、增加单位长度的线圈匝数或在螺线管内插入软铁芯来增强磁场。铁芯成为感应磁体，有极强地集中磁通量的作用，从而使电磁铁大大增强。
7. Electromagnets | 电磁铁
An electromagnet consists of a solenoid with a soft iron core. When current flows, the iron core becomes strongly magnetised; when the current is switched off, the core loses most of its magnetism quickly. This temporary magnetism is what makes electromagnets so useful in devices that need to switch magnetic forces on and off.
电磁铁由带有软铁芯的螺线管构成。当有电流流过时，铁芯被强烈磁化；当电流断开时，铁芯迅速失去大部分磁性。这种暂时磁性使得电磁铁在需要开关磁力的设备中非常有用。
The magnetic field of an electromagnet can be controlled by varying the current. A larger current produces a stronger magnetic field, but there is a limit because the iron core can reach magnetic saturation. Reversing the current reverses the polarity of the electromagnet.
电磁铁的磁场可以通过改变电流大小来控制。电流越大，磁场越强，但由于铁芯可能达到磁饱和而存在上限。改变电流方向会使电磁铁的磁极反转。
8. Applications of Electromagnets | 电磁铁的应用
Electromagnets are found in many everyday devices. In a scrap yard crane, a powerful electromagnet lifts heavy magnetic materials such as iron and steel. When the current is switched off, the load is released. In an electric bell, an electromagnet attracts a hammer that strikes the gong, and the circuit is automatically broken, causing the hammer to spring back and the cycle to repeat.
电磁铁出现在许多日常设备中。在废料场起重机中，强大的电磁铁能吸起铁和钢等沉重的磁性材料，当电流断开时，重物就被释放。在电铃中，电磁铁吸引小锤敲击铃盖，同时电路自动断开，小锤弹回，如此循环往复。
Another key application is the relay. A relay uses an electromagnet to operate a switch in a secondary circuit, allowing a small current in the primary circuit to control a much larger current in the secondary circuit safely. This is widely used in motor vehicles and industrial control systems.
另一个重要应用是继电器。继电器利用电磁铁来操控次级电路中的开关，使得初级电路中的小电流能够安全地控制次级电路中的大电流。这广泛应用于机动车辆和工业控制系统中。
9. Force on a Current-Carrying Conductor in a Magnetic Field | 磁场对载流导体的力（电动机效应）
When a current-carrying wire is placed perpendicular to a magnetic field, it experiences a force. This is known as the motor effect. The force is at a maximum when the wire is perpendicular to the field lines and is zero when the wire is parallel to the field. The magnitude of the force is given by the equation:
当载流导线垂直于磁场放置时，会受到一个力的作用，这就是电动机效应。当导线与磁感线垂直时力最大，平行时力为零。力的大小由以下公式给出：
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).
其中 F 是力，单位牛顿 (N)；B 是磁通量密度，单位特斯拉 (T)；I 是电流，单位安培 (A)；L 是导体在磁场中的长度，单位米 (m)。
Magnetic flux density B is a measure of the strength of the magnetic field. 1 T = 1 N A⁻¹ m⁻¹. Higher flux density means a stronger field, and therefore a larger force for a given current and length.
磁通量密度 B 是衡量磁场强弱的一个量。1 T = 1 N A⁻¹ m⁻¹。磁通量密度越高，磁场越强，因此在电流和长度一定时，受力也越大。
You can increase the force by increasing any of the three factors: a stronger magnet (higher B), a larger current (I), or a longer conductor inside the field (L). In practice, coils with many turns are used to multiply the effective length.
你可以通过增大三个因素中的任何一个来增大作用力：使用更强的磁铁（提高 B）、增大电流 (I) 或增加磁场中导体的长度 (L)。实际应用中常使用多匝线圈来增加有效长度。
10. Fleming’s Left-Hand Rule | 左手定则
Fleming’s left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field. Hold your left hand with the thumb, first finger and second finger mutually at right angles. The first finger points in the direction of the magnetic field (from N to S), the second finger points in the direction of the conventional current (from + to –), and the thumb then shows the direction of the force (motion) on the conductor.
左手定则可以判断磁场中载流导体的受力方向。伸出左手，让拇指、食指和中指相互垂直。食指指向磁场方向（从 N 到 S），中指指向常规电流方向（从 + 到 –），则拇指所指的方向就是导体受力的方向（运动方向）。
Remember that the magnetic field direction is always taken as north to south, and conventional current is opposite to electron flow. If the current or field is reversed, the force reverses. If both are reversed, the force direction remains unchanged.
请记住，磁场方向总是规定为从北极到南极，而常规电流方向与电子流动方向相反。如果电流方向或磁场方向反转，力的方向也随之反转。如果两者同时反转，力的方向保持不变。
11. The DC Electric Motor | 直流电动机
A simple DC motor consists of a coil of wire placed between the poles of a permanent magnet. When current flows through the coil, the two sides of the coil perpendicular to the field experience forces in opposite directions according to Fleming’s left-hand rule. These forces create a turning effect (torque), causing the coil to rotate.
简单的直流电动机由置于永磁体磁极之间的一匝线圈构成。当电流流过线圈时，根据左手定则，线圈中垂直于磁场的两边受到方向相反的力。这两个力产生一个转动效应（转矩），使线圈转动。
To keep the coil rotating continuously, a split-ring commutator is used. Each half of the split ring connects to one end of the coil and makes sliding contact with carbon brushes. As the coil passes the vertical position, the commutator reverses the direction of the current in the coil every half turn. This ensures the forces always act to push the coil in the same rotational direction, producing continuous rotation.
为了使线圈持续旋转，电动机使用了开口环换向器。开口环的两半分别连接线圈的两端，并与碳刷滑动接触。当线圈越过竖直位置时，换向器每半圈改变一次线圈中的电流方向，这使得力的作用方向始终推动线圈朝同一个方向旋转，从而实现持续转动。
Practical motors use several coils wound on a laminated soft iron armature, along with curved pole pieces to create a radial magnetic field, which improves torque and smoothness.
实用电动机使用绕在叠层软铁电枢上的多组线圈，并采用弧形磁极片来产生径向磁场，从而提高转矩和运转平稳性。
12. Loudspeakers | 扬声器
A loudspeaker is another common application of the motor effect. It contains a coil of wire (the voice coil) placed within a radial magnetic field produced by a permanent magnet. When an alternating current (AC) representing a sound signal passes through the coil, the coil experiences a varying force and moves back and forth. The coil is attached to a paper cone, which vibrates and produces sound waves in the air.
扬声器是电动机效应的另一个常见应用。它内部有一个线圈（音圈），置于永磁体产生的径向磁场中。当代表声音信号的交流电通过线圈时，线圈受到变化的力，从而前后运动。线圈连接着一个纸盆，纸盆振动并在空气中产生声波。
The frequency of the alternating current determines the frequency (pitch) of the sound produced, while the amplitude of the current determines the loudness. The precision of the cone movement reproduces the original sound accurately.
交流电的频率决定了所产生声音的频率（音调），而电流的振幅决定了声音的响度。纸盆运动的精确性能够准确地重现原始声音。
Published by TutorHao | Physics Revision Series | aleveler.com
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