📚 GCSE CIE Physics: Magnetic Fields | GCSE CIE 物理：磁场考点精讲
Magnetism is a fundamental topic in GCSE CIE Physics, covering everything from simple permanent magnets to electric motors and transformers. Understanding magnetic fields and electromagnetic induction is essential for tackling exam questions confidently.
磁学是 GCSE CIE 物理的基础话题，涵盖从简单永磁体到电动机和变压器的全部内容。理解磁场与电磁感应对于充满信心地解决考试问题至关重要。
1. Magnets and Magnetic Materials | 磁铁与磁性材料
A magnet attracts objects made of iron, steel, nickel and cobalt. These substances are called ferromagnetic materials.
磁铁吸引由铁、钢、镍和钴制成的物体。这些物质称为铁磁材料。
Permanent magnets are made of hard magnetic materials like steel, which retain their magnetism for a long time. Temporary magnets are made of soft magnetic materials like soft iron, which lose their magnetism easily.
永磁体由硬磁性材料（如钢）制成，能长时间保持磁性。暂时磁体由软磁性材料（如软铁）制成，容易失磁。
A material can be magnetised by stroking it with a permanent magnet, or by placing it inside a solenoid carrying a direct current. It can be demagnetised by heating, hammering or using an alternating current field.
材料可以通过用永磁体摩擦，或者把它放在通有直流电的螺线管内部而被磁化。通过加热、锤击或使用交变电流可以使它去磁。
Hard magnetic materials are used for permanent magnets because they are difficult to demagnetise. Soft magnetic materials are used for electromagnets because they can be easily magnetised and demagnetised.
硬磁性材料难以退磁，因此用来制作永磁体。软磁性材料容易磁化和退磁，因此用于电磁铁。
2. Magnetic Poles and Their Interactions | 磁极及其相互作用
Every magnet has a north-seeking pole (N-pole) and a south-seeking pole (S-pole). These are commonly called the north and south poles.
每块磁铁都有一个指北极（N 极）和一个指南极（S 极），通常简称为北极和南极。
Like poles repel each other, whereas unlike poles attract. This is similar to the behaviour of electric charges.
同名磁极相互排斥，异名磁极相互吸引。这与电荷的行为相似。
If you break a bar magnet into two pieces, each piece becomes a smaller magnet with its own north and south pole. You cannot obtain an isolated magnetic pole; magnetism is a dipole phenomenon.
如果将一根条形磁铁分成两段，每一段都会变成一块具有自己 N 极和 S 极的小磁铁。无法得到孤立的磁单极；磁性是一种偶极现象。
The strength of magnetic attraction or repulsion decreases rapidly as the distance between the poles increases.
磁极间的引力或斥力随距离的增大而迅速减小。
3. Magnetic Field Lines | 磁感线
A magnetic field is the region around a magnet where a magnetic material or another magnet experiences a force. The direction of a magnetic field at a point is the direction of the force on an isolated N-pole placed there.
磁场是磁铁周围能使磁性材料或另一磁铁受到力的区域。磁场在某点的方向就是放在该点的孤立 N 极所受力的方向。
Magnetic field lines are continuous, they leave the N-pole and enter the S-pole. They never cross each other. The strength of the field is indicated by how close the lines are – the closer the lines, the stronger the field.
磁感线是连续的，从 N 极出发进入 S 极。它们永不相交。磁场的强弱由磁感线的疏密来表示——线越密，磁场越强。
A uniform magnetic field is represented by a set of parallel, equally spaced field lines. You can produce a uniform field between two opposite poles of flat bar magnets or inside a horseshoe magnet.
匀强磁场用一组平行且等距的磁感线表示。可以在两块扁平的条形磁铁的异名极之间，或在蹄形磁铁内部产生匀强磁场。
You can plot field lines using a plotting compass or by sprinkling iron filings on a card placed above a magnet.
可以使用指南针来描绘磁感线，或者在磁铁上方放置一张卡片并撒上铁屑来显示磁感线。
4. Earth’s Magnetic Field | 地磁场
The Earth itself behaves as a giant magnet with its magnetic south pole located near the geographic North Pole, and its magnetic north pole near the geographic South Pole.
地球本身像一个巨大的磁铁，其地磁南极位于地理北极附近，地磁北极位于地理南极附近。
A freely suspended bar magnet or a compass needle aligns itself roughly in the north–south direction. The N-pole of the compass points towards the geographic North because it is attracted by the Earth’s magnetic south pole.
自由悬挂的条形磁铁或指南针的 N 极大致指向南北方向。指南针的 N 极指向地理北方，是因为它被地球的磁南极所吸引。
The Earth’s magnetic field protects us from harmful solar wind particles and is thought to be generated by movements in the Earth’s liquid outer core.
地磁场保护我们免受有害太阳风粒子的伤害，一般认为它是由地球液态外核的运动产生的。
In magnetic navigation, knowledge of the angle of declination (difference between true north and magnetic north) is important, though not required in quantitative detail at GCSE.
