Electricity and Magnetism Key Concepts | IGCSE WJEC 科学：电与磁考点精讲

📚 Electricity and Magnetism Key Concepts | IGCSE WJEC 科学：电与磁考点精讲

Electricity and magnetism are fundamental topics in the IGCSE WJEC Science specification. Understanding current, voltage, resistance, circuits, and the link between electricity and magnetism is essential for success in the exam. This article breaks down the key concepts, common graphs, and practical applications with clear explanations and Chinese translations to boost your revision.

电与磁是 IGCSE WJEC 科学考试大纲中的基础主题。理解电流、电压、电阻、电路以及电与磁之间的联系对于考试成功至关重要。本文通过清晰的解释和中文翻译，分解关键概念、常见图表和实际应用，帮助你的复习更高效。

1. Static Electricity | 静电学

Static electricity is the build-up of electric charge on the surface of an insulator. When two insulating materials are rubbed together, electrons transfer from one to the other, causing one to become positively charged and the other negatively charged. Like charges repel; opposite charges attract. A charged object can attract neutral objects by inducing a charge separation.

静电是绝缘体表面电荷的积聚。当两种绝缘材料相互摩擦时，电子从一个物体转移到另一个物体上，导致一个带正电，另一个带负电。同种电荷相互排斥，异种电荷相互吸引。带电物体可以通过感应电荷分离来吸引中性物体。

2. Current, Voltage, and Resistance | 电流、电压和电阻

Electric current is the rate of flow of charge. It is measured in amperes (A). In a metal wire, the current is carried by moving electrons. Conventional current flows from positive to negative, opposite to electron flow. Voltage (potential difference) is the energy transferred per unit charge, measured in volts (V). Resistance opposes the flow of current and is measured in ohms (Ω).

电流是电荷流动的速率，以安培（A）为单位。在金属导线中，电流由移动的电子携带。传统电流方向从正极到负极，与电子流动方向相反。电压（电势差）是每单位电荷传递的能量，以伏特（V）为单位。电阻阻碍电流流动，以欧姆（Ω）为单位。

Ohm’s Law states that for a metallic conductor at constant temperature, the current through it is directly proportional to the voltage across it: V = I × R. The resistance of a wire depends on its length (longer wire, higher resistance), cross-sectional area (thicker wire, lower resistance), and material. A filament lamp does not obey Ohm’s law because its resistance increases as it gets hot.

欧姆定律指出，对于恒定温度下的金属导体，通过其中的电流与两端电压成正比：V = I × R。导线的电阻取决于其长度（越长电阻越大）、横截面积（越粗电阻越小）和材料。白炽灯不遵守欧姆定律，因为它变热时电阻会增大。

3. I-V Characteristics Graphs | 电流-电压特性曲线图

For a fixed resistor, the I-V graph is a straight line through the origin, showing constant resistance. For a filament lamp, the graph curves because resistance increases with temperature. For a diode, current flows only when the voltage is in the forward direction above a threshold; reverse bias current is negligible.

对于固定电阻器，I-V 图是一条穿过原点的直线，显示电阻恒定。对于白炽灯，图形弯曲，因为电阻随温度升高而增大。对于二极管，只有当电压正向偏置且超过阈值时电流才流动，反向偏置下电流可以忽略不计。

4. Series and Parallel Circuits | 串联和并联电路

In a series circuit, components are connected end-to-end. Current is the same everywhere. Total resistance is the sum of individual resistances: R_total = R₁ + R₂ + … Total voltage is shared across components. If one component breaks, the circuit is open and all stop working.

在串联电路中，元件首尾相连。各处电流相同。总电阻为各电阻之和：R_total = R₁ + R₂ + … 总电压在各元件上分配。如果一个元件损坏，电路断开，所有元件停止工作。

In a parallel circuit, components are connected on separate branches. Total current is the sum of branch currents. Voltage across each branch is the same as the source voltage. Total resistance is less than the smallest individual resistance. If one branch breaks, others still work.

在并联电路中，元件连接在单独的分支上。总电流为各支路电流之和。各支路两端电压相同，等于电源电压。总电阻小于最小的单个电阻。如果一个分支断开，其他支路仍工作。

5. Electrical Power and Energy | 电功率和电能

Power is the rate of energy transfer. In an electrical circuit, power can be calculated using P = I × V, or by combining with Ohm’s law: P = I²R = V²/R. Energy transferred E = P × t, measured in joules (J). In homes, energy is often measured in kilowatt-hours (kWh). 1 kWh = 3.6 × 10⁶ J.

