电磁学
爱德思国际普通中等教育证书（IGCSE）物理· 爱德思4PH1第6(c)部分，大纲考点6.8-6.14· 15 分钟阅读
1. 载流导体产生的磁场★★☆☆☆⏱ 4 min
🚫 计算器禁用
当电流流过导体时，会在导体周围产生磁场，这是本主题所有考核的电磁学应用的基础原理。该磁场的形状与强度取决于载流导体本身的形状。
电磁铁
由螺线管（紧密缠绕的圆柱形导线线圈）包裹软铁芯制成的临时磁体，软铁芯可以放大电流通过线圈时产生的磁场。
绘制常规电流向上流动的直载流导线周围的磁场分布图案。
- 1
画一条竖直线代表导线，添加向上的箭头标注电流方向。
- 2
在导线周围绘制同心圆，随着与导线距离增加，圆的间距逐渐变大，代表磁场强度逐渐减弱。
- 3
使用右手螺旋定则：用右手握住导线，大拇指向上（指向电流方向），四指的环绕方向为逆时针。
- 4
在所有同心圆场线上添加逆时针箭头，标注磁场方向。
Exam tip:
在解答螺线管磁场分布相关题目时，请清晰标注N极与S极：用右手握住螺线管，四指沿电流方向弯曲，大拇指指向的就是螺线管的N极。
2. 电动机效应与弗莱明左手定则★★★☆☆⏱ 4 min
🚫 计算器禁用
当载流导线置于外磁场中时，导线自身磁场与外磁场的相互作用会对导线产生作用力，该效应称为电动机效应。当电流方向与磁场方向垂直时，作用力达到最大值；当电流方向与磁场方向平行时，作用力为零。
弗莱明左手定则
用于预测垂直于磁场放置的载流导线上受力方向的记忆口诀：伸出左手，使大拇指、食指与中指两两垂直。大拇指代表力/运动方向，食指代表磁场方向（从N极指向S极），中指代表常规电流方向（从正极流向负极）。
一根导线置于竖直向下的磁场中，电流沿导线从左向右流动。请预测导线上受到的力的方向。
- 1
伸出左手，让食指竖直向下，与磁场方向保持一致。
- 2
将中指水平指向右侧，与常规电流的方向保持一致。
- 3
你伸展的拇指将垂直指向纸内（朝向远离你的方向）：这就是导线上受力的方向。
Exam tip:
解答电动机效应相关题目时切勿使用右手：右手定则仅用于后续子主题介绍的电磁感应相关内容。
3. 电动机效应的应用：直流电动机与扬声器★★★☆☆⏱ 4 min
🚫 计算器禁用
考试中经常要求你解释两种利用电动机效应的常见装置：简易直流电动机和扬声器。二者都依靠磁场中的通电电流产生的力来驱动运动。
简易直流电动机的线圈放置在永磁体的两个磁极之间。当电流流过线圈时，线圈的两条对边会受到方向相反的力，使线圈产生转动效果。分裂环换向器每转动半圈就会反向切换线圈中的电流，确保线圈每一侧的受力方向保持不变，因此线圈可以持续沿同一方向旋转。
扬声器包含一个连接在纸盆上的线圈，该线圈套在永磁体的磁极外侧。当代表声音信号的交变电流流过线圈时，线圈受到的力的大小和方向都会随电流发生变化，带动线圈和与之相连的纸盆来回振动，在周围空气中产生声波。
解释直流电动机要实现持续旋转为什么必须使用分裂环换向器。
- 1
如果没有换向器，线圈旋转过程中内部的电流方向将保持不变。
- 2
线圈转动半圈之后，线圈每一侧的受力方向都会反向，将线圈往回推，最终使转动停止。
- 3
分裂环换向器每转动半圈就切换一次与电源的连接，使线圈内的电流方向反向。
- 4
这一设计让线圈每一侧的受力方向保持一致，因此线圈可以沿单一方向持续旋转。
Exam tip:
解释扬声器工作原理时，务必按照“交变电流→变化的力→纸盆振动→产生声音”的逻辑串联内容：遵循该顺序才能拿到满分。
4. 影响通电导体受力的因素★★☆☆☆⏱ 3 min
🚫 计算器禁用
你需要能够描述改变电流或磁场会如何影响导体受力的大小和方向；对于高阶层级的考生，还需要掌握运动带电粒子的受力相关内容。
Reversing either the direction of the current or the direction of the magnetic field reverses the direction of the force
Reversing both the current and field direction leaves the force direction unchanged
Increasing the size of the current or the strength of the magnetic field increases the size of the force
[Higher Only] A moving charged particle (e.g. an electron) experiences a force in a magnetic field unless it is moving parallel to the field lines, as the moving charge constitutes an electric current
说出两种可以增大电磁铁对铁针的提拉力的调整方式。
- 1
增大流过电磁铁螺线管的电流大小
- 2
增加螺线管上的导线匝数，或者确保螺线管内部安装了软铁芯
Exam tip:
答题时不要引用这类公式，因为这些内容不属于IGCSE考纲范围，不会为你带来额外分数。
5. 常见陷阱
错误做法:
Using the right hand instead of left hand for motor effect direction questions
原因:
Right hand rules apply to electromagnetic induction, not the motor effect, so you will get the opposite direction result
正确做法:
Always use your LEFT hand for all motor effect and force direction questions in this sub-topic
错误做法:
Drawing magnetic field lines without arrows, or arrows pointing from South to North
原因:
Field lines show the direction a North pole would move, so they must point from North to South to be correct