在磁导航中，磁偏角（真北与磁北之间的夹角）的知识很重要，虽然 GCSE 阶段不要求定量细节。
5. Magnetic Effect of a Current – Straight Wire | 电流的磁效应——直导线
A current-carrying wire produces a magnetic field around it. This was first discovered by Hans Christian Oersted, who noticed a compass needle deflected near a current-carrying wire.
载流导线周围会产生磁场。最初由奥斯特发现，他注意到靠近通电导线的指南针发生了偏转。
The magnetic field lines around a straight current-carrying wire are concentric circles. The direction of the magnetic field can be determined using the right-hand grip rule: grasp the wire with your right hand, thumb pointing in the direction of conventional current; the fingers curl in the direction of the magnetic field.
通电直导线周围的磁感线是一些同心圆。磁场方向可用右手螺旋定则判断：用右手握住导线，拇指指向常规电流方向，那么四指弯曲的方向就是磁场方向。
The strength of the magnetic field increases with the current and decreases with distance from the wire. The field is also stronger if the wire is coiled into a solenoid.
磁场强度随电流增大而增强，随与导线的距离增大而减弱。若把导线绕成螺线管，磁场会更强。
On diagrams, a dot (•) in a wire symbolises current coming out of the page, and a cross (×) symbolises current going into the page.
在图中，导线中的点（•）表示电流垂直从纸面流出，叉（×）表示电流垂直流入纸面。
6. Magnetic Field of a Solenoid and Electromagnets | 螺线管和电磁铁
A solenoid is a long coil of insulated wire. When a current flows through it, the magnetic field pattern is similar to that of a bar magnet, with a distinct N-pole and S-pole.
螺线管是一个长的绝缘导线线圈。当有电流通过时，其磁感线图案与条形磁铁相似，具有清晰的 N 极和 S 极。
The polarity of a solenoid can be found using the right-hand rule: curl the fingers of your right hand around the solenoid in the direction of conventional current; your thumb then points to the N-pole.
螺线管的磁极极性可用右手定则判定：用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 N 极。
Inserting a soft iron core inside the solenoid greatly strengthens the magnetic field. This arrangement is called an electromagnet. When the current is switched off, the iron core loses most of its magnetism.
在螺线管内插入软铁芯可以大大增强磁场。这种装置称为电磁铁。当电流切断时，铁芯几乎失去全部磁性。
Electromagnets are used in relays, electric bells, circuit breakers and for lifting heavy scrap iron in recycling plants. Their strength can be increased by increasing the current or the number of turns in the coil.
电磁铁用于继电器、电铃、断路器和在回收厂中起吊重型废铁。通过增加电流或线圈匝数可以增强其磁性。
7. Fleming’s Left-Hand Rule and the Motor Effect | 弗莱明左手定则与电动机效应
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 current is perpendicular to the magnetic field.
当载流导体放在磁场中时，它会受到力的作用，这就是电动机效应。当电流方向与磁场方向垂直时，力最大。
The magnitude of the force is given by:
F = B I L
where F is the force (N), B is the magnetic flux density (T or N/A m), I is the current (A), and L is the length of the conductor in the field (m).
力的大小可用公式F = B I L计算，其中 F 为力（牛），B 为磁通量密度（特或牛/安·米），I 为电流（安），L 为导体在磁场中的长度（米）。
The direction of the force is given by Fleming’s left-hand rule: hold the thumb, forefinger and second finger of your left hand mutually at right angles. The First finger points in the direction of the magnetic Field (N → S), the seCond finger in the direction of the Current (conventional current), then the ThuMb points in the direction of the Motion (force).
力的方向由弗莱明左手定则判断：将左手拇指、食指和中指互成直角。食指指向磁场方向（N 到 S），中指指向常规电流方向，那么拇指就指向导体运动（力）的方向。
A simple direct current (d.c.) motor consists of a coil placed in a magnetic field. The commutator (split-ring) reverses the current direction every half turn, ensuring that the coil continues to rotate in the same direction.
简单的直流电动机由一个放在磁场中的线圈组成。换向器（开口环）每半圈反转一次电流方向，确保线圈持续朝同一方向旋转。
The speed of the motor can be increased by increasing the current, using a stronger magnetic field or adding more turns to the coil. Reversing the current or the magnetic field reverses the direction of rotation.
电动机的转速可以通过增大电流、使用更强的磁场或增加线圈匝数来提高。反转电流或磁场方向会使旋转方向反转。
8. Electromagnetic Induction | 电磁感应
Electromagnetic induction is the generation of an induced e.m.f. (voltage) when a conductor cuts magnetic field lines. It was discovered by Michael Faraday.
电磁感应是指当导体切割磁感线时产生感应电动势（电压）的现象。该现象由法拉第发现。
An induced e.m.f. can be produced by moving a magnet into a coil or by moving a conductor (such as a wire) through a magnetic field. The size of the induced e.m.f. increases when the relative speed of movement is greater, the magnet or magnetic field is stronger, or the coil has more turns.
