📚 Magnets and Magnetic Fields | 磁铁与磁场
Magnets are objects that produce an invisible region called a magnetic field around them. Understanding magnets and magnetic fields explains how compasses work, why only some materials are attracted to a magnet, and how forces can act at a distance without the two objects ever touching.
磁铁是能在周围产生一种看不见的区域——磁场的物体。理解磁铁与磁场，可以解释指南针的工作原理、为什么只有某些材料会被吸引，以及力如何在不接触的情况下发生作用。
This revision guide follows the KS3 and Cambridge lower secondary science syllabus. It covers magnetic poles, field lines, induced magnetism, the Earth's magnetic field, electromagnets and the key practical investigations, and it ends with exam-style tips and common mistakes.
本复习指南依照 KS3 与剑桥初中科学大纲编写，涵盖磁极、磁感线、感应磁性、地磁场、电磁铁以及核心实验探究，并在最后给出考试技巧与常见错误。
1. What is a Magnet? | 什么是磁铁？
A magnet is a material or object that produces a magnetic field and can attract certain metals, especially iron, nickel and cobalt. Steel is also attracted, because steel is mainly iron. Magnets do not attract copper, aluminium, brass, plastic, wood or glass.
磁铁是一种能产生磁场并吸引某些金属（尤其是铁、镍和钴）的材料或物体。钢同样会被吸引，因为钢的主要成分是铁。磁铁不吸引铜、铝、黄铜、塑料、木材或玻璃。
Permanent magnets keep their magnetism for a long time, while temporary magnets behave like magnets only when they are placed inside a strong magnetic field. A permanent magnet does not need electricity to work, which makes it different from an electromagnet.
永磁体可以长期保持磁性，而暂时磁体只有在放入强磁场中时才会表现出磁性。永磁体不需要电流就能工作，这一点与电磁铁不同。
In KS3 experiments, bar magnets and horseshoe magnets are commonly used because their poles are easy to identify and their shape makes the field pattern easy to plot. A magnet can also attract magnetic materials through paper, water, plastic and wood, which shows that the force acts at a distance rather than by contact.
在 KS3 实验中，条形磁铁和蹄形磁铁很常用，因为它们的磁极容易辨认，而且形状使磁场分布容易描绘。磁铁还能隔着纸、水、塑料和木材吸引磁性材料，这说明磁力是隔空作用的，并不需要接触。
2. Magnetic Poles | 磁极
Every magnet has two poles: a north-seeking pole (N) and a south-seeking pole (S). The magnetic force is strongest at these poles and weaker in the middle of a bar magnet. If a bar magnet is hung freely from a thread, it slowly turns so that its N pole points roughly towards the Earth's geographic north.
每块磁铁都有两个磁极：指北极（N）和指南极（S）。磁力在这些磁极处最强，在条形磁铁的中部较弱。如果用细线把条形磁铁自由悬挂起来，它会慢慢转动，使 N 极大致指向地球的地理北方。
The two poles cannot be separated. If you cut a magnet in half, each piece will still have its own N pole and S pole. Cutting the pieces again simply produces more small magnets, each with a complete pair of poles. A single magnetic pole on its own has never been found.
两个磁极无法分离。如果把一块磁铁切成两半，每一半仍会各自具有 N 极和 S 极。继续切下去，只会得到更多完整的小磁铁。单独存在的磁极从未被发现。
This is why the pole names are based on direction rather than on any special material: the end that turns towards the north of the Earth is called the north-seeking pole, and the opposite end is the south-seeking pole.
这也是磁极名称由指向决定、而非由特殊材质决定的原因：转向地球北方的一端叫指北极，另一端叫指南极。
3. Magnetic and Non-Magnetic Materials | 磁性材料与非磁性材料
Materials can be divided into magnetic materials, which are attracted by a magnet, and non-magnetic materials, which are not attracted at all. Only iron, nickel and cobalt, plus alloys that contain them, are strongly magnetic.
材料可分为磁性材料（会被磁铁吸引）和非磁性材料（完全不被吸引）。只有铁、镍、钴以及含有这些金属的合金才具有很强的磁性。
Soft iron is easy to magnetise but loses its magnetism quickly, so it is used for the core of an electromagnet. Steel is harder to magnetise but keeps its magnetism, so it is used to make permanent magnets.
软铁容易磁化，但很快失去磁性，因此常用于电磁铁的铁芯。钢较难磁化，但能保持磁性，因此常用于制造永磁体。
A simple test is to bring the material close to one pole of a bar magnet and watch for attraction. Attraction alone only proves that the material is magnetic; it does not prove that the material is itself a magnet.
