📚 KS3 CIE Electromagnets 2.1.3: Electromagnets | KS3 CIE 电磁铁 2.1.3
Electromagnets are temporary magnets created by an electric current flowing through a coil of wire. They sit at the heart of many devices you use every day, from electric bells and door locks to scrapyard cranes and loudspeakers. In KS3 CIE topic 2.1.3 you will learn how to build an electromagnet, how to change its strength, and why a soft iron core makes such a big difference.
电磁铁是由电流通过线圈而产生的临时磁铁。它们是许多日常设备的核心，例如电铃、门锁、废铁场起重机和扬声器。在 KS3 CIE 2.1.3 这一主题中，你将学习如何制作电磁铁、如何改变它的强度，以及软铁芯为什么如此重要。
This topic also builds on your earlier knowledge of circuits, current and magnetic fields. By the end of it, you should be able to describe the magnetic effect of a current, explain how the strength of an electromagnet can be controlled, and give real-life examples of where electromagnets are used and why they are chosen instead of permanent magnets.
这一主题也建立在你此前对电路、电流和磁场的知识之上。学完之后，你应该能够描述电流的磁效应，解释如何控制电磁铁的强度，并举出电磁铁在现实生活中应用的例子，说明为什么这些场合会选择电磁铁而不是永磁体。
1. What Is an Electromagnet? | 什么是电磁铁？
An electromagnet is a solenoid - usually a coil of insulated copper wire - that becomes magnetic when an electric current passes through it. If the coil is wrapped around a soft iron core, the magnetic effect becomes much stronger. The magnet is temporary because it loses almost all of its magnetism as soon as the current stops.
电磁铁是一个螺线管——通常由绝缘铜线绕成的线圈——当电流通过时它便具有磁性。如果把线圈绕在软铁芯上，磁效应会强得多。这种磁铁是临时的，因为一旦电流停止，它几乎立刻失去全部磁性。
Key vocabulary you must be able to use accurately in this topic:
- Solenoid - 螺线管：a long coil of wire with many turns.
- Core - 铁芯：the material placed inside the coil, usually soft iron.
- Temporary magnet - 临时磁铁：a magnet that only works while the current flows.
- Magnetic field - 磁场：the region around a magnet where a magnetic force can be detected.
- Insulated wire - 绝缘导线：wire coated in plastic so that neighbouring turns do not short-circuit.
2. The Magnetic Effect of a Current | 电流的磁效应
In 1820 Hans Christian Oersted noticed that a compass needle twitched when it was placed near a wire carrying a current. This simple observation proved that an electric current produces a magnetic field around it. The effect is reversed if the direction of the current is reversed, and it disappears completely when the current is switched off.
1820 年，奥斯特发现把指南针放在通电导线附近时，指针会发生偏转。这个简单的观察证明了电流会在其周围产生磁场。如果电流方向相反，磁场方向也随之相反；当电流断开时，磁场完全消失。
Around a single straight wire the field lines form circles. Around a coil, the circles join up to form a pattern that looks exactly like the field of a bar magnet, with a north pole at one end and a south pole at the other.
在单根直导线周围，磁感线形成一个个圆圈。在线圈周围，这些圆圈连接起来，形成的磁场分布与条形磁铁完全相同，一端为北极、另一端为南极。
Field strength of a solenoid: B ∝ n × I
B ∝ nI（n = 每米匝数，I = 电流）
This means the field gets stronger if you increase the current I, or if you pack more turns of wire n into each centimetre of the coil.
也就是说，增大电流 I，或者在每厘米线圈中绕入更多匝数 n，磁场都会变强。
3. Building an Electromagnet Step by Step | 一步步制作电磁铁
You can build a working electromagnet in a school laboratory with only a few items: a long soft iron nail, about one metre of thin insulated copper wire, a low-voltage DC power supply (or two 1.5 V cells), a switch, connecting leads, a paper clip or small steel pins, and a compass.
