📚 The Motor Effect and Electric Motors | 电动机效应与电动机
When a current-carrying conductor is placed in an external magnetic field, it experiences a force. This phenomenon is called the motor effect and forms the basis of how electric motors convert electrical energy into mechanical work. Understanding the relationship between magnetic fields, current, and force—guided by Fleming's left-hand rule—is a core requirement for the Edexcel IGCSE Science (Double Award) Physics component. In this article, we will explore the principles of the motor effect, the factors affecting the force on a wire, the construction of a simple DC motor, and the role of the split-ring commutator, all presented in clear bilingual notes.
当通电导线置于外部磁场中时，会受到力的作用。这一现象称为电动机效应，也是电动机将电能转化为机械能的基础。理解磁场、电流与力之间的关系——借助弗莱明左手定则——是爱德思 IGCSE 科学（双奖）物理部分的核心要求。本文将深入探究电动机效应原理、影响导线受力大小的因素、简单直流电动机的结构以及换向器的作用，全部以清晰的双语笔记呈现。
1. Magnetic Fields Around Current-Carrying Wires | 通电导线周围的磁场
A straight wire carrying an electric current produces a circular magnetic field around it. The direction of this field can be found using the right-hand grip rule: if the thumb points in the direction of conventional current (positive to negative), the curled fingers show the direction of the magnetic field lines.
通电直导线会在其周围产生环形磁场。该磁场的方向可用右手握线定则判断：若拇指指向常规电流方向（正到负），弯曲的四指即代表磁场线的环绕方向。
When the wire is formed into a coil (solenoid), the magnetic field becomes similar to that of a bar magnet, with a distinct north and south pole. Placing a soft iron core inside the coil greatly strengthens the magnetic field, creating an electromagnet.
当导线绕成线圈（螺线管）时，磁场变得与条形磁铁相似，有明确的北极和南极。在线圈内部放置软铁芯能极大增强磁场，形成电磁铁。
The interaction between the magnetic field around the current-carrying conductor and an external magnetic field is what leads to the motor effect.
通电导体周围的磁场与外部磁场之间的相互作用，正是导致电动机效应的原因。
2. The Motor Effect Explained | 电动机效应详解
The motor effect is the force experienced by a current-carrying conductor when it is placed inside an external magnetic field. The force arises because the conductor's own magnetic field interacts with the external field, producing a region of stronger field on one side and weaker field on the other, resulting in a net push.
电动机效应是指通电导体置于外部磁场中时所受到的力。该力产生的原因是导体自身的磁场与外部磁场相互作用，使得一侧磁场增强、另一侧减弱，从而产生净推力。
This force is always perpendicular to both the direction of the current and the direction of the magnetic field. The maximum force occurs when the current is perpendicular to the magnetic field lines. If the wire is parallel to the field, no force is experienced.
这个力始终既垂直于电流方向，又垂直于磁场方向。当电流与磁场线垂直时，力达到最大值。如果导线与磁场平行，则不受力。
Mathematically, for a straight conductor of length L carrying current I in a magnetic field of flux density B, the magnitude of the force F is given by:
从数学角度看，对于长度为 L、通有电流 I 且处于磁感应强度为 B 的磁场中的直导体，其受力大小 F 表示为：
F = B × I × L
where F is in newtons (N), B in teslas (T), I in amperes (A), and L in metres (m).
其中 F 的单位为牛顿 (N)，B 为特斯拉 (T)，I 为安培 (A)，L 为米 (m)。
3. Fleming's Left-Hand Rule | 弗莱明左手定则
To predict the direction of the force on a current-carrying conductor in a magnetic field, we use Fleming's left-hand rule. Hold the thumb, first finger, and second finger of the left hand mutually at right angles:
要判断通电导体在磁场中受力方向，我们使用弗莱明左手定则。将左手的拇指、食指和中指相互垂直张开：
- First finger points in the direction of the external magnetic field (North to South).
- Second finger points in the direction of conventional current (positive to negative).
- Thumb then points in the direction of the force (motion).
- 食指指向外部磁场方向（北到南）。
- 中指指向常规电流方向（正到负）。
- 拇指所指即为导体受力（运动）方向。
This rule is essential for analysing motor rotation and for solving exam questions that ask for the direction of movement of a wire or coil side.
这一定则对于分析电动机旋转以及解答考试中问及导线或线圈边运动方向的题目至关重要。
4. Factors Affecting the Force | 影响力大小的因素
hat increasing the magnetic flux density, the current, or the length of conductor inside the field will produce a larger force. These factors are often tested by asking how to increase the turning effect in a motor.
由方程 F = B I L 可知，增大磁感应强度、电流或处于磁场中的导体长度，都会产生更大的力。这些因素常出现在如何提高电动机转动效果的考题中。
The table below summarises the key factors and their effects on the force experienced by a straight wire.