一个简单的检验方法是把材料靠近条形磁铁的一极，观察是否被吸引。单凭吸引只能说明该材料是磁性材料，并不能证明它本身就是一块磁铁。
| Attracted by a magnet | 会被吸引 | Not attracted | 不会被吸引 |
| Iron 铁, steel 钢, nickel 镍, cobalt 钴, alnico 铝镍钴合金, some stainless steels 部分不锈钢 | Copper 铜, aluminium 铝, brass 黄铜, zinc 锌, plastic 塑料, wood 木材, glass 玻璃, water 水, paper 纸 |
4. The Magnetic Field | 磁场
A magnetic field is the region around a magnet where a magnetic force can be detected. The field is invisible, but its shape can be shown using iron filings or a plotting compass. Because magnetism is a non-contact force, a magnet can pull a steel pin without touching it.
磁场是磁铁周围能够检测到磁力的区域。磁场看不见，但可以用铁屑或小磁针显示它的形状。由于磁力属于非接触力，磁铁可以在不接触的情况下吸起钢针。
he force on a magnetic material placed in it. Magnetic field strength is measured in tesla (T). The Earth' strong permanent magnet near one of its poles can produce a field of the order of 10⁻² T.
磁场越强，放在其中的磁性材料受到的力就越大。磁感应强度的单位是特斯拉（T）。地磁场约为 5 × 10⁻⁵ T，而强永磁体在磁极附近产生的磁场可达约 10⁻² T 的数量级。
Fields pass straight through non-magnetic materials such as paper, plastic, glass, wood, water and aluminium. Putting a sheet of paper between a magnet and a paper clip does not weaken the force enough for the clip to fall.
磁场可以穿过纸张、塑料、玻璃、木材、水和铝等非磁性材料。把一张纸夹在磁铁和回形针之间，磁力并不会明显减弱到让回形针掉下来。
5. Magnetic Field Lines and Field Patterns | 磁感线与磁场分布
Field lines are drawn to show the shape of a magnetic field and the direction of the force. Outside a magnet, the lines run from the N pole to the S pole; inside the magnet they continue from S back to N, so every line forms a complete loop.
磁感线用来表示磁场的形状和磁力的方向。在磁铁外部，磁感线从 N 极指向 S 极；在磁铁内部，它们由 S 极回到 N 极，因此每条磁感线都构成一个闭合回路。
Where the lines are close together, the field is strong; where they are spread out, the field is weak. Field lines never cross each other, because at any point the field can only have one direction.
磁感线越密集的地方磁场越强；越稀疏的地方磁场越弱。磁感线永不相交，因为在同一点上磁场只能有一个方向。
Field lines always leave the N pole and enter the S pole outside the magnet.
Around a bar magnet the pattern is like a set of curved loops that are closest together at the two poles. Between the poles of a horseshoe magnet the lines are almost straight and equally spaced, producing a nearly uniform field.
条形磁铁周围的磁感线是一组弯曲的环线，在两个磁极附近最密集。在蹄形磁铁两极之间，磁感线几乎平直且间距相等，形成近似匀强的磁场。
Remember that a magnetic field is three-dimensional. Iron filings on a sheet of paper only show one slice through the field, so a full description should mention the field spreading out in all directions around the magnet.
要注意磁场是三维的。纸上的铁屑只显示了磁场的一个截面，因此完整的描述应当指出磁场是朝磁铁四周各个方向延伸的。
6. Attraction and Repulsion Between Magnets | 磁铁之间的吸引与排斥
When two magnets are brought together, like poles repel and unlike poles attract. Two N poles push each other apart, two S poles push each other apart, but an N pole and an S pole pull towards each other.
把两块磁铁靠近时，同极相斥、异极相吸。两个 N 极互相推开，两个 S 极互相推开，而 N 极与 S 极则互相吸引。
The forces are equal in size and opposite in direction on the two magnets, so both magnets feel a push or a pull. The force becomes much weaker as the distance between the magnets increases.
作用在两块磁铁上的力大小相等、方向相反，因此两块磁铁都会感受到推力或拉力。随着两块磁铁之间的距离增大，这个力会迅速减弱。
Repulsion is the only reliable test that an object is a magnet. A magnetic material is always attracted to either pole, but only a magnet can be repelled by a pole of the same kind.
排斥是判断一个物体是否为磁铁的唯一可靠方法。磁性材料对任何一极都表现为被吸引，但只有磁铁才会被同名磁极排斥。
- N versus N: repulsion | N 与 N：排斥
- S versus S: repulsion | S 与 S：排斥
- N versus S: attraction | N 与 S：吸引
- Magnet versus unmagnetised iron: attraction only | 磁铁与未磁化的铁：只会被吸引
7. Induced Magnetism | 感应磁性
When a piece of magnetic material such as an iron nail is placed near a magnet, the material itself becomes a magnet for as long as it stays in the field. This is called induced magnetism, and the end of the nail nearest the magnet always develops the opposite pole.
