📚 Electromagnets 1.1.2 - Current and Potential Difference Experiment | 电磁铁 1.1.2 – 电流与电势差实验探究
In this experiment, we investigate the relationship between current and potential difference (voltage) for an electromagnet. An electromagnet is a coil of wire wrapped around a soft iron core; when current passes through it, a strong magnetic field is produced. For the purpose of studying basic circuit concepts, we treat the electromagnet as a resistive component. By varying the voltage supplied to it and measuring the resulting current, we can determine whether the electromagnet obeys Ohm's law and examine its electrical resistance characteristics.
本实验探究通过电磁铁的电流与电势差（电压）之间的关系。电磁铁是由绕在软铁芯上的线圈构成的，当电流通过时会产生强磁场。为了研究基本电路概念，我们将电磁铁作为一个电阻元件来处理。通过改变其两端的电压并测量相应的电流，我们可以判断电磁铁是否遵循欧姆定律，并考察其电阻特性。
1. Experiment Objective | 实验目标
To investigate how the current through an electromagnet varies with the potential difference applied across it, and to determine the resistance of its coil from the collected data.
探究通过电磁铁的电流如何随其两端电势差的变化而变化，并从采集的数据中确定线圈的电阻。
2. Apparatus and Materials | 器材与材料
A DC power supply (variable, 0–12 V), a demonstration electromagnet (enamelled copper wire wound on a soft iron core, typical resistance 5–20 Ω), an ammeter (0–1 A or 0–5 A range), a voltmeter (0–15 V range), a rheostat (variable resistor, 10–50 Ω), a switch, connecting leads, and a thermometer (optional for monitoring temperature).
直流电源（可调，0–12 V）、演示用电磁铁（漆包铜线绕在软铁芯上，典型电阻5–20 Ω）、安培表（量程0–1 A或0–5 A）、伏特表（量程0–15 V）、变阻器（10–50 Ω）、开关、连接导线，以及温度计（可选，用于监测温度）。
3. Safety Precautions | 安全注意事项
Do not exceed the current rating of the electromagnet to avoid overheating. The coil may become warm after prolonged use; allow it to cool between measurements. Ensure all connections are secure to prevent short circuits. Keep the work area dry and avoid handling bare wires when the circuit is live.
不要超过电磁铁的额定电流，以免过热。长时间使用后线圈可能会发热；测量之间应让其冷却。确保所有连接牢固，防止短路。保持工作区域干燥，电路通电时避免触碰裸线。
4. Circuit Setup | 电路连接
Connect the power supply, switch, ammeter, electromagnet, and rheostat in series. The ammeter must be placed in series to measure the current through the electromagnet. Connect the voltmeter in parallel across the terminals of the electromagnet to measure the potential difference. A circuit diagram would show the electromagnet symbol (coil with iron core) with the voltmeter bridging it. Set the rheostat to its maximum resistance before closing the switch to limit initial current.
将电源、开关、安培表、电磁铁和变阻器串联连接。安培表必须串联在电路中以测量通过电磁铁的电流。将伏特表并联在电磁铁的两端，测量其电势差。电路图中会用带铁芯的线圈符号表示电磁铁，伏特表跨接在其两端。闭合开关前，将变阻器调至最大电阻以限制初始电流。
5. Experimental Procedure | 实验步骤
1. Set up the circuit as described and have it checked by a supervisor.
2. Close the switch and adjust the rheostat to obtain a convenient low voltage reading (e.g. around 1.0 V).
3. Record the ammeter reading (I) and the voltmeter reading (V) simultaneously.
4. Without changing the power supply dial, vary the rheostat to change the current in steps. Record at least six pairs of I and V values.
5. To extend the range, adjust the power supply to a higher voltage and repeat step 4, collecting data over a wide span (e.g. from 0.1 A to 0.5 A).
6. After completing measurements, open the switch and disconnect the circuit.
1. 按上述方式连接电路，并经老师检查。
2. 闭合开关，调节变阻器，获得一个方便的低电压读数（如约1.0 V）。
3. 同时记录安培表读数（I）和伏特表读数（V）。
4. 保持电源旋钮不变，逐步调节变阻器以改变电流。至少记录六组I和V值。
5. 为了扩展测量范围，将电源调至较高电压，重复步骤4，在较宽范围内收集数据（如从0.1 A到0.5 A）。
6. 测量完成后，断开开关，拆除电路。
6. Recording Data | 数据记录
Design a results table with clear headings and units. Record the current in amperes (A) and the potential difference in volts (V). An example is shown below. Ensure each reading is taken with care, avoiding parallax errors on analogue meters.
