📚 Observing and Demonstrating Magnetic Force in A-Level Physics | A-Level 物理：磁场力的观察与演示实验
Magnetic force is a fundamental interaction in A-Level physics, and seeing it in action is far more convincing than memorising formulas. This article walks through classic observations and demonstration experiments — from permanent magnets to current-carrying wires and charged particles in a magnetic field.
磁场力是 A-Level 物理中的基本相互作用。亲眼看它起作用，远比死记公式更有说服力。本文带你逐一梳理经典观察现象与演示实验——从永磁体、载流导线到磁场中的带电粒子。
1. The Magnetic Force Between Two Permanent Magnets | 永磁体之间的磁场力
The simplest observation of magnetic force involves two bar magnets. When unlike poles are brought close, they attract; when like poles are brought close, they repel. The force is a non-contact force: it acts through space without any physical connection.
磁场力最简单的观察来自两根条形磁铁。当异名磁极靠近时相互吸引；同名磁极靠近时相互排斥。磁场力是一种非接触力：它不需要任何实物连接，就能在空间中起作用。
To demonstrate this in class, suspend one bar magnet from a thread so that it can rotate freely. Bring another magnet slowly toward it. The suspended magnet will swing to show both attraction and repulsion, clearly indicating the direction of the magnetic force.
课堂演示时，可用细线悬挂一根条形磁铁，让它能自由转动。手持另一根磁铁慢慢靠近，悬挂的磁铁便会摆动，既能展示吸引也能展示排斥，清晰说明磁场力的方向。
2. Plotting Magnetic Field Lines | 描绘磁感线
Magnetic field lines are a visual tool used to represent the direction and strength of a magnetic field. Around a bar magnet, they emerge from the north pole and enter the south pole. The spacing of the lines indicates the field strength: closer lines mean a stronger field.
磁感线是表示磁场方向与强弱的可视化工具。在条形磁铁周围，磁感线从 N 极发出，进入 S 极。线的疏密反映磁场的强弱：越密则场越强。
A simple demonstration uses iron filings sprinkled on a sheet of paper placed over a bar magnet. Gently tap the paper; the filings align along the field lines. This works because each iron filing becomes a tiny temporary magnet and aligns with the local magnetic field direction.
一个简单的演示是把铁粉撒在覆盖条形磁铁的纸面上，轻轻敲击纸面，铁粉会沿磁感线排列。这是因为每个铁屑都变成微小临时磁体，沿当地磁场方向排列。
Alternatively, use a small plotting compass. Place the compass at one point, mark the direction of its needle, then move the compass so that the tail of the needle follows the previous mark. Repeating this traces one continuous field line.
另一种方法是使用小型罗盘磁针。在一点标记磁针方向，然后移动罗盘使磁针尾部对准前一个标记，重复操作即可画出一条连续的磁感线。
3. Force on a Current-Carrying Wire in a Magnetic Field | 载流导线在磁场中受力
A current-carrying wire placed in a magnetic field experiences a magnetic force. This is the basis of electric motors and is described by the equation F = BIL sin θ, where B is the magnetic flux density, I is the current, L is the length of the wire in the field, and θ is the angle between the wire and the magnetic field.
置于磁场中的载流导体会受到磁场力。这是电动机的基础，其公式为 F = BIL sin θ，其中 B 是磁通密度，I 是电流，L 是处于磁场中的导线长度，θ 是导线与磁场方向的夹角。
F = BIL sin θ
For the maximum force, the wire must be perpendicular to the field (θ = 90°). If the wire is parallel to the field, the force is zero.
当导线与磁场垂直（θ = 90°）时力最大；若导线与磁场平行，则力为零。
4. The Classic Cookbook Demonstration | 经典“天平”演示实验
One well-known classroom demonstration is the “cookbook” setup: a stiff copper wire is placed horizontally between the poles of a strong U-shaped magnet. A large current is passed through the wire. When the current is switched on, the wire jumps upward or downward, depending on the direction of the current.
一个著名的课堂演示是“天平”装置：将一根硬铜线水平置于强 U 形磁铁的两极之间，通以大电流。当电流接通时，铜线会向上或向下跳动，具体方向取决于电流方向。
This experiment clearly shows that the force is perpendicular to both the current direction and the magnetic field direction. Reversing the current reverses the force, which is a direct confirmation of Fleming’s left-hand rule.
该实验清楚表明，力的方向同时垂直于电流方向和磁场方向。改变电流方向会改变受力方向，这直接验证了弗莱明左手定则。
5. Fleming’s Left-Hand Rule | 弗莱明左手定则
To predict the direction of the magnetic force on a current-carrying conductor, use Fleming’s left-hand rule: hold the thumb, first finger, and second finger of your left hand mutually at right angles. The First finger points in the direction of the magnetic Field, the seCond finger points in the direction of the Current, and the thuMb points in the direction of the Motion (force).
要判断载流导体所受磁场力的方向，可用弗莱明左手定则：左手拇指、食指与中指相互垂直。食指指向磁场方向（Field），中指指向电流方向（Current），拇指指向运动（受力）方向（Motion）。
A memorable way to demonstrate this is with a simple wire suspended between two vertical magnets. When current flows, the wire deflects sideways. Students can verify the rule by predicting the direction before switching on the current.