正确做法:
Add clear arrows to all field lines, pointing from North to South pole for magnets and solenoids
错误做法:
Stating that reversing both current and magnetic field direction reverses the force
原因:
The two changes cancel each other out, so force direction stays the same
正确做法:
Force direction only reverses if you change one of current or field direction, not both
错误做法:
Forgetting that force on a current-carrying wire is zero when current is parallel to the field
原因:
The interaction between the two magnetic fields is zero when they are aligned
正确做法:
State that force is maximum when current is perpendicular to the field, and zero when parallel
错误做法:
[Higher Only] Describing electromagnets as just a coil of wire with no soft iron core
原因:
The soft iron core is a key component that greatly increases the strength of the electromagnet
正确做法:
Define an electromagnet as a solenoid (coil of wire) wrapped around a soft iron core
6. 速查表
Concept | Core / Higher | Key Details |
|---|---|---|
Magnetic field from current | Core | Current through a conductor produces a magnetic field around it |
Fleming's Left Hand Rule | Core | Thumb=Force, First finger=Field, Second finger=Current (all right angles) |
DC Motor function | Core | Split-ring commutator reverses current every half turn to maintain rotation |
Loudspeaker function | Core | Varying current → varying force on coil → cone vibrates to produce sound |
Force size factors | Core | Increases with larger current, stronger magnetic field |
Electromagnet construction | Higher | Solenoid + soft iron core; stronger with more turns, larger current |
Solenoid field pattern | Higher | Same as bar magnet, uniform field inside the coil |
Force on charged particles | Higher | Force exists unless particle moves parallel to magnetic field |
深入阅读
下一步
Now you have mastered the fundamentals of electromagnetism and the motor effect, you are ready to move on to the next sub-topic in Section 6: electromagnetic induction, generators and transformers, which covers how magnetic fields can be used to produce electric current. You can also practice applying Fleming's left-hand rule and field diagram drawing skills with past paper questions for this topic, to make sure you are familiar with the exam phrasing and mark scheme requirements. For Higher Tier students, make sure you revise the electromagnet construction and field pattern content specifically, as these are commonly tested 2-3 mark questions on Paper 2. Core tier and Double Award students can focus on the motor effect, left-hand rule and applications as these are the only parts assessed in Paper 1 and Double Award exams.