📚 Force on a Current-Carrying Conductor | 载流导体在磁场中的受力
When an electric current flows through a wire placed in a magnetic field, the wire experiences a force. This is the motor effect, one of the most important interactions in electromagnetism. It explains how electric motors, loudspeakers and many other devices convert electrical energy into motion. Understanding the factors that affect this force is key to IGCSE Edexcel Science, especially topic 6.13, which requires you to describe how the force depends on the strength of the magnetic field and the size of the current.
当电流通过放在磁场中的导线时，导线会受到一个力。这就是电动机效应，是电磁学中最重要的相互作用之一。它解释了电动机、扬声器和许多其他设备如何将电能转换为运动。理解影响这个力的因素对 IGCSE Edexcel 科学课程至关重要，尤其是主题 6.13，要求你描述力是如何依赖于磁场强度和电流大小的。
1. Introduction to Magnetic Force on Conductors | 磁场对导体的作用力简介
A stationary charged particle does not experience a magnetic force, but moving charges do. In a current-carrying conductor, countless electrons drift along the wire. When this conductor sits inside an external magnetic field, each moving charge is pushed sideways. The net result is a macroscopic force on the whole wire, provided the current flows at an angle to the field lines. This phenomenon is often called the motor effect.
静止的带电粒子不会受到磁力，但运动的带电粒子会受到。在载流导体中，大量电子沿导线漂移。当这根导体处于外部磁场中时，每个运动电荷都会受到侧向的推力。只要电流方向与磁感线不平行，整根导线就会受到一个宏观的合力，这种现象通常被称为电动机效应。
2. Magnetic Field Lines and Flux | 磁感线与磁通量
A magnetic field is a region where magnetic materials and moving charges experience a force. Field lines show the direction of the field: they run from north to south outside a magnet. The strength of the field – the magnetic flux density – is measured in tesla (T). A stronger field has more lines per unit area. For the motor effect, it is the component of the field perpendicular to the wire that matters most.
磁场是磁性材料和运动电荷受到磁力的区域。磁感线显示磁场的方向：在磁铁外部，它们从 N 极指向 S 极。磁场的强度——磁通量密度——以特斯拉（T）为单位测量。越强的磁场，单位面积内的磁感线越多。对于电动机效应，最关键的是垂直于导线的那个磁场分量。
3. Current and Magnetic Field Interaction | 电流与磁场的相互作用
The interaction can be visualised by combining the uniform field between two magnets with the circular field around a straight current-carrying wire. On one side of the wire, the two fields add together, creating a stronger field; on the other side they oppose, giving a weaker field. The wire is then pushed from the region of strong field towards the region of weak field. This simple picture helps to predict the direction of the force.
我们可以通过将两块磁铁间的均匀磁场与直导线周围的环形磁场叠加来直观理解这种相互作用。在导线的一侧，两个磁场相互叠加，合成较强的磁场；在另一侧，它们相互抵消，产生较弱的磁场。于是导线就从强场区域被推向弱场区域。这个简单的图像有助于预测力的方向。
4. Fleming's Left-Hand Rule | 弗莱明左手定则
To find the direction of the force, use Fleming's left-hand rule. Hold your left hand with the thumb, first finger and second finger all at right angles to each other. The First finger represents the magnetic Field direction (North to South). The seCond finger shows the conventional Current direction (positive to negative). The ThuMb then gives the direction of the Motion (Force). Practice this rule until it becomes automatic.
要判断力的方向，可使用弗莱明左手定则。伸出左手，让拇指、食指和中指彼此成直角。食指（First finger）表示磁场方向（从 N 指向 S）。中指（seCond finger）表示常规电流的方向（从正到负）。拇指（ThuMb）就给出了运动（力）的方向。反复练习这个定则，直到能熟练应用为止。
5. Factors Affecting the Force: Magnetic Field Strength | 影响力的因素：磁场强度
The force on the conductor is directly proportional to the magnetic flux density, B. If you double the strength of the magnetic field (for example by using stronger magnets or moving the poles closer), the force doubles as long as current and length remain unchanged. This relationship is at the heart of the specification point 6.13 – you must be able to describe how increasing the field strength increases the force.
导体上的力与磁通量密度 B 成正比。如果磁场强度加倍（例如通过使用更强的磁铁或使磁极靠得更近），只要电流和导线长度不变，力也会加倍。这一关系正是考纲 6.13 的核心——你必须能够描述增加磁场强度如何增大作用力。
6. Factors Affecting the Force: Current | 影响力的因素：电流
The force also increases linearly with the electric current, I. A larger current means more charge carriers move past a point each second, each experiencing a magnetic force. Consequently, doubling the current doubles the total force exerted on the wire. Again, the condition is that the magnetic field and the length of the conductor inside the field stay constant.