在学校的实验室里，只需几样材料就能做出可用的电磁铁：一根长软铁钉、约一米长的细绝缘铜线、一个低压直流电源（或两节 1.5 V 电池）、一个开关、导线夹、一枚回形针或几枚小钢针，以及一个指南针。
Follow these steps in order:
- Step 1 - Leave about 15 cm of wire free at one end, then wind the wire tightly and evenly around the nail. 先在导线一端留出约 15 厘米，然后把导线紧密、均匀地绕在铁钉上。
- Step 2 - Count and record the number of turns. Wind at least 20 turns for a visible effect. 数出并记录匝数，至少要绕 20 匝才能看到明显效果。
- Step 3 - Connect the two free ends to the cell and switch, so that the circuit is complete only when the switch is closed. 把导线两端接到电池和开关上，使电路只有闭合开关时才接通。
- Step 4 - Close the switch and bring the electromagnet near the paper clips. Record how many it picks up. 闭合开关，把电磁铁靠近回形针，记录它能吸起几枚。
- Step 5 - Open the switch and check that the clips fall off immediately. 断开开关，确认回形针立刻掉落。
Safety note: use a low voltage (never mains electricity), keep the switch open between readings, and do not leave the coil connected for long, because the wire heats up and the current then falls.
安全提示：使用低压电源（绝不要使用市电），每次读数之间断开开关，不要让线圈长时间通电，因为导线会发热并且电流会随之下降。
4. Why a Soft Iron Core Matters | 软铁芯为何重要
An empty coil already produces a magnetic field, but the field is weak. When you slide a soft iron core inside the coil, the iron itself becomes an induced magnet: its tiny magnetic regions, called domains, line up with the field of the coil. The core then adds its own magnetism to the coil's field, so the total field may be many times stronger.
空心线圈本身已经能产生磁场，但磁场很弱。当你把软铁芯插入线圈时，铁本身被感应磁化：它内部称为磁畴的微小磁性区域会沿线圈磁场方向排列。铁芯于是把自己的磁性叠加到线圈的磁场上，总磁场可能强出许多倍。
Soft iron is chosen because it magnetises easily and demagnetises just as easily. This is exactly what you want for a temporary magnet. Steel would stay magnetically aligned after the current is switched off, which is useful for making permanent magnets but not for a device that must switch on and off thousands of times a day.
选择软铁，是因为它容易被磁化，也同样容易退磁。这正是临时磁铁需要的性质。钢在断电后仍会保持磁化排列，这对制作永磁体很有用，但对一个每天要开关上千次的装置却不合适。
5. How to Change the Strength of an Electromagnet | 如何改变电磁铁的强度
There are three main variables you can change in a school experiment, plus one extra factor that is often overlooked. Each one is a classic exam question, so learn the direction of the change as well as the reason for it.
在学校的实验中，主要有三个可改变的变量，另外还有一个常被忽略的因素。每一个都是经典考题，因此不仅要记住变化的方向，还要记住变化的原因。
| Variable 变量 | Change 改变方式 | Effect on strength 对强度的影响 |
| Current I 电流 | Increase the current 增大电流 | Stronger 变强 |
| Number of turns n 匝数 | Wind more turns 增加绕线圈数 | Stronger 变强 |
| Core material 铁芯材料 | Add a soft iron core 加入软铁芯 | Much stronger 显著变强 |
| Turns per cm 每厘米匝数 | Squeeze the turns closer 把线圈绕得更紧 | Stronger 变强 |
Notice that the coil's length also matters. The same 100 turns spread over 20 cm give a weaker field than the same 100 turns packed into 5 cm, because the turns per centimetre value is larger.
注意线圈长度也有影响。同样 100 匝绕在 20 厘米上，其磁场要比绕在 5 厘米上弱，因为每厘米的匝数更大。
6. The Field Pattern and the Right-Hand Grip Rule | 磁场分布与右手螺旋定则
Outside the coil, the field lines run from the north pole to the south pole, curving around exactly like the field of a bar magnet. Inside the coil, the field lines are straight, parallel and evenly spaced, which means the field inside is uniform - the same strength everywhere.
