📚 Observational Experiments in Electromagnetic Induction | A-Level 物理：电磁感应现象的观察实验
Electromagnetic induction is one of the core topics in CIE A-Level Physics. Observational experiments help students connect the abstract idea of magnetic flux to real current and force effects.
电磁感应是 CIE A-Level 物理的核心内容之一。观察实验能帮助学生将“磁通量”这一抽象概念与真实的电流和受力效果联系起来。
1. Faraday's Discovery and the Basic Observation | 1. 法拉第的发现与基本现象
Faraday discovered that a changing magnetic field near a conductor induces an electromotive force (emf) and, if the circuit is closed, a current flows.
法拉第发现，导体附近的磁场发生变化时，会在导体中感应出电动势；若电路闭合，还会形成电流。
The key observation is that no battery is connected to the coil; the current appears only while the magnetic field through the coil is changing.
关键观测是：线圈上没有连接电池，电流只在线圈中的磁场发生变化时出现。
When a bar magnet is pushed into a coil connected to a sensitive galvanometer, the galvanometer deflects.
当条形磁铁插入连接灵敏电流计的线圈时，电流计发生偏转。
When the magnet is held stationary inside the coil, the galvanometer returns to zero.
当磁铁在线圈中静止不动时，电流计指针回到零位。
When the magnet is pulled out, the galvanometer deflects in the opposite direction.
当磁铁被拉出时，电流计指针向相反方向偏转。
This proves that the induced current depends on the change of magnetic field, not on the presence of the field alone.
这证明感应电流取决于磁场的变化，而不是仅仅取决于磁场的存在。
2. Required Apparatus and Circuit Arrangement | 2. 所需器材与电路布置
To carry out reliable observations, choose equipment that gives a clear and visible deflection.
为了获得可靠且明显的观察结果，应选择能产生清晰可见偏转的器材。
A strong bar magnet with clearly marked north and south poles.
一根南北极标记清晰的强条形磁铁。
A coil or solenoid with a known number of turns, such as 200 turns.
一个已知匝数的线圈或螺线管，例如 200 匝。
A centre-zero sensitive galvanometer, which allows current direction to be seen.
一个中心为零刻度的灵敏电流计，以便观察电流方向。
Connecting wires fitted with clean terminals to avoid loose contacts.
接线端清洁的连接导线，避免接触不良。
For mutual induction, a second coil, a switch, a low-voltage cell and a variable resistor are required.
对于互感实验，还需要第二个线圈、开关、低压电池和可变电阻器。
For motional emf experiments, a horseshoe magnet and a movable straight conductor are useful.
对于动生电动势实验，可使用蹄形磁铁和一截可移动的直导体。
A digital data logger with a voltage sensor can be used to record the induced emf, but an analogue galvanometer is more useful for observing direction changes.
可以用带电压传感器的数据采集器记录感应电动势，但观察方向变化时，指针式电流计更为直观。
3. Experiment 1: Bar Magnet and Coil | 3. 实验一：条形磁铁与线圈
This is the simplest demonstration of electromagnetic induction.
这是电磁感应最简单的演示实验。
Connect the coil directly to the galvanometer and then move the magnet relative to the coil.
将线圈直接接在电流计上，然后使磁铁相对线圈运动。
Push the north pole of the magnet rapidly into the coil: the galvanometer deflects to one side.
将磁铁北极快速插入线圈：电流计向一侧偏转。
Stop the magnet inside the coil: the deflection returns to zero.
让磁铁在线圈内停止：偏转回零。
Pull the magnet outwards: the galvanometer deflects to the opposite side.
将磁铁向外拉出：电流计向另一侧偏转。
Repeat using the south pole first: the directions of deflection are reversed.
换成南极先插入重复实验：偏转方向相反。
Move the coil towards the stationary magnet instead: the same effect is produced.
改为让线圈移向静止磁铁：产生相同效果。
Therefore, the induced emf depends on relative motion between the magnet and the coil.
因此，感应电动势取决于磁铁与线圈之间的相对运动。
4. Experiment 2: Mutual Induction Between Two Coils | 4. 实验二：两个线圈之间的互感
Put two coils on the same iron core or place them close together. Connect one coil to a battery and the other to a galvanometer.
将两个线圈放在同一铁芯上，或彼此靠近。一个线圈接电池，另一个线圈接电流计。
When the switch in the primary circuit is closed, the galvanometer in the secondary circuit deflects momentarily.
闭合原电路开关的瞬间，副线圈回路中的电流计发生瞬时偏转。
With the switch closed and the current steady, the secondary galvanometer shows zero deflection.
开关保持闭合且电流稳定时，副线圈电流计不偏转。
When the switch is opened, the galvanometer deflects momentarily in the opposite direction.
断开开关的瞬间，电流计向相反方向瞬时偏转。
If the primary current is changed continuously with a variable resistor, the galvanometer deflects only while the current is changing.