功率是能量传递的速率。在电路中，功率可用 P = I × V 计算，或结合欧姆定律：P = I²R = V²/R。传递的能量 E = P × t，以焦耳（J）为单位。在家庭中，电能通常以千瓦时（kWh）计量。1 kWh = 3.6 × 10⁶ J。

6. Domestic Electricity and Safety | 家庭用电与安全

Mains electricity is alternating current (a.c.) at 230 V, 50 Hz in the UK. The live wire (brown) carries the voltage; the neutral wire (blue) completes the circuit; the earth wire (green/yellow) is a safety connection that carries current away if a fault occurs. Fuses and circuit breakers protect against overcurrent. Double insulation eliminates the need for an earth wire in some appliances.

市电在英国是交流电，230 V，50 Hz。火线（棕色）承载电压；零线（蓝色）构成回路；地线（绿/黄色）是安全连接，在故障时将电流导出。熔断器和断路器防止过电流。双重绝缘使得某些电器不需要地线。

7. Magnetism and Magnetic Fields | 磁性与磁场

A magnet has a north pole and a south pole. Like poles repel, unlike poles attract. Magnetic fields are regions where magnetic forces act. Field lines go from north to south outside the magnet. The field is strongest where lines are closest together. A plotting compass can map the field.

磁体有北极和南极。同极相斥，异极相吸。磁场是磁力作用的区域。磁感线在磁体外部从北极指向南极。磁感线最密集的地方磁场最强。用罗盘可以描绘磁场分布。

8. Electromagnetism | 电磁学

When an electric current flows through a wire, it produces a magnetic field around the wire. The right-hand grip rule shows the field direction: thumb points in current direction, fingers curl in field direction. A solenoid (coil of wire) produces a magnetic field similar to a bar magnet. An electromagnet is a solenoid with a soft iron core; it can be switched on and off, and its strength increased by more turns, higher current, or adding an iron core.

当电流通过导线时，会在导线周围产生磁场。右手螺旋定则确定磁场方向：拇指指向电流方向，弯曲的手指指向磁场方向。螺线管（线圈）产生的磁场与条形磁铁相似。电磁铁是带有软铁芯的螺线管，可以打开和关闭，通过增加匝数、提高电流或添加铁芯来增强磁场。

9. Motor Effect | 电动机效应

A current-carrying conductor in a magnetic field experiences a force. This is the motor effect. The force is maximum when the wire is perpendicular to the field, and zero when parallel. Fleming’s left-hand rule predicts the direction of force: thumb = force (motion), first finger = field (N to S), second finger = current (plus to minus). The size of the force depends on magnetic flux density (B), current (I), and length of wire in the field (L): F = B I L.

磁场中的通电导体会受到力的作用，这就是电动机效应。当导线与磁场垂直时力最大，平行时力为零。弗莱明左手定则预测力的方向：拇指 — 力（运动），食指 — 磁场（N 到 S），中指 — 电流（正到负）。力的大小依赖于磁通密度（B）、电流（I）和导线在磁场中的长度（L）：F = B I L。

10. Electromagnetic Induction | 电磁感应

Electromagnetic induction is the generation of voltage across a conductor when it is exposed to a changing magnetic field. If a magnet is moved into a coil, a voltage is induced; if the circuit is complete, current flows. The induced voltage increases with faster movement, stronger magnet, or more turns on the coil. This is the principle of generators and dynamos.

电磁感应是当导体处于变化的磁场中时，在其两端产生电压的现象。如果将磁铁移入线圈，会产生感应电压；如果电路闭合，就会有电流。感应电压随移动速度加快、磁铁增强或线圈匝数增多而增大。这是发电机和发电机的原理。

11. Transformers | 变压器

A transformer changes the voltage of an alternating current. It consists of two coils wound on a soft iron core. The primary coil is connected to the input a.c. voltage. The changing magnetic field in the core induces a voltage in the secondary coil. For an ideal transformer, V_p / V_s = N_p / N_s. A step-up transformer increases voltage; a step-down reduces it. Transformers only work with a.c., not d.c.

变压器改变交流电的电压。它由绕在软铁芯上的两个线圈组成。初级线圈连接输入交流电压。铁芯中变化的磁场在次级线圈中感应出电压。对于理想变压器，V_p / V_s = N_p / N_s。升压变压器增加电压；降压变压器降低电压。变压器仅适用于交流电，不适用于直流电。

12. The National Grid | 国家电网

The National Grid transmits electricity from power stations to consumers. Electricity is stepped up to high voltages (e.g., 400 000 V) for long-distance transmission to reduce energy loss as heat in the cables (since for a given power, higher voltage means lower current, and smaller I²R losses). Sub-stations step the voltage down safely for homes (230 V).

国家电网将电力从发电站输送到用户。电力被升至高电压（例如 400,000 V）进行远距离传输，以减少电缆中的热损耗（因为在给定功率下，电压越高电流越小，I²R 损耗越小）。变电站将电压安全降压到家庭用电（230 V）。

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Electricity and Magnetism Key Concepts | IGCSE WJEC 科学：电与磁 考点精讲