Electricity and Magnetism for IGCSE Science | IGCSE 科学：电与磁考点精讲

📚 Electricity and Magnetism for IGCSE Science | IGCSE 科学：电与磁考点精讲

Electricity and magnetism are fundamental pillars of IGCSE Science, bridging the abstract world of charges and currents with real-life applications. This revision guide systematically walks you through static charge, circuit analysis, electromagnetism, electromagnetic induction, and their practical uses. Every concept is explained in a clear, exam-focused manner to help you tackle multiple-choice, structured, and calculation questions with confidence.

电与磁是 IGCSE 科学的核心支柱，将电荷与电流的抽象世界同实际应用紧密联系起来。本篇考点精讲系统梳理了静电、电路分析、电磁学、电磁感应及其实际应用，每个概念都以清晰、贴近考试的方式讲解，助你从容应对选择题、结构化题目和计算题。

1. Static Electricity | 静电

When two insulating materials are rubbed together, electrons are transferred from one material to the other. The material that gains electrons becomes negatively charged, while the one that loses electrons becomes positively charged. This is charging by friction.

当两种绝缘材料相互摩擦时，电子会从一个材料转移到另一个材料。获得电子的材料带负电，失去电子的材料带正电。这就是摩擦起电。

Like charges repel each other; unlike charges attract. A charged object can also attract a neutral conductor by inducing opposite charge on the near side. This is called electrostatic induction.

同种电荷相互排斥，异种电荷相互吸引。带电物体也可以通过感应让中性导体近端带异种电荷而产生吸引，这称为静电感应。

The SI unit of electric charge is the coulomb (C). The charge of a single electron is approximately -1.6 × 10⁻¹⁹ C. In any charging process, the total charge is conserved.

电荷的 SI 单位是库仑 (C)。单个电子的电荷约为 -1.6 × 10⁻¹⁹ C。在任何带电过程中，总电荷是守恒的。

2. Electric Current | 电流

Electric current is the rate of flow of electric charge. In a metal wire, current is carried by negatively charged electrons moving from the negative terminal to the positive terminal. The conventional current direction is opposite to the electron flow – from positive to negative.

电流是电荷流动的速率。在金属导线中，电流由带负电的电子从负极移向正极而形成。而约定的电流方向与电子流动方向相反——从正极流向负极。

Current (I) is measured in amperes (A). The equation is I = Q / t, where Q is the charge in coulombs and t is time in seconds. An ammeter is always connected in series to measure current.

电流 (I) 的单位是安培 (A)。公式为 I = Q / t，其中 Q 是电荷（库仑），t 是时间（秒）。测量电流的电流表必须串联在电路中。

For a current to flow, there must be a complete closed circuit and a source of potential difference (voltage). Current in a series circuit is the same at all points.

要有电流通过，必须有完整的闭合回路和电位差（电压）的来源。在串联电路中，各处电流均相等。

3. Potential Difference and EMF | 电位差与电动势

Potential difference (p.d.) or voltage is the energy transferred per unit charge as it moves between two points. The unit is the volt (V). 1 V = 1 J/C. A voltmeter is connected in parallel to measure potential difference.

电位差（p.d.）或电压是单位电荷在两点间移动时所传递的能量。单位是伏特 (V)。1 V = 1 J/C。测量电压的电压表必须并联。

Electromotive force (EMF) is the total energy supplied by a source (like a battery) to drive a unit charge around a complete circuit. It is also measured in volts. EMF is not a force; it is a measure of energy per charge.

电动势 (EMF) 是电源（如电池）驱动单位电荷绕完整电路一周所提供的总能量。单位也是伏特。电动势并不是力，而是单位电荷能量的一种量度。

The equation relating energy, charge, and voltage is W = QV. Combined with I = Q/t, we also get W = ItV and for power: P = IV.

能量、电荷和电压的关系是 W = QV。结合 I = Q/t，可进一步得到 W = ItV，功率则为 P = IV。

4. Resistance and Ohm’s Law | 电阻与欧姆定律

Resistance is the opposition to current flow. It is measured in ohms (Ω). Ohm’s law states that, at constant temperature, the current through a conductor is directly proportional to the p.d. across it. This is written as V = IR.

电阻是对电流的阻碍作用，单位为欧姆 (Ω)。欧姆定律指出，在温度不变的情况下，通过导体的电流与其两端的电位差成正比，写作 V = IR。

Factors affecting resistance include: length of wire (longer → higher resistance), cross-sectional area (thicker → lower resistance), material (resistivity), and temperature. For a metal, resistance increases with rising temperature because ions vibrate more, causing more collisions with free electrons.

影响电阻的因素有：导线长度（越长电阻越大）、横截面积（越粗电阻越小）、材料（电阻率）以及温度。对金属而言，温度升高电阻增大，这是因为离子振动加剧，导致自由电子碰撞增多。

A resistor that obeys Ohm’s law is called an ohmic conductor; its V-I graph is a straight line through the origin. A filament lamp is non-ohmic because its resistance increases as it heats up.

