磁学
物理· 6(b)（2017年考纲，6.2–6.7小节）· 12 分钟阅读
1. 磁的基础特性与材料分类★★☆☆☆⏱ 3 min
✓ 计算器
磁体拥有两个磁极：北极（N极）和南极（S极）。磁力的基本规律为：同名磁极相斥，异名磁极相吸。磁体还可以吸引未磁化的磁性物质，包括铁、钢、镍、钴。
硬磁材料与软磁材料
硬磁材料难以磁化也难以退磁，非常适合用于制作永磁体。软磁材料极易磁化，且离开磁场后会快速失去磁性，适合用于临时磁体场景。
例:
Hard material = steel; Soft material = iron
某厂商需要制作一款永久冰箱贴和一个电磁铁铁芯，请说明两种产品分别应选用钢还是铁，并给出理由。
- 1
永久冰箱贴应选用钢（硬磁材料）。
- 2
理由：钢可以长期保留磁性，因此无需外磁场也能持续吸附在冰箱表面。
- 3
电磁铁铁芯应选用铁（软磁材料）。
- 4
理由：断开电磁铁的电流后，铁会立刻失去磁性，因此电磁铁可以根据需求随时开启和关闭。
Exam tip:
作答时请始终将材料选择与硬磁/软磁特性直接关联，不要仅罗列特性而不结合题目给定场景，才能拿到全部分数。
2. 磁感线：规则与绘制规范★★☆☆☆⏱ 3 min
磁场是磁体周围的区域，处于该区域内的磁性材料或其他磁体都会受到磁力作用。我们使用磁感线来表示磁场，考试中磁感线的绘制需要严格遵守既定规范。
磁感线
用于表示自由北极置于磁场中时的运动方向，以及不同位置磁场相对强弱的线条。
Field lines run from North to South outside the magnet, and South to North inside the magnet
Closer lines = stronger magnetic field; the strongest field is always at the poles of a magnet
Field lines never cross each other
Arrows on lines always point in the direction of the magnetic field
绘制单个条形永磁体周围的磁场分布示意图，并标注出磁场最强和最弱的区域。
- 1
绘制一个矩形条形磁铁，将左端标记为N极，右端标记为S极。
- 2
在磁铁外部绘制从N极指向S极的曲线，在每条线上添加指向S极的箭头。
- 3
在N极和S极处绘制间距最小的磁感线，将这些区域标注为磁场最强区域。
- 4
在远离磁铁的位置绘制间距最大的磁感线，将这些区域标注为磁场最弱区域。
Exam tip:
如果漏画磁感线上的箭头，或者在磁体外部将箭头方向画为从S极指向N极，每道题都会被扣掉1分。
3. 感应磁现象★★★☆☆⏱ 2 min
当未磁化的磁性材料置于磁场中时，其内部会产生临时磁性，转变为临时磁体。该材料距离永磁体最近的一端始终会形成异名磁极，因此磁体与该材料之间总会发生吸引作用。
感应磁现象
未磁化的磁性材料置于外磁场中时产生的临时磁性。感应磁现象只会引发吸引作用，绝不会产生排斥作用。
解释为什么条形永磁体可以吸引未磁化的钢制回形针。
- 1
当回形针被移近永磁体时，它会进入该磁体的磁场范围内。
- 2
钢制回形针内部产生感应磁性，转变为临时磁体。
- 3
回形针距离永磁体磁极最近的一端会形成异名磁极（例如，如果磁体的该端为N极，回形针最近的一端就会变为S极）。
- 4
异名磁极相互吸引，因此回形针会被拉向永磁体。
4. 磁场测绘实验与匀强磁场★★★☆☆⏱ 4 min
你需要掌握两种探究条形永磁体磁场分布的方法，并能够解释如何使用两个永磁体生成匀强磁场。
Iron filings method: Place the magnet under a sheet of paper, sprinkle thin iron filings evenly over the paper, tap the paper gently to reduce friction. The filings will align along the magnetic field lines, showing the full pattern.
Plotting compass method: Draw around the magnet on a sheet of paper. Place the compass near one pole, mark the direction the needle points. Move the compass so the tail of the needle lines up with the previous mark, mark the new direction. Repeat until you reach the opposite pole, then draw a line through the marks. Repeat for multiple lines around the magnet.
