📚 KS3 CIE Electromagnets: Exam Question 2 Walkthrough | KS3 CIE 电磁铁：考题2精讲
This article walks through a typical CIE KS3 exam question on electromagnets. It covers key ideas such as the iron core, coil turns, current, field strength, fair testing, graph interpretation and comparison of core materials.
本文精讲一道典型的 CIE KS3 电磁铁考题，涵盖铁芯、线圈匝数、电流、磁场强度、公平实验、图表解读和铁芯材料比较等核心考点。
1. The Exam Question | 考题重现
The question below is a typical CIE KS3 electromagnetism task. It tests fair-test variables, reliability, graph reading, prediction using the solenoid relationship, and comparison of core materials.
下面这道题是 CIE KS3 电磁学典型考题，考查公平实验变量、测量可靠性、图表解读、利用螺线管关系进行预测，以及铁芯材料的比较。
A student makes an electromagnet by winding a coil of insulated copper wire around an iron nail. She connects the coil to a variable power supply, an ammeter and a switch. She changes the current in the coil and counts the number of steel paper clips the nail can hold.
一名学生将绝缘铜线绕在铁钉上制成电磁铁。她把线圈连接到可变电源、电流表和开关上。她改变线圈中的电流，并记录铁钉能吸住的钢制回形针数量。
| Current / A | Number of paper clips |
|---|---|
| 0.5 | 2 |
| 1.0 | 5 |
| 1.5 | 9 |
| 2.0 | 14 |
| 2.5 | 18 |
(a) Give one variable the student must keep constant to make the investigation a fair test. [1]
(b) Describe how the student can make sure the number of paper clips is counted reliably. [2]
(c) Describe the trend shown by the data in Table 1. [2]
(d) The student then doubles the number of turns on the coil. Predict and explain what happens to the number of paper clips picked up at 1.0 A. [2]
(e) State one advantage of an electromagnet compared with a permanent magnet. [1]
(f) The iron nail becomes magnetised when the current is switched on. Explain why soft iron is used rather than steel for the core. [2]
(a) 给出一个学生必须保持不变的变量，以使实验公平。[1分]
(b) 描述学生如何可靠地记录回形针数量。[2分]
(c) 描述表1中数据呈现的趋势。[2分]
(d) 学生随后将线圈匝数增加一倍。预测并解释在 1.0 A 时吸住的回形针数量会发生什么变化。[2分]
(e) 写出电磁铁相比永磁体的一个优点。[1分]
(f) 当电流接通时，铁钉被磁化。解释为什么铁芯用软铁而不用钢。[2分]
2. Part (a): Fair Test Variables | (a) 公平实验变量
The independent variable is current, because that is what the student changes. The dependent variable is the number of paper clips picked up. Control variables must include the number of turns on the coil, the type and size of the iron nail, the type and size of paper clips, and the room temperature.
自变量是电流，因为这是学生改变的量。因变量是吸引的回形针数量。控制变量必须包括线圈匝数、铁钉类型和尺寸、回形针类型和大小，以及室温。
A one-mark answer can simply be: number of turns on the coil, or type of nail, or size of paper clips. The key is to keep everything the same except the current.
一分的答案可以简单写成：线圈匝数、铁钉类型或回形针大小。关键是要除电流外的其他条件保持不变。
fair test = change only one variable, keep all others constant
公平实验 = 只改变一个变量，其余全部保持不变
3. Part (b): Measuring Strength Reliably | (b) 可靠地测量磁性强度
To improve reliability, the student should repeat each current reading three times and calculate a mean. The same size and type of paper clips should be used each time, and the nail should be lowered into the clips in the same way for every trial.
为了提高可靠性，学生应对每个电流值重复读数三次并计算平均值。每次应使用相同大小和类型的回形针，并且每次以相同方式将铁钉放入回形针中。
It is also important to switch off the circuit between readings. This prevents the coil from heating up, because resistance changes with temperature and can affect the current.
在两次读数之间断开电路也很重要。这样可以防止线圈发热，因为电阻随温度变化，会影响电流。
A reliable method also means counting the clips held for a fixed time, such as 5 seconds after lifting, to avoid extra clips being pulled up by movement.
可靠的方法还意味着在固定时间内计算吸住的回形针，例如提起后 5 秒计数，以避免因移动而额外吸上回形针。
4. Part (c): Graph Trend and Proportionality | (c) 图表趋势与比例关系
The data show that as current increases, the number of paper clips increases. The increase is not perfectly linear: from 0.5 A to 1.0 A the clip count rises by 3, but from 2.0 A to 2.5 A it rises by 4, so the trend is upward and slightly steepening within this range.
数据显示，随着电流增大，吸住的回形针数量增多。这种增长并非完全线性：从 0.5 A 到 1.0 A 回形针数增加 3 个，但从 2.0 A 到 2.5 A 增加 4 个，因此在这个范围内趋势是上升且略微变陡。
For a solenoid with a soft iron core, the magnetic field strength is approximately proportional to the product of the number of turns N and the current I, as long as the core is not saturated.
对于带软铁芯的螺线管，只要铁芯未饱和，磁场强度大致与匝数 N 和电流 I 的乘积成正比。
magnetic strength ∝ N × I
磁性强弱 ∝ 匝数 × 电流
In a graph, examiners expect students to describe the general pattern first, then quote data from the table, for example: “As current increases, the number of paper clips increases from 2 at 0.5 A to 18 at 2.5 A.”
在图表题中，考官希望学生先描述总体规律，再引用表格中的数据，例如：”随着电流增大，回形针数量从 0.5 A 时的 2 个增加到 2.5 A 时的 18 个。”
5. Part (d): More Turns, More Clips | (d) 增加匝数，吸引更多回形针
Doubling the number of turns increases the magnetic field strength because each turn adds its own magnetic field. As a result, at the same current of 1.0 A, the nail can attract more paper clips.