遵循欧姆定律的电阻器称为欧姆导体，其 V-I 图像为过原点的直线。白炽灯属于非欧姆导体，因为发热后其电阻上升，图像弯曲。

5. Series and Parallel Circuits | 串联与并联电路

In a series circuit, there is only one path for current. The current is the same everywhere. The total resistance is the sum of individual resistances: R_total = R₁ + R₂ + … . The supply voltage is split across components proportionally to their resistances.

串联电路只有一条电流路径。各处电流相等。总电阻为各电阻之和：R_total = R₁ + R₂ + … 。电源电压按电阻值的比例分配到各元件上。

In a parallel circuit, the current divides among branches. The p.d. across each branch is the same as the supply voltage. The total resistance is less than the smallest branch resistance and can be calculated using 1/R_total = 1/R₁ + 1/R₂ + … .

并联电路中，电流分配到各支路。各支路两端的电压与电源电压相等。总电阻小于任何一个支路电阻，可由 1/R_total = 1/R₁ + 1/R₂ + … 求得。

Connecting bulbs in parallel ensures they receive full voltage and shine with normal brightness; in series, they share voltage and are dimmer. Household circuits are wired in parallel for exactly this reason.

灯泡并联能获得全额电压，正常亮度；串联时分压，灯泡变暗。家庭电路正是基于这个原因采用并联布线。

6. Electrical Power and Energy | 电功率与电能

Power is the rate of energy transfer. The power P of an electrical device is given by P = IV, P = I²R, and P = V² / R. The unit of power is the watt (W).

功率是能量传递的速率。电器的功率 P 可由 P = IV、P = I²R 和 P = V² / R 求出，单位是瓦特 (W)。

Energy transferred E = Pt = IVt. Domestic energy consumption is measured in kilowatt-hours (kWh). 1 kWh = 1000 W consumed for 1 hour = 3.6 × 10⁶ J.

传输的能量 E = Pt = IVt。家庭用电量以千瓦时 (kWh) 计量。1 kWh = 1000 W × 1 小时 = 3.6 × 10⁶ J。

Understanding power equations helps in choosing correct fuses and cables. High power appliances require thicker cables to carry larger currents safely, and fuses are rated to break the circuit if current exceeds safe limits.

理解功率公式有助于选择正确的保险丝和电缆。大功率电器需要较粗的导线以安全承载较大电流；当电流超过安全限值时，保险丝会熔断以切断电路。

7. Magnetism | 磁学基础

A magnet has two poles: north (N) and south (S). Like poles repel, unlike poles attract. Magnetic field lines show the direction of the force that would act on a north pole; they emerge from the north pole and enter the south pole. The field is strongest where the lines are closest (near the poles).

磁铁有两个极：北极 (N) 和南极 (S)。同名极相斥，异名极相吸。磁感线显示作用在北极上的力的方向，从北极出发进入南极。磁感线越密，磁场越强（两极附近最强）。

Soft magnetic materials (e.g., iron) are easily magnetised but lose magnetism quickly; they are used for temporary magnets and electromagnets. Hard magnetic materials (e.g., steel) retain magnetism and are used for permanent magnets.

软磁材料（如铁）容易磁化但磁性也容易消失，常用于临时磁铁和电磁铁。硬磁材料（如钢）能长期保留磁性，用于制造永磁铁。

The Earth itself acts as a huge magnet with a magnetic south pole near the geographic North Pole. A freely suspended bar magnet aligns itself roughly north-south due to the Earth’s magnetic field.

地球本身是一个巨大的磁体，其磁南极位于地理北极附近。自由悬挂的条形磁铁因地球磁场大致指向南北方向。

8. Electromagnetism | 电磁铁与磁场

When an electric current passes through a straight wire, a circular magnetic field is produced around it. The direction of the field is given by the right-hand grip rule: pointing the thumb in the direction of current, the fingers curl in the direction of the magnetic field lines.

当电流通过直导线时，其周围会产生环形磁场。磁场方向可用右手握线定则判断：拇指指向电流方向，弯曲的四指则指向磁感线方向。

A solenoid (coil of wire) produces a strong and uniform magnetic field inside when current flows. Its field pattern is similar to that of a bar magnet. Placing a soft iron core inside the solenoid makes an electromagnet, greatly increasing field strength.

通电的螺线管（线圈）内部产生强而均匀的磁场，其磁场分布类似条形磁铁。在螺线管中插入软铁心就构成电磁铁，磁场强度大大增强。

Electromagnets are used in relays, electric bells, loudspeakers, and magnetic resonance imaging (MRI). The ability to switch the magnetism on and off makes them incredibly versatile.

电磁铁用于继电器、电铃、扬声器和磁共振成像 (MRI)。可以方便地通断磁性，使得电磁铁应用极为广泛。

9. Motor Effect (Fleming’s Left-Hand Rule) | 电动机效应（左手定则）

A current-carrying conductor placed in a magnetic field experiences a force. The magnitude of this force is given by F = BIL, where B is magnetic flux density (T), I is current (A), and L is the length of conductor within the field (m). The force is maximum when the conductor is perpendicular to the field.