当铁钉等磁性材料靠近磁铁时，它本身会在处于磁场中期间变成磁铁，这叫感应磁性，而且靠近磁铁的一端总会形成相反的磁极。
Because the near end has the opposite pole, the nail is attracted towards the magnet. This explains why a magnet can pick up a chain of paper clips: each clip magnetises the next one in the chain.
由于近端出现相反磁极，铁钉会被磁铁吸引。这也能解释为什么磁铁可以吸起一串回形针：每一个回形针都会把下一个磁化。
With soft iron the induced magnetism disappears as soon as the magnet is removed, but with steel the material may keep some magnetism and become a weak permanent magnet. This is one method of making a magnet without electricity.
对于软铁，移走磁铁后感应磁性立即消失；对于钢，材料可能保留一部分磁性，成为较弱的永磁体。这也是不用电就能制作磁铁的一种方法。
Induced magnetism also explains why a strong magnet can attract a steel paper clip that is not magnetised at all, and why iron filings cling in tufts around the poles of a magnet.
感应磁性同样解释了为什么强磁铁能吸引完全没有磁化的钢制回形针，以及为什么铁屑会成簇地附在磁铁两极周围。
8. Magnetising and Demagnetising | 磁化与退磁
There are three common ways to magnetise a steel object: stroke it many times in the same direction with one pole of a permanent magnet, place it inside a coil of wire carrying direct current, or hammer it while it lies along the direction of the Earth's magnetic field.
磁化钢制物体有三种常用方法：用永磁体的同一极沿同一方向反复摩擦；把它放入通有直流电的线圈中；或者让它沿地磁场方向放置并敲击它。
To demagnetise a magnet, heat it strongly, hammer it repeatedly, or drop it onto a hard floor. These actions shake the internal magnetic regions out of alignment so that the magnetism is lost.
要使磁铁退磁，可以把它强烈加热、反复敲击，或者把它摔在坚硬的表面上。这些操作会打乱内部磁性区域的整齐排列，使磁性消失。
Magnets should be stored carefully, for example in pairs with opposite poles touching and with a soft iron keeper across the ends. This reduces the effect of the Earth's field and the field of nearby magnets, so the magnets keep their strength for longer.
磁铁需要妥善存放，例如成对放置、异极相对，并在两端横放一块软铁衔铁。这样可以减弱地磁场和邻近磁铁磁场的影响，使磁铁保持磁性的时间更长。
9. The Earth's Magnetic Field and Compasses | 地磁场与指南针
The Earth behaves as if it had a huge bar magnet inside it. The Earth's magnetic south pole lies near the geographic north pole, which is why the north-seeking pole of a compass needle turns towards the north.
地球的表现就像内部有一根巨大的条形磁铁。地球的磁南极位于地理北极附近，这正是指南针的指北极转向北方的原因。
A compass needle is a small, lightly balanced bar magnet that is free to turn. It always settles along the direction of the magnetic field at that place, so it can be used to find direction when travelling or map reading.
指南针的指针是一根轻巧、能自由转动的小条形磁铁。它总会沿着当地磁场的方向静止下来，因此可以在旅行或看地图时用来辨别方向。
The magnetic poles are not exactly at the geographic poles, and the angle between magnetic north and true north is called magnetic declination. It is small in the UK but can be more than 20° in some parts of the world.
磁极并不恰好位于地理极点，磁北与真北之间的夹角叫做磁偏角。在英国这个角度很小，但在世界某些地区可以超过 20°。
When using a compass in an experiment, keep it away from magnets, steel objects, phones, speakers and electrical cables, otherwise it will point in the wrong direction. Near the Earth's magnetic poles a compass becomes unreliable because the field is almost vertical there.
在实验中使用指南针时，要让它远离磁铁、钢铁物品、手机、扬声器和电缆，否则指针会指向错误的方向。在地磁极附近，由于磁场几乎垂直于地面，指南针会变得不可靠。
10. Electromagnets | 电磁铁
When an electric current flows through a wire, a magnetic field is produced around the wire. Winding the wire into a coil (a solenoid) concentrates the field, and placing a soft iron core inside the coil makes the field much stronger. The result is an electromagnet.
当电流通过导线时，导线周围会产生磁场。把导线绕成线圈（螺线管）可以使磁场集中，而在线圈中放入软铁芯则能大大增强磁场。这样就得到了电磁铁。
The strength of an electromagnet is increased by using more turns of wire,
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