下表总结了关键因素及其对直导线受力大小的影响。
| Factor (因素) | How to increase force (如何增大力) | Effect on force F (对力 F 的影响) |
|---|---|---|
| Magnetic flux density B (磁感应强度 B) | Use stronger magnets (使用更强的磁铁) | F increases proportionally (F 成正比增加) |
| Current I (电流 I) | Increase the current through the wire (增大导线中的电流) | F increases proportionally (F 成正比增加) |
| Length of conductor L (导体长度 L) | Use a longer wire inside the field (使用更长的导线置于磁场中) | F increases proportionally (F 成正比增加) |
| Angle between wire and field (导线与磁场夹角) | Make the wire perpendicular to the field (使导线与磁场垂直) | F is maximum at 90°; zero at 0° (90°时 F 最大；0°时为零) |
In a motor, the coil is positioned so that its sides are perpendicular to the magnetic field for much of the rotation, maximising the force and turning effect.
在电动机中，线圈的位置使其大部分旋转过程中侧边与磁场垂直，从而最大化力和转动效果。
5. Simple DC Motor: Construction | 简单直流电动机的结构
A basic direct current (DC) motor consists of several key components: a rectangular coil of insulated copper wire, a pair of permanent magnets providing a uniform magnetic field, a split-ring commutator, and two carbon brushes that press against the commutator to deliver current from a power supply.
简单的直流电动机由几个关键部件组成：一个矩形绝缘铜线圈、提供匀强磁场的一对永磁体、一个裂环换向器，以及两个压在换向器上用于从电源输送电流的碳刷。
The coil, also called the armature, is mounted on an axle so it can rotate freely. The magnetic field is directed from the north pole to the south pole of the permanent magnets. The sides of the coil that are perpendicular to the field experience the motor effect force, creating a pair of forces that act as a couple, causing rotation.
线圈也称为电枢，安装在转轴上以自由旋转。磁场从永磁体的北极指向南极。垂直于磁场的线圈侧边受到电动机效应力，产生一对力偶，导致旋转。
6. Role of the Split-Ring Commutator | 裂环换向器的作用
The split-ring commutator is a crucial component that allows the motor to keep rotating in one direction. It consists of a copper ring split into two halves, each connected to one end of the coil. As the coil rotates, the halves swap electrical contact with the brushes every half turn.
裂环换向器是确保电动机持续单向旋转的关键部件。它是一个分成两半的铜环，每半分别连接线圈的一端。随着线圈旋转，每半圈两个半环就互换与电刷的电接触。
This swapping reverses the direction of current in the coil every half rotation. Consequently, the direction of the force on each side of the coil is reversed relative to the magnetic field, so the side that was pushed up is now pushed down, and the coil continues to spin in the same direction. Without the commutator, the coil would stop and then try to turn back, resulting in oscillation rather than continuous rotation.
这种交换每半圈反转线圈中的电流方向。结果，相对于磁场，线圈每个侧边的受力方向发生反转，原来被向上推的一侧现在被向下推，线圈得以继续同向旋转。如果没有换向器，线圈会停顿并试图回转，造成摆动而非连续旋转。
7. How the Coil Rotates Step by Step | 线圈旋转的逐步过程
When the coil is horizontal, its sides are perpendicular to the magnetic field. Using Fleming's left-hand rule with current flowing in opposite directions in the two sides, one side experiences an upward force and the other a downward force. This pair of forces produces a turning effect (torque) that rotates the coil clockwise.
当线圈处于水平位置时，其侧边与磁场垂直。利用弗莱明左手定则，两个侧边中电流方向相反，一侧受到向上的力，另一侧受到向下的力。这对力产生使线圈顺时针旋转的转动效果（扭矩）。
As the coil reaches the vertical position, the forces are still present but their lines of action now pass through the axis, so the turning effect momentarily becomes zero. The coil continues moving past the vertical due to its own momentum. At this instant, the split-ring commutator reverses the current.
当线圈到达竖直位置时，力仍然存在但作用线通过转轴，因此转动效果瞬间变为零。线圈凭借自身动量继续转过竖直位置。在这一瞬间，裂环换向器反转了电流。
With the current now reversed, the forces on the two sides switch direction. The side that was moving upward now experiences a downward force, and vice versa. This keeps the coil rotating in the same clockwise direction, completing the cycle every 180 degrees.
随着电流反转，两个侧边的受力方向互换。原来向上运动的一侧现在受到向下的力，反之亦然。这使线圈继续沿同一顺时针方向旋转，每180度完成一个循环。
8. Working of the Carbon Brushes | 碳刷的工作方式
Carbon brushes are spring-loaded graphite blocks that maintain electrical contact with the rotating commutator segments. Graphite is used because it is an electrical conductor, reduces friction, and is soft enough not to damage the copper commutator. The brushes are connected to the terminals of the DC power supply.
碳刷是带弹簧的石墨块，维持与旋转换向器片段的电接触。使用石墨是因为它导电、减小摩擦且质地柔软，不会损坏铜换向器。电刷与直流电源的接线柱相连。
As the motor runs, the brushes rub against the commutator, delivering current to the coil. Good brush contact is essential for reliable motor operation. Worn brushes can be replaced easily, making this design practical for real motors.