通电导体放置在磁场中会受到力的作用。该力的大小为 F = BIL，B 为磁通密度 (T)，I 为电流 (A)，L 为处在场中的导体长度 (m)。当导体与磁场垂直时，力最大。

The direction of the force is found using Fleming’s left-hand rule: thuMb → Motion (force), First finger → Field (North to South), seCond finger → Current (+ to -). All three are mutually perpendicular.

力的方向可用弗莱明左手定则判断：拇指 → 运动（力）方向，食指 → 磁场（从 N 到 S），中指 → 电流（+ 到 -）。三者互相垂直。

This principle is used in electric motors. A rectangular coil in a magnetic field rotates when current passes through it. A split-ring commutator reverses the current every half-turn, ensuring the coil continues to rotate in the same direction.

这一原理应用于电动机。磁场中的矩形线圈通电后会旋转。裂环换向器每半圈反转一次电流，让线圈能持续朝同一方向旋转。

10. Electromagnetic Induction | 电磁感应

When a conductor moves relative to a magnetic field (or the magnetic field through a coil changes), an EMF is induced across the conductor. If the conductor forms part of a complete circuit, an induced current flows. This is electromagnetic induction, discovered by Faraday.

当导体与磁场发生相对运动（或穿过线圈的磁场发生变化）时，导体两端会感应出电动势。如果导体是闭合回路的一部分，就会产生感应电流。这就是法拉第发现的电磁感应。

The magnitude of the induced EMF is proportional to the rate of change of magnetic flux linkage (Faraday’s law). Moving the magnet faster, using a stronger magnet, or increasing the number of coil turns all increase the induced EMF.

感应电动势的大小与磁通链的变化率成正比（法拉第定律）。加快磁铁移动速度、使用更强磁铁或增加线圈匝数，都能增大感应电动势。

The direction of the induced current opposes the change that produced it (Lenz’s law). This is why a magnet falling through a coil experiences a retarding force.

感应电流的方向总是试图阻碍引起它的那个变化（楞次定律）。这就是磁铁穿过线圈下落时会受到阻碍的原因。

11. Generators and Transformers | 发电机与变压器

A generator converts mechanical energy into electrical energy. In a simple AC generator (alternator), a coil rotates in a magnetic field, producing an alternating EMF. Slip rings and brushes maintain continuous connection. The output is a sinusoidal voltage.

发电机将机械能转换为电能。简单的交流发电机中，线圈在磁场里旋转，产生交变电动势。滑环和电刷保持持续连接，输出正弦波形电压。

A transformer consists of two coils wound on a soft iron core. An alternating current in the primary coil creates a changing magnetic field, which induces an EMF in the secondary coil. The voltage ratio equals the turns ratio: Vp / Vs = Np / Ns.

变压器由绕在软铁心上的两个线圈组成。初级线圈的交流电产生变化的磁场，从而在次级线圈感应出电动势。电压比等于匝数比：Vp / Vs = Np / Ns。

For an ideal transformer (100% efficient), power input = power output, so Vp Ip = Vs Is. Step-up transformers increase voltage and decrease current, essential for long-distance power transmission to reduce energy loss as heat (I²R losses). Step-down transformers reduce voltage for safe domestic use.

对于理想变压器（效率 100%），输入功率等于输出功率，即 Vp Ip = Vs Is。升压变压器升高电压并降低电流，这对远距离输电至关重要，可减少因热效应产生的能量损耗 (I²R)。降压变压器则将电压降低至安全家庭用电水平。

12. Safety and Applications | 安全与应用

Fuses, circuit breakers, and earthing are critical safety devices. A fuse contains a thin wire that melts if the current exceeds the rated value, breaking the circuit. A circuit breaker does the same but can be reset. Earthing provides a low-resistance path for fault current to the ground, preventing electric shock.

保险丝、断路器和接地是至关重要的安全装置。保险丝内含细金属丝，当电流超过额定值时会熔断，切断电路。断路器功能类似，但可以复位。接地为故障电流提供低电阻路径导入大地，防止触电。

Double insulation eliminates the need for an earth wire in many modern appliances. The symbol (a square within a square) indicates the appliance does not require earthing.

双重绝缘使许多现代电器无需接地线。符号（框中框）表示该设备不需要接地。

Practical applications of electromagnetism surround us: loudspeakers use the motor effect to produce sound, generators supply our electricity grid, and transformers enable efficient power distribution. From charging your phone to powering an electric train, electricity and magnetism are inseparable in technology.

电磁学的实际应用无处不在：扬声器利用电动机效应发声，发电机为电网供电，变压器实现高效配电。从手机充电到驱动电力火车，电与磁在技术中密不可分。

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Electricity and Magnetism for IGCSE Science | IGCSE 科学：电与磁 考点精讲