若用可变电阻连续改变原线圈电流，则只有在电流变化时电流计才会偏转。
This shows that a changing current in one coil induces an emf in a nearby coil without any direct connection.
这说明，一个线圈中变化的电流无需直接连接，就能在邻近线圈中感应出电动势。
5. Experiment 3: Motional Emf in a Straight Conductor | 5. 实验三：直导体运动产生的动生电动势
Place a straight metal conductor between the poles of a horseshoe magnet and connect its ends to a galvanometer.
将一段直金属导体放在蹄形磁铁的两极之间，并将两端连接到电流计。
Move the conductor perpendicularly across the magnetic field: the galvanometer deflects.
让导体垂直切割磁感线运动：电流计发生偏转。
Keep the conductor stationary inside the magnetic field: there is no deflection.
让导体在磁场中静止：无偏转。
Move the conductor back in the opposite direction: the deflection is reversed.
使导体沿相反方向运动：偏转方向相反。
Move the conductor parallel to the magnetic field: no induced current is observed because the conductor does not cut magnetic field lines.
使导体沿平行于磁感线的方向运动：无感应电流，因为导体没有切割磁感线。
For a conductor of length L moving at speed v in a uniform field B, the magnitude of the induced emf is:
对于长度为 L、以速度 v 在匀强磁场 B 中运动的导体，感应电动势的大小为：
ε = B L v sin θ
where θ is the angle between the velocity and the magnetic field.
其中 θ 是速度与磁场之间的夹角。
6. Observing Lenz's Law in the Coil | 6. 在实验中观察楞次定律
Lenz's law states that the induced current always flows in a direction that opposes the change producing it.
楞次定律指出：感应电流的方向总是阻碍引起它的磁通量变化。
Use the deflection of the galvanometer together with the right-hand grip rule to determine the polarity of the induced magnetic field.
利用电流计偏转方向结合右手螺旋定则，可以判断感应磁场的方向。
Push the north pole of a magnet into the coil: the end of the coil facing the magnet becomes a north pole, so it repels the approaching magnet.
将磁铁北极插入线圈：线圈靠近磁铁的一端变为北极，因此排斥靠近的磁铁。
Pull the north pole out of the coil: the same end becomes a south pole, so it attracts the magnet and opposes its motion away.
将磁铁北极向外拉：该端变为南极，因此吸引磁铁，阻碍磁铁远离。
Use a force sensor to measure the push and pull force: the force is larger when the coil circuit is closed than when it is open.
用力传感器测量推力和拉力：闭合线圈电路时所需力比断开电路时更大。
Drop a strong magnet through a vertical copper or aluminium tube: it falls slowly because the eddy currents oppose its motion.
让强力磁铁从竖直铜管或铝管中下落：它会缓慢下落，因为涡流阻碍其运动。
Lenz's law is actually a consequence of conservation of energy.
楞次定律实际上是能量守恒的必然结果。
7. Factors Affecting the Magnitude of Induced Emf | 7. 影响感应电动势大小的因素
By changing one variable at a time, the factors that affect the induced emf can be observed.
每次只改变一个变量，就能观察影响感应电动势的因素。
Movement speed: move the magnet faster; the galvanometer deflection becomes larger.
运动速度：更快地移动磁铁，电流计偏转更大。
Magnet strength: use a stronger magnet; the deflection increases because the magnetic flux is larger.
磁铁强度：使用更强的磁铁，偏转增大，因为磁通量更大。
Number of turns: use a coil with more turns; the total flux linkage increases, so the induced emf increases.
线圈匝数：使用匝数更多的线圈，总磁通匝连数增大，因此感应电动势增大。
Iron core: place a soft iron core inside the coil; the flux linkage increases and the induced emf becomes larger.
铁芯：在线圈中插入软铁芯，磁通匝连数增大，感应电动势变大。
The relationship is summarised by Faraday's law:
上述关系由法拉第定律概括：
ε = -N ΔΦ / Δt
Here N is the number of turns and ΔΦ/Δt is the rate of change of flux.
其中 N 是匝数，ΔΦ/Δt 是磁通量变化率。
8. Flux Linkage Graphs and Gradient Interpretation | 8. 磁通匝连数图像与斜率解读
Magnetic flux Φ through a coil of area A in a uniform field is given by:
在匀强磁场中，通过面积为 A 的线圈的磁通量为：
Φ = B A cos θ
where θ is the angle between the field direction and the normal to the coil.
其中 θ 是磁场方向与线圈法线之间的夹角。
Flux linkage is NΦ. When data are plotted as flux linkage against time, the induced emf is equal to the negative gradient of the graph.
磁通匝连数为 NΦ。若绘制磁通匝连数随时间变化的图像，则感应电动势等于图像斜率的负值。
A steep graph means a rapid change of flux linkage, so a large induced emf is produced.
图像越陡，说明磁通匝连数变化越快，产生的感应电动势越大。
A horizontal graph means no change of flux linkage, so the induced emf is zero.
图像水平，说明磁通匝连数不变，感应电动势为零。
If the slope is negative, the induced emf is positive according to the sign convention used in the equation.