匀强磁场
所有位置的磁场强度和方向都完全相同的磁场，可用等间距分布的平行直线磁感线表示。
描述如何使用两个条形永磁体生成匀强磁场，并说明该磁场的磁感线形态。
- 1
取两个完全相同的条形永磁体。
- 2
将二者平行放置，间距保持2-3厘米，使其中一个磁体的N极正对另一个磁体的S极。
- 3
两个磁体之间的区域就会形成匀强磁场。
- 4
该区域内的磁感线为平行等距的直线，方向从N极指向S极。
Exam tip:
在描述铁屑实验时，务必提到轻敲纸张以减小铁屑与纸张之间的摩擦力，这是考试中的常见得分点。
5. 常见陷阱
错误做法:
Drawing magnetic field lines pointing from South to North outside the magnet
原因:
The standard convention is lines run North to South outside the magnet, only running South to North inside the magnet
正确做法:
Add arrows pointing from North to South on all field lines drawn outside of a permanent magnet
错误做法:
Stating repulsion can occur between a permanent magnet and an unmagnetised magnetic material
原因:
Induced magnetism always creates an opposite pole nearest the permanent magnet, so only attraction occurs in this case
正确做法:
Remember repulsion is the only test for a permanent magnet; attraction can happen between a magnet and any unmagnetised magnetic material
错误做法:
Using iron (soft magnetic material) to make a permanent magnet
原因:
Iron loses its magnetism quickly when removed from an external field, so it cannot retain magnetism long-term for permanent applications
正确做法:
Use steel (hard magnetic material) for permanent magnets, as it retains its magnetism for long periods
错误做法:
Drawing curved, unevenly spaced lines for a uniform magnetic field
原因:
Uniform fields have identical strength and direction at all points, so lines must be straight, parallel and equally spaced
正确做法:
When drawing a uniform field between two opposite poles, draw straight, evenly spaced parallel lines with arrows pointing from N to S
错误做法:
Forgetting to mention tapping the paper when describing the iron filings field mapping practical
原因:
Iron filings have friction with the paper, so they will not align with field lines unless the paper is tapped to reduce friction
正确做法:
Always state that you tap the paper gently after sprinkling iron filings to allow them to align with the magnetic field lines
6. 速查表
Term/Rule | Key Property | Exam Application |
|---|---|---|
Magnetic force | Like poles repel, unlike attract | Identify permanent magnets if repulsion is observed |
Hard magnetic material (steel) | Hard to magnetise/demagnetise | Used for permanent fridge magnets, compass needles |
Soft magnetic material (iron) | Easily magnetised/demagnetised | Used for electromagnet cores, temporary magnets |
Magnetic field lines | N→S outside, closer = stronger, no crossing | Draw correct field patterns for bar magnets |
Induced magnetism | Temporary, always causes attraction | Explain why magnets attract unmagnetised metal objects |
Uniform field | Straight, parallel, evenly spaced lines | Produce using two opposite parallel poles facing each other |
7. 常见问题
判定某物体为永磁体的唯一可靠检测方法是什么？
排斥作用是检测永磁体的唯一可靠方法。永磁体既可以通过异名磁极吸引其他永磁体，也可以通过磁感应吸引未磁化的磁性材料，因此仅存在吸引作用无法证明某物体是永磁体。只有两个物体之间发生排斥作用，才能确认二者均为永磁体。
如何正确绘制匀强磁场？
绘制匀强磁场的步骤：
- 绘制两条平行直线，代表两个正对磁极之间的空隙（左侧为N极，右侧为S极）
- 在两极之间添加等距分布的平行直线
- 在每一条直线上添加从N极指向S极的箭头 请勿在匀强区域内绘制曲线，否则会代表该区域为非匀强磁场。
深入阅读
下一步
Now that you have mastered permanent magnetism fundamentals, you are ready to move on to electromagnetism, the next subtopic in the Edexcel IGCSE Physics Magnetism and Electromagnetism unit. You will learn how electric currents produce magnetic fields, how electromagnets work, and their real-world applications. This foundation in magnetic fields and material properties will be critical to understanding how electromagnet strength can be controlled, and how they are used in devices like electric bells and circuit breakers. After electromagnetism, you will cover the motor effect and electromagnetic induction, including generators and transformers, which are high-weightage topics in both Paper 1 and Paper 2 exams. Make sure you can correctly draw and label magnetic field patterns before moving on, as these are frequently assessed alongside electromagnet content.