一个便于记忆的演示方式是用一根导线悬挂在两块竖直磁铁之间，电流通过时导线会向侧面偏转。学生可先预测受力方向，再接通电流验证定则。
6. Turning Effect on a Coil: The Electric Motor Effect | 线圈的转动效应：电动机原理
If a rectangular coil is placed in a magnetic field and current is passed through it, two opposite sides of the coil experience forces in opposite directions. This creates a couple that rotates the coil. This is the electric motor effect.
若将矩形线圈置于磁场中并通入电流，线圈两条对边会受到方向相反的力，形成力偶使线圈转动。这就是电动机效应。
In the demonstration, a coil of wire is mounted on an axle between the poles of a magnet. When current flows, the coil rotates until it reaches the vertical position. A split-ring commutator reverses the current direction every half turn, so the coil continues to rotate.
演示时，将线圈装在转轴上，置于磁铁两极之间。通电后线圈转动，直到竖直位置。换向器（半环）每半圈改变一次电流方向，使线圈持续转动。
For A-Level, you should be able to calculate the maximum torque acting on a coil: τ = BANI, where A is the area of the coil and N is the number of turns.
在 A-Level 中，你需要会计算线圈所受的最大转矩：τ = BANI，其中 A 是线圈面积，N 是匝数。
τ = BANI
7. Demonstrating the Force with a Cathode-Ray Tube | 用阴极射线管观察磁场力
A beam of electrons is invisible, but when it strikes a fluorescent screen it produces a bright spot. If a bar magnet is brought near the tube, the spot moves, showing that a magnetic field exerts a force on moving charged particles.
电子束本身不可见，但打在荧光屏上会形成亮点。当磁铁靠近阴极射线管时，亮点会移动，说明磁场对运动的带电粒子施加了力。
The direction of the force can be predicted using Fleming’s right-hand rule, since electrons move opposite to the conventional current direction. This experiment beautifully links electric current to the motion of charges.
受力方向可用弗莱明右手定则判断，因为电子运动方向与常规电流方向相反。该实验巧妙地将电流与电荷运动联系起来。
8. Circular Motion of Charged Particles in a Magnetic Field | 带电粒子在磁场中的圆周运动
When a charged particle, such as an electron, enters a uniform magnetic field perpendicular to its velocity, the magnetic force acts as a centripetal force. The particle moves in a circle of radius r given by r = mv/(Bq), where m is the mass, q is the charge, B is the flux density, and v is the speed.
当带电粒子（如电子）垂直于磁场方向进入匀强磁场时，磁场力充当向心力，粒子做圆周运动。轨道半径 r = mv/(Bq)，其中 m 是质量，q 是电荷量，B 是磁通密度，v 是速度。
r = mv / (Bq)
A low-pressure gas tube can be used to make the electron beam visible. With Helmholtz coils producing a known magnetic field, the circular path of the beam is clearly seen. Varying B changes r inversely, confirming the mathematical relationship.
使用低压气体管可使电子束的轨迹可见。利用亥姆霍兹线圈产生已知磁场，能清晰看到电子束的圆形轨迹。改变 B 时 r 随之成反比变化，印证了上述数学关系。
9. The Force Between Two Parallel Current-Carrying Wires | 两根平行载流导线之间的力
Two parallel wires carrying currents exert a magnetic force on each other. If the currents are in the same direction, the wires attract; if they are in opposite directions, they repel. This is because each wire creates a magnetic field that acts on the other wire.
两相互平行的载流导线之间会产生磁场力。当电流方向相同时，导线相互吸引；电流方向相反时，导线相互排斥。这是因为每根导线产生的磁场会对另一根导线施加作用力。
To demonstrate, suspend two flexible strips of metal foil vertically with a small separation. Connect them in series or parallel to control the current direction. When a large current passes, the strips visibly move together or apart. This effect is the definition of the ampere in SI units.
演示时，将两条柔性金属箔竖直悬挂，间隔很小。通过串联或并联来控制电流方向。当大电流通过时，箔条会明显靠近或分开。这一效应正是国际单位制中安培的定义基础。
10. Using a Search Coil and CRO to Observe Induced Magnetic Effects | 用探测线圈与示波器观察感生磁场效应
Although magnetic force is the focus of this article, observing induction with a search coil can also illustrate magnetic field changes. When a magnet is moved into and out of a coil, the galvanometer deflects in opposite directions. The faster the motion, the larger the deflection.
虽然本文以磁场力为主线，但用探测线圈观察电磁感应也能帮助理解磁场的变化。当磁铁插入或拔出线圈时，电流计指针会向相反方向偏转；运动越快，偏转越大。
This demonstration introduces Faraday’s law and Lenz’s law, both of which link magnetic fields to forces on charges. A-level students are expected to explain the direction of the induced current using energy conservation.
这一实验引入法拉第定律和楞次定律，两者都将磁场与电荷受力联系起来。A-Level 学生应会用能量守恒解释感应电流的方向。
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