若斜率为负，则按该方程所用的符号约定，感应电动势为正。
This graphical interpretation is a common examination skill in CIE A-Level Physics.
这种图像解读是 CIE A-Level 物理中常见的考查技能。
9. Observing Eddy Currents | 9. 观察涡流
Eddy currents are loops of current induced inside a solid conductor when the magnetic flux through it changes.
涡流是实心导体内，因磁通量变化而感应出的闭合回路电流。
A simple experiment uses a metal pendulum swinging between the poles of a strong magnet.
一个简单实验是用金属摆锤在强磁铁两极之间摆动。
Without the magnet, the pendulum continues swinging for a long time.
没有磁铁时，摆锤能长时间摆动。
With the magnet, the pendulum stops quickly because eddy currents induce magnetic forces that oppose the motion.
有磁铁时，摆锤很快停止，因为涡流感应出的磁力阻碍运动。
Use a pendulum with slots cut in the metal; the damping effect becomes much smaller because the slots break the eddy current paths.
使用开有狭缝的金属摆，阻尼效应明显减小，因为狭缝切断了涡流路径。
Hold an aluminium ring near an alternating current coil; the ring becomes warm, showing that eddy currents dissipate energy as heat.
将铝环靠近通有交变电流的线圈，铝环会发热，说明涡流将能量转化为热能。
Eddy currents are important in devices such as induction cookers and electromagnetic brakes.
涡流在电磁炉和电磁制动器等设备中有重要应用。
10. Applications Seen in the Laboratory | 10. 在实验室中观察到的应用
Two common applications of electromagnetic induction are the transformer and the generator.
电磁感应的两个常见应用是变压器和发电机。
In a transformer, an alternating current in the primary coil creates a changing magnetic flux in the iron core, which induces an emf in the secondary coil.
变压器中，原线圈的交变电流在铁芯中产生变化的磁通量，从而在副线圈中感应出电动势。
A simple demonstration transformer with lamps can show that energy is transferred without a direct electrical connection.
用带小灯泡的简易演示变压器，可以展示能量在没有直接电连接的情况下被传递。
In a bicycle dynamo, a rotating magnet induces an emf in a fixed coil, lighting a lamp when the wheel turns.
在自行车发电机中，旋转磁铁在固定线圈中感应出电动势，车轮转动时小灯泡发光。
In a hand-crank generator, the brightness of the lamp increases as the crank turns faster.
在手摇发电机实验中，摇得越快，灯泡越亮。
These demonstrations show that the rate of rotation controls the induced emf through Faraday and Lenz effects.
这些演示表明，旋转快慢通过法拉第效应和楞次效应控制感应电动势。
11. Common Errors and Exam Tips | 11. 常见错误与考试提示
Students often lose marks because of simple conceptual mistakes in electromagnetic induction.
在电磁感应中，学生常常因为简单的概念错误而丢分。
Do not say current is induced when the flux is constant; the flux must be changing.
不要认为磁通量不变时会有感应电流；磁通量必须变化。
Distinguish between magnetic flux and flux linkage: flux linkage includes the number of turns N.
区分磁通量与磁通匝连数：磁通匝连数包含匝数 N。
Remember the direction from Lenz's law: the induced current opposes the change of flux, not necessarily the pole of the magnet.
牢记楞次定律的方向：感应电流阻碍磁通量的变化，而不一定是阻碍磁铁的磁极。
Use the gradient of a flux-time graph to calculate emf, not the flux itself.
要用磁通量-时间图像的斜率来计算电动势，而不是用磁通量本身。
If the conductor is open-circuit, an emf may still be induced, but no current flows.
若导体断开，仍然可以感应出电动势，但没有电流流动。
calculation questions, convert units correctly: 1 mWb = 1 × 10⁻³ Wb and 1 ms = 1 × 10⁻³ s.
在计算题中正确换算单位：1 mWb = 1 × 10⁻³ Wb，1 ms = 1 × 10⁻³ s。
Reading each question carefully and sketching a flux-linkage graph can help you avoid many of these mistakes.
仔细审题并画出磁通匝连数草图，有助于避免上述许多错误。
12. Conclusion: Observing is Understanding | 12. 结论：观察即理解
The observation experiments for electromagnetic induction are simple to perform but rich in physical meaning.
电磁感应的观察实验操作简单，但物理含义丰富。
By moving a magnet near a coil, changing current in one coil or moving a conductor through a field, key ideas such as induced emf, Lenz's law and eddy currents become visible.
通过让磁铁靠近线圈运动、改变一个线圈中的电流，或让导体在磁场中运动，感应电动势、楞次定律和涡流等关键概念都可以变得直观可见。
Mastering these observations will strengthen your conceptual understanding and prepare you for both practical-based questions and theoretical problems in the CIE examination.
掌握这些观察实验，不仅能加深概念理解，还能帮助你应对 CIE 考试中的实验类问题与理论题。
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