把磁铁移入线圈，或者让导体（如导线）在磁场中运动，都会产生感应电动势。当相对运动速度越大、磁铁或磁场越强、线圈匝数越多时，感应电动势也越大。
Fleming’s right-hand rule (the generator rule) gives the direction of induced current: if the thumb, forefinger and second finger of the right hand are held at right angles, the First finger shows the magnetic Field, the thuMb the Motion of the conductor, and the seCond finger the induced Current direction.
弗莱明右手定则（发电机定则）给出感应电流的方向：将右手拇指、食指和中指互成直角，食指表示磁场方向，拇指表示导体的运动方向，那么中指就表示感应电流的方向。
The table below summarises the two Fleming’s rules you need to remember:
| Rule | Hand | Use | Fingers represent |
|---|---|---|---|
| Motor effect | Left | Force on current-carrying wire | Thumb = Motion (force) First finger = Field Second finger = Current |
| Generator effect | Right | Induced current when conductor moves | Thumb = Motion First finger = Field Second finger = Current |
Electromagnetic induction is used in generators, microphones, and in the operation of transformers, as you will see in the next sections.
电磁感应应用于发电机、麦克风和变压器的工作中，你将在后续小节中看到。
9. AC and DC Generators | 交流与直流发电机
A generator converts mechanical energy into electrical energy using electromagnetic induction. The simplest form is a coil rotating in a magnetic field, connected to an external circuit via slip rings or a commutator.
发电机利用电磁感应将机械能转化为电能。最简单的形式是一个线圈在磁场中转动，通过滑环或换向器连接到外电路。
If the coil is connected to two separate slip rings, the output voltage is alternating (a.c.) because the direction of e.m.f. reverses each half turn. This alternating voltage can be shown as a sinusoidal graph.
如果线圈连接到两个彼此绝缘的滑环，输出的是交流电压，因为每半圈电动势的方向会反转。这种交变电压可用正弦图形表示。
If a split-ring commutator is used instead of slip rings, the connections reverse each half turn so that the external circuit always receives current in the same direction, producing a varying direct voltage (d.c.).
如果使用裂环换向器代替滑环，连接关系每半圈反转一次，这样外电路总是得到同一方向的电流，从而产生脉动直流电压。
The frequency of the a.c. voltage depends on the speed of rotation: the faster the coil spins, the higher the frequency. The peak voltage increases if the coil spins faster, the magnetic field is stronger, or the coil has more turns and a larger area.
交流电压的频率取决于转动速度：线圈转得越快，频率越高。如果线圈转得更快、磁场更强、线圈匝数更多且面积更大，峰值电压就会增大。
In a simple dynamo, a magnet is often rotated near a coil, while in large power station generators, coils are rotated inside huge electromagnets.
在简易发电机中，通常让磁铁绕线圈转动，而在大型电站发电机中，线圈则在巨大的电磁铁内部旋转。
10. Transformers | 变压器
A transformer is a device that changes the size of an alternating voltage. It consists of two coils of wire, the primary coil and the secondary coil, wound on a common soft iron core.
变压器是一种改变交流电压大小的装置。它由两个线圈（初级线圈和次级线圈）绕在同一软铁芯上构成。
When an alternating current flows in the primary coil, it produces a changing magnetic field in the core. This changing field links with the secondary coil and induces an alternating voltage across it.
当初级线圈中有交流电通过时，会在铁芯中产生变化的磁场。这个变化的磁场与次级线圈交链，从而在次级线圈两端感应出交流电压。
For an ideal transformer (100% efficient), the ratio of voltages is equal to the ratio of the number of turns:
Vp / Vs = Np / Ns
where Vp and Vs are the primary and secondary voltages, Np and Ns are the number of turns on the primary and secondary coils.
对于理想变压器（100% 效率），电压之比等于匝数之比：Vp / Vs = Np / Ns，其中 Vp、Vs 分别为初级和次级电压，Np、Ns 分别为初级和次级线圈的匝数。
A step-up transformer has more turns on the secondary coil, so Vs > Vp. A step-down transformer has fewer turns on the secondary coil, so Vs < Vp.
升压变压器次级线圈匝数较多，因此 Vs > Vp。降压变压器次级线圈匝数较少，因此 Vs < Vp。
Transformer efficiency can be improved by using a laminated soft iron core, which reduces energy losses due to eddy currents. In an ideal transformer, input power equals output power, so Vp Ip = Vs Is (where I is current).
通过使用叠层软铁芯可以减少涡流造成的能量损耗，从而提高变压器效率。在理想变压器中，输入功率等于输出功率，即 Vp Ip = Vs Is（其中 I 为电流）。
Transformers are essential in the National Grid for stepping up voltage before transmission (to reduce current and therefore I²R heating losses) and stepping down voltage for safe use at home and in industry.
变压器在国家电网中至关重要：在输电前升高电压（以减小电流，从而减少 I²R 的热损耗），并在家庭和工业用电时降压以保证安全。
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