力也随电流 I 线性增加。更大的电流意味着每秒钟经过某点的载流子更多，每一个都会受到磁力。因此，电流加倍会使作用在导线上的总力加倍。同样，条件是磁场和磁场中导体的长度保持不变。
7. Factors Affecting the Force: Length of Conductor in the Field | 影响力的因素：导体在磁场中的长度
The force is proportional to the length of the wire that lies within the magnetic field, l. A longer wire contains more moving charges, so the total sideways push adds up. If you double the length of the conductor inside the uniform field region, the force becomes twice as large, assuming B and I are fixed. This is why many practical motors use coils of many turns instead of a single straight wire.
力与处在磁场中的导线长度 l 成正比。更长的导线含有更多的运动电荷，总的侧向推力也就更大。如果使处于均匀磁场区域内的导线长度加倍，在 B 和 I 不变的情况下，力也会变为原来的两倍。这就是为什么许多实际的电动机都采用多匝线圈，而不是单根直线。
8. Force Equation F = B I l | 力的公式 F = B I l
Combining these three proportionalities gives the equation for the force on a straight current-carrying conductor placed perpendicular to a uniform magnetic field:
F = B I l
F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A) and l is the length of the conductor inside the field in metres (m). This equation only holds when the wire is exactly perpendicular to the field lines.
将这三个正比关系结合在一起，就得到了垂直于均匀磁场放置的直载流导线所受的力的公式。F 是力，单位为牛顿 (N)；B 是磁通量密度，单位为特斯拉 (T)；I 是电流，单位为安培 (A)；l 是处于磁场中的导体长度，单位为米 (m)。只有在导线与磁感线完全垂直时，该公式才成立。
9. Angle Between Conductor and Field | 导体与磁场间的夹角
If the wire is not perpendicular to the magnetic field, only the component of the field perpendicular to the current contributes to the force. The general formula is:
F = B I l sin θ
d there is no force. For Edexcel IGCSE, focusing on the perpendicular case is most important, but understanding this angle dependence deepens your grasp.
如果导线不与磁场垂直，就只有垂直于电流的磁场分量才对力有贡献。普适公式为 F = B I l sin θ，其中 θ 是电流方向与磁场方向之间的夹角。当 θ = 90°（导线垂直）时，sin 90° = 1，F = B I l；当 θ = 0°（导线与磁场平行）时，sin 0° = 0，力为零。对于 Edexcel IGCSE，垂直情况是考查重点，但理解这一夹角关系可以加深对知识的掌握。
10. Applications: The Electric Motor | 应用：电动机
A simple DC electric motor uses the motor effect to spin a coil. The coil is placed between the poles of a magnet, and current passes through it. The forces on opposite sides of the coil act in opposite directions because the current flows one way on one side and the opposite way on the other. This produces a turning effect (torque). A split-ring commutator reverses the current every half turn, ensuring the coil keeps rotating in the same direction.
简单的直流电动机利用电动机效应让线圈旋转起来。线圈被置于磁铁的磁极之间，并有电流通过。电流在线圈的两对边沿相反方向流动，因此这两边受到的力方向相反，从而产生了一个转动效果（转矩）。换向器（整流子）每半圈改变一次电流方向，确保线圈能持续沿同一方向旋转。
11. Practical Investigation | 实验探究
d uses a rigid wire placed between two strong magnets, connected to a variable power supply and an ammeter. The magnets rest on a top-pan balance, and the wire is fixed just above the balance. When current flows, the wire experiences a force; by Newton' nges the balance reading. By varying the current, the magnetic field strength (changing magnets), or the length of wire in the field, students can confirm the proportionalities and even estimate B.
可以通过实验验证关系 F = B I l。常用方法是将一根硬直导线固定在两块强磁铁之间，并连接到可调电源和电流表上。磁铁放在电子天平上，导线恰好悬于天平上方。通电时导线受到力，根据牛顿第三定律，磁铁受到等大反向的力，这会改变天平的读数。通过改变电流、磁场强度（更换磁铁）或处于磁场中的导线长度，学生可以证实各比例关系，甚至估算出 B 的数值。
12. Key Points Summary | 要点总结
d in a magnetic field experiences a force if the current is not parallel to the field. The force is perpendicular to both the current and the field, given by Fleming' B I l when the conductor is perpendicular to the field, and increases with larger B, larger I and longer l. This motor effect is the basis of electric motors and many other electromagnetic devices.
载流导体置于磁场中时，如果电流不平行于磁场，它就会受到一个力。这个力既垂直于电流，也垂直于磁场，可由弗莱明左手定则判定其方向。当导体与磁场垂直时，力的大小由 F = B I l 给出，并随 B 的增大、I 的增大和 l 的增长而增大。这种电动机效应是电动机和许多其他电磁装置的基础。
Published by TutorHao | IGCSE Edexcel 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