匝数增加一倍会增强磁场，因为每一匝线圈都会增加自己的磁场。因此，在相同电流 1.0 A 下，铁钉能吸引更多回形针。
At 1.0 A the original result was 5 clips. Doubling the turns may approximately double the magnetic strength, so we can predict about 10 paper clips, although saturation and friction may make the actual value slightly lower.
在 1.0 A 时原来的结果是 5 个回形针。匝数加倍可能使磁性强弱大致加倍，因此我们可预测约 10 个回形针，但磁饱和和摩擦力可能会使实际数值略低。
Examiners want to see both a prediction and an explanation linked to magnetic field strength. Saying “more turns makes it stronger” is not enough unless the connection to attracting more clips is clear.
考官希望看到预测和与磁场强度相关的解释。只说 “匝数更多就更强” 还不够，必须清楚说明因此能吸引更多回形针。
6. Part (e): Advantage of Electromagnets | (e) 电磁铁的优点
An electromagnet can be switched on and off, while a permanent magnet is always magnetic. Its strength can also be changed by adjusting the current or the number of turns.
电磁铁可以接通和断开，而永磁体始终具有磁性。它的强弱还可以通过调节电流或线圈匝数来改变。
This makes electromagnets useful in scrapyard cranes, relays, electric bells and magnetic locks, where controlled magnetic force is needed.
这使得电磁铁广泛用于废料场起重机、继电器、电铃和磁力锁等需要可控磁力的场合。
A simple one-mark answer is: “It can be switched off.” Do not say “it is stronger” because some permanent magnets are stronger than small electromagnets.
一个简单的一分答案是：”它可以被关闭。” 不要说 “它更强”，因为有些永磁体比小型电磁铁更强。
7. Key Concept: How an Electromagnet Works | 核心概念：电磁铁工作原理
When current flows through a wire, a magnetic field is created around it. Coiling the wire into a solenoid concentrates the field, and inserting a soft iron core intensifies it because the iron becomes an induced magnet.
当电流通过导线时，导线周围会产生磁场。将导线绕成螺线管可以集中磁场，插入软铁芯会增强磁场，因为铁被感应磁化。
The direction of the magnetic field depends on the direction of the current. Reversing the current reverses the poles of the electromagnet. This is important in applications like electric motors.
磁场的方向取决于电流方向。改变电流方向会反转电磁铁的磁极。这在电动机等应用中非常重要。
If the current stops, the soft iron core quickly loses most of its magnetism, so the electromagnet releases the paper clips. This property is essential for a switchable magnet.
如果电流停止，软铁芯会迅速失去大部分磁性，于是电磁铁松开回形针。这一特性对于可开关的磁铁至关重要。
8. Core Materials: Soft Iron vs Steel | 铁芯材料：软铁与钢
Soft iron is used for electromagnet cores because it magnetises easily and demagnetises quickly when the current is switched off. Steel retains its magnetism and would keep attracting objects even after the circuit is broken.
电磁铁铁芯使用软铁，因为软铁容易磁化，并且在切断电流后能迅速退磁。钢会保留磁性，即使电路断开仍会继续吸引物体。
| Property | Soft iron | Steel |
|---|---|---|
| Magnetises | easily | less easily |
| Demagnetises | quickly | slowly |
| Use in electromagnet | ideal core | not suitable |
In the exam answer, you should say: soft iron is used because it loses its magnetism quickly when the current is switched off, while steel would stay magnetised.
在考试答案中，应写出：使用软铁是因为电流断开后它能迅速退磁，而钢会保持磁性。
9. Common Mistakes and Examiner Tips | 常见错误与考官提示
- Do not say “add more batteries” without linking it to increased current or stronger field. – 不要只说 “加更多电池” 而不说明电流增大或磁场增强。
- Do not claim the iron core blocks the magnetic field; it actually concentrates and strengthens it. – 不要声称铁芯阻挡磁场；实际上铁芯集中并增强了磁场。
- In fair-test questions, do not forget to name a specific control variable such as coil turns or nail size. – 在公平实验题中，不要忘记写出具体的控制变量，如线圈匝数或铁钉大小。
- Use scientific terms: current, magnetic field strength, induced magnetism, turns, soft iron core. – 使用科学术语：电流、磁场强度、感应磁化、匝数、软铁芯。
- When describing a graph, always state the trend first, then support it with data. – 描述图表时，先说明趋势，再用数据支持。
Examiners award marks for clear linking statements, for example: “More turns means a stronger magnetic field, so the nail can attract more paper clips.” This is better than two separate facts.
考官会给清晰的因果连接句加分，例如：”匝数越多意味着磁场越强，所以铁钉能吸引更多回形针。” 这比写两个独立事实更好。
10. Practice Extension Question | 拓展练习题
Try this exam-style question: A student uses an electromagnet to separate iron from aluminium in a mixture of metal pieces. Explain why the electromagnet can separate these metals and describe one safety precaution the student should take.
请尝试这道考试风格的问题：一名学生使用电磁铁从金属混合物中分离铁和铝。解释为什么电磁铁能分离这些金属，并写出学生应采取的一个安全注意事项。
Suggested points: Iron is magnetic, but aluminium is not, so only iron is attracted to the electromagnet. A safety precaution is to switch off the circuit after use to prevent the coil from overheating.
参考要点：铁具有磁性，铝没有，因此只有铁被电磁铁吸引。安全注意事项是使用后关闭电路，防止线圈过热。
iron attracted to magnet, aluminium not attracted
铁被磁铁吸引，铝不被吸引
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导