设计一个有清楚标题和单位的结果表。记录电流（安培，A）和电势差（伏特，V）。示例如下。每次读数时都要仔细，避免模拟表头的视差误差。
| Current I (A) | Potential Difference V (V) |
|---|---|
| 0.10 | 0.85 |
| 0.20 | 1.70 |
| 0.30 | 2.55 |
| 0.40 | 3.40 |
| 0.50 | 4.25 |
7. Data Analysis | 数据分析
For each pair of readings, calculate the ratio V/I. This gives the resistance R of the electromagnet coil at that particular current. Record these values in an extra column. Use the relation R = V / I and note whether R remains constant. Compute the average resistance if the variation is small. If the resistance appears to increase with current, discuss the possible heating effect.
对每一组读数，计算V/I的比值。该比值即为在该电流下电磁铁线圈的电阻R。将这些值记录在附加栏中。使用关系式 R = V / I，并注意R是否保持不变。若变化很小，计算平均电阻。若电阻随电流增大而增大，则讨论可能的发热效应。
R = V / I
8. Graph Plotting | 绘制图表
ve through the data points. For an ohmic conductor, the graph should be a straight line passing through the origin, with the gradient giving the resistance (gradient = R). If the line curves at higher currents, it indicates that resistance is not constant due to temperature rise.
使用合适的标度绘制电势差V（纵轴）对电流I（横轴）的图线。通过数据点画出最佳拟合直线或光滑曲线。对于欧姆导体，图线应为通过原点的直线，其斜率即为电阻（斜率 = R）。若在高电流处线条弯曲，则表明由于温度升高，电阻不是常数。
9. Conclusion and Ohm's Law | 结论及欧姆定律
If the V–I graph is a straight line, we conclude that the electromagnet behaves as an ohmic conductor within the tested range, and its resistance is equal to the gradient of the line. The experiment demonstrates that for a metallic wire at constant temperature, the current is directly proportional to the potential difference (I ∝ V). However, the electromagnet's coil may deviate from perfect linearity if it heats up significantly, showing the limitations of Ohm's law when temperature is not constant.
如果V–I图是一条直线，我们可以得出结论：在测试范围内电磁铁表现为欧姆导体，其电阻等于直线的斜率。该实验表明，对于恒温下的金属导线，电流与电势差成正比（I ∝ V）。然而，如果电磁铁线圈显著发热，可能会偏离完全的线性，表明当温度不恒定时欧姆定律的局限性。
10. Evaluation and Improvements | 评估与改进
Possible sources of error include the internal resistance of meters (though usually negligible), contact resistance at connections, and temperature changes in the coil caused by continuous current. To improve accuracy, use digital meters, keep the current low to minimise heating, take readings quickly, and allow the coil to cool between measurements. Repeating the experiment with a cooling fan or immersing the coil in a temperature-controlled water bath could yield more constant-temperature results. Additionally, reversing the current direction can check for any diode-like behaviour, which should not occur in a simple coil.
可能的误差来源包括表头的内阻（尽管通常可忽略）、连接处的接触电阻，以及由持续电流引起的线圈温度变化。为了提高精度，可使用数字表头，保持低电流以减少发热，快速读数，并在各次测量之间让线圈冷却。使用风扇冷却或将线圈浸入温控水浴中重复实验，可以获得更恒温的结果。此外，反向电流方向可以检验是否存在类似二极管的行为，这对于简单的线圈是不应出现的。
11. Extension: Magnetic Effect and Current | 拓展：磁场效应与电流
While the core of this experiment is the electrical relationship, remember that an electromagnet's primary purpose is to produce a magnetic field. The strength of the magnetic field is also proportional to the current (B ∝ I for a given core and coil geometry). If the experiment is extended to measure magnetic field strength using a Hall probe or by counting the number of paper clips attracted, you would find a linear relationship up to magnetic saturation of the iron core. This links the measured current and voltage behaviour with the device's intended function.
虽然本实验的核心是电学关系，但要记住电磁铁的主要目的是产生磁场。磁场强度也与电流成正比（对于给定的铁芯和线圈几何形状，B ∝ I）。如果将实验扩展，使用霍尔探头或通过计数吸引的回形针数量来测量磁场强度，你会发现直到铁芯磁饱和之前都是线性关系。这就将测量到的电流电压行为与器件本身的功能联系了起来。
12. Summary | 总结
This investigation solidifies the fundamental concept of current and potential difference in a practical context. By handling real components such as an electromagnet, students observe Ohm's law in action, learn to design and analyse a simple electrical experiment, and appreciate how material properties like temperature coefficient of resistance can influence results. The electromagnet thus serves as both a circuit element and a bridge to electromagnetism topics.
本次探究在实际背景下巩固了电流与电势差的基本概念。通过操作电磁铁这类真实元件，学生观察到欧姆定律的实际表现，学会设计和分析一个简单的电学实验，并认识到电阻温度系数等材料性质如何影响结果。因此，电磁铁既是电路元件，也是通往电磁学主题的桥梁。
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