在线圈外部，磁感线从北极出发指向南极，弯曲的形状与条形磁铁的磁场完全一样。在线圈内部，磁感线是笔直、平行且间隔均匀的，这意味着内部磁场是匀强的——各处强度相同。
The right-hand grip rule tells you which end is north. Point the thumb of your right hand along the coil axis in the direction the current flows around the coil (the direction your fingers curl), and your thumb points to the north pole.
右手螺旋定则可以判断哪一端是北极。用右手握住线圈，让四指指向电流环绕线圈的方向，大拇指所指的一端就是北极。
If you reverse the connections to the battery, the current direction reverses, so the north and south poles swap ends. The strength stays the same.
如果把连接电池的两根导线对调，电流方向相反，南北极也随之互换；但磁场强度不变。
7. Electromagnets vs Permanent Magnets | 电磁铁与永磁体的比较
Both produce magnetic fields, but they behave very differently in practice. Examiners often ask you to compare them in a table or to explain why an electromagnet is chosen for a particular job.
两者都能产生磁场，但在实际使用中表现大不相同。考官常要求你用表格比较它们，或解释某个装置为什么选用电磁铁。
| Feature 特征 | Electromagnet 电磁铁 | Permanent magnet 永磁体 |
| Magnetism 磁性 | Only while current flows 仅在通电时 | Always 始终存在 |
| Strength 强度 | Can be changed 可以调节 | Fixed 固定不变 |
| Poles 磁极 | Can be swapped by reversing current 反接电流可互换 | Cannot be swapped 不能互换 |
| Typical use 典型用途 | Cranes, bells, relays, loudspeakers 起重机、电铃、继电器、扬声器 | Fridge magnets, compass needles 冰箱贴、指南针 |
8. Everyday Uses of Electromagnets | 电磁铁的日常应用
Electromagnets are used wherever a magnetic field must be switched on and off, or where its strength must be controlled. Recognising the reason behind each use earns marks that a simple list of devices does not.
凡是需要把磁场开、关，或需要控制磁场强弱的地方，都会用到电磁铁。说出每个用途背后的原因，比单纯罗列装置名称更容易得分。
- Electric bell 电铃：the electromagnet pulls an armature, which strikes the gong and breaks the circuit. 电磁铁吸引衔铁，敲击铃碗并断开电路。
- Scrapyard crane 废铁场起重机：a huge electromagnet lifts steel scrap and drops it by switching the current off. 巨型电磁铁吸起废钢，断电即可放下。
- Relay 继电器：a small current in one circuit switches a larger current in another. 用小电流电路控制另一个大电流电路。
- Loudspeaker 扬声器：the varying current in the coil makes the cone vibrate and produce sound. 线圈中变化的电流使纸盆振动发声。
- Maglev train 磁悬浮列车：alternating electromagnets both lift and push the train, so there is no friction with the track. 交替通电的电磁铁既抬升又推动列车，与轨道之间没有摩擦。
- Automatic door lock and MRI scanner 自动门锁与核磁共振扫描仪：both rely on strong, controllable magnetic fields. 两者都依赖强大且可控的磁场。
9. Worked Example: The Electric Bell | 实例分析：电铃
The electric bell is the standard worked example in this topic because it combines a circuit, a switch and an electromagnet in one repeating cycle. Learn the sequence in order.