电动机运行时，电刷在换向器上滑动，向线圈输送电流。良好的电刷接触对于电动机的可靠运行至关重要。磨损的电刷可轻松更换，使得该设计在实际电动机中可行。
9. Factors That Increase Motor Speed | 提高电动机转速的因素
The speed of a DC motor depends on the size of the force on the coil and the number of turns on the coil. A larger force increases the torque and therefore the acceleration of the coil, leading to higher speed. This can be achieved by using stronger magnets, increasing the current, or adding more turns of wire on the coil (effectively increasing L for the same side length).
直流电动机的转速取决于线圈受力的大小和线圈的匝数。更大的力可增大扭矩，进而增加线圈的加速度，提高转速。这可通过使用更强的磁铁、增大电流或增加线圈匝数（实际上增大了同等边长下的 L）来实现。
A coil with many turns (e.g., 200 turns) experiences a force multiplied by the number of turns, because each turn carries the same current and contributes to the total force. Using a soft iron core inside the coil also concentrates the magnetic field, effectively increasing B and the resulting torque.
具有多匝数的线圈（如200匝）所受的力与匝数相乘，因为每一匝载有相同电流，共同贡献于总力。在线圈内部放置软铁芯也能集中磁场，有效地增大 B 和由此产生的扭矩。
The voltage of the power supply can be increased to drive more current through the coil, but this must stay within safe limits to avoid overheating. These principles are frequently examined through data-response questions comparing different motor designs.
可提高电源电压以驱动更多电流通过线圈，但必须控制在安全范围内以防过热。这些原理常通过数据反映题在考试中对比不同电动机设计来考查。
10. Comparing AC and DC Motors | 交流与直流电动机的比较
The motor described in this article is a DC motor, which requires a direct current and a split-ring commutator to produce continuous rotation. In contrast, an AC motor runs on alternating current and uses slip rings instead of a split-ring commutator, because the current naturally reverses direction every half cycle, keeping the coil rotating in synchronism with the supply frequency.
本文描述的电动机为直流电动机，需要直流电和裂环换向器来产生持续旋转。相比之下，交流电动机使用交流电，配以滑环而不是裂环换向器，因为电流每半个周期自然反向，使线圈与电源频率同步持续旋转。
DC motors are common in battery-operated devices and portable tools, while AC motors dominate household appliances and industrial machinery. For the IGCSE syllabus, the emphasis is on the DC motor and understanding how the commutator solves the problem of current reversal.
直流电动机常见于电池供电设备和便携工具，而交流电动机主导家电和工业机器。对于 IGCSE 教学大纲，重点在于直流电动机以及理解换向器如何解决电流反向问题。
11. Common Exam Pitfalls and Tips | 常见考试误区与技巧
Students often misapply Fleming's left-hand rule by confusing the fingers. Remember: "Father (field), Mother (motion), Child (current)" or "FBI": First finger Field, seCond finger Current, thuMb Motion. Practice with a physical left hand to determine force direction for a given field and current direction.
学生常因混淆手指而用错弗莱明左手定则。记住口诀 "FBI"：First finger 磁场 (Field)，seCond finger 电流 (Current)，thuMb 运动 (Motion)。用左手实物练习，针对给定的磁场和电流方向判断受力方向。
When describing motor rotation, always state the direction of the force on each side independently and how the split-ring commutator reverses the current. Examiners look for the link between current reversal and continuous rotation. Avoid vague statements like "the motor spins".
在描述电动机旋转时，务必分别说明每个侧边的受力方向以及裂环换向器如何反转电流。考官期待看到电流反转与持续旋转之间的关联。避免诸如“电动机转动”这样的模糊表述。
nsure units are consistent. If a question gives length in cm, convert to metres. If a magnet' mT (millitesla), convert to tesla by dividing by 1000. Always show the substitution step.
涉及 F = B I L 的计算时，确保单位一致。若题目给出的长度以厘米为单位，需转换为米。若磁感应强度以毫特斯拉 (mT) 给出，应除以1000转换为特斯拉。务必展示代入数值的步骤。
12. Summary and Key Points | 总结与关键要点
magnetic field, resulting in a force F = B I L (for wire perpendicular to field). Fleming' tor reverses current every half turn to maintain rotation in a single direction. Factors that increase motor speed include stronger magnets, higher current, more coil turns, and a soft iron core. Understanding each component's function is essential for success in IGCSE Physics questions on electromagnetism.
当通电导线置于磁场中时发生电动机效应，产生力 F = B I L（适用于导线垂直于磁场的情况）。弗莱明左手定则给出该力的方向。简单直流电动机将一个矩形线圈置于匀强磁场中，裂环换向器每半圈反转电流以维持单向旋转。提高电动机转速的因素包括更强的磁铁、更大的电流、更多的线圈匝数以及软铁芯。理解每个部件的功能对成功解答 IGCSE 物理电磁学题目至关重要。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图