电铃是本主题的标准实例，因为它把电路、开关和电磁铁结合在一个不断重复的循环中。请按顺序记住整个过程。
- 1. The push switch is pressed, completing the circuit, so current flows through the coil. 按下按钮，电路接通，电流通过线圈。
- 2. The soft iron core becomes a magnet and attracts the iron armature. 软铁芯被磁化，吸引铁质衔铁。
- 3. The hammer on the armature strikes the gong and produces a note. 衔铁上的小锤敲击铃碗，发出声音。
- 4. The same movement pulls the contact screw away, so the circuit breaks and the current stops. 这一运动同时使触点螺钉分离，电路断开，电流停止。
- 5. The core loses its magnetism at once, and a spring pulls the armature back. 铁芯立即失去磁性，弹簧把衔铁拉回原位。
- 6. The contact is remade, the cycle repeats, and the bell buzzes continuously. 触点重新接触，循环重复，电铃持续作响。
The reason the bell buzzes rather than giving one long ring is precisely this make-and-break action of the armature.
电铃之所以发出连续的嗡嗡声而不是一声长响，正是因为衔铁这种"通—断"反复的动作。
10. Practical Investigation and Data Handling | 实验探究与数据处理
A typical investigation asks: how does the number of turns affect the strength of an electromagnet? Strength is measured indirectly by counting how many steel pins or paper clips the electromagnet can lift.
典型的探究题目是：线圈匝数如何影响电磁铁的强度？强度通常用间接方法测量，即数出电磁铁能吸起多少枚钢针或回形针。
Identify the variables clearly before you start:
- Independent variable 自变量：the number of turns on the coil. 线圈的匝数。
- Dependent variable 因变量：the number of paper clips lifted. 吸起的回形针数量。
- Control variables 控制变量：the current, the core material and size, and the coil length. 电流、铁芯材料与尺寸、线圈长度。
To improve reliability, repeat each reading three times and calculate a mean, and use the same paper clips every time. Plot a graph of clips lifted (y-axis) against number of turns (x-axis). You should obtain a straight line through the origin, showing that the strength is directly proportional to the number of turns.
为提高可靠性，每个读数重复三次并取平均值，并且每次使用同样的回形针。以吸起枚数（y 轴）对匝数（x 轴）作图，应得到一条过原点的直线，说明强度与匝数成正比。
clips lifted ∝ number of turns
吸起枚数 ∝ 匝数
If the line curves and flattens at high turn numbers, the iron core has probably become saturated, or the current has dropped because the coil got hot.
如果曲线在高匝数处变弯并趋于平缓，很可能是铁芯已经磁饱和，或者线圈发热导致电流下降。
11. Common Mistakes and Quick Summary | 常见错误与快速小结
These are the errors that cost marks most often in KS3 CIE assessments. Read them once more before a test.
以下是 KS3 CIE 测评中最常失分的错误，考前请再读一遍。
- Saying the electromagnet is a permanent magnet. 错误：把电磁铁说成永磁体。It is temporary - 它是临时的。
- Forgetting that the magnetic field reverses when the current reverses. 忘记电流反向时磁场也反向。
- Using steel instead of soft iron for the core. 用钢而不是软铁做铁芯。
- Using the word "power" instead of "current". 把"电流"说成"功率"。
- Claiming the coil stays magnetic after the switch opens. 误以为开关断开后线圈仍然有磁性。
- Describing the field inside the coil as circular; inside it is uniform and straight. 误说线圈内部磁感线是圆的；内部其实是匀强且笔直的。
Quick summary of the whole topic:
- A current-carrying coil produces a magnetic field; a soft iron core makes it far stronger. 通电线圈产生磁场，软铁芯使其强得多。
- Strength increases with current, with the number of turns, and with a soft iron core. 强度随电流、匝数和软铁芯而增大。
- An electromagnet is temporary and its poles can be reversed; a permanent magnet is neither. 电磁铁是临时的，磁极可互换；永磁体两者都不行。
- Applications include electric bells, relays, cranes and loudspeakers. 应用包括电铃、继电器、起重机和扬声器。
If you can explain each of these four statements in your own words, and draw the field pattern of a solenoid with the north pole labelled, you have mastered KS3 CIE topic 2.1.3.
如果你能用自己的话解释以上四点，并画出螺线管的磁场分布并标出北极，那么你已经掌握了 KS3 CIE 2.1.3 这一主题。
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