📚 Electricity and Magnetism | 电与磁
Electricity and magnetism are closely related areas of physics that explain how charges move, how circuits deliver energy, and how magnetic fields can be produced and used. In the Cambridge IGCSE Science syllabus, this topic connects everyday electrical devices with fundamental concepts such as current, voltage, resistance, energy transfer, electromagnetic induction, and transformers.
电与磁是物理学中密切相关的领域，解释电荷如何运动、电路如何输送能量，以及磁场如何产生和应用。在剑桥 IGCSE 科学课程中，本主题将日常电器与电流、电压、电阻、能量转移、电磁感应和变压器等基本概念联系起来。
1. Electric Charge and Current | 电荷与电流
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, while conventional current is shown as flowing 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向正极移动形成，而传统电流方向规定为正极流向负极。
Q = I × t
The unit of charge is the coulomb (C), current is measured in amperes (A), and time in seconds (s). One coulomb is the charge passing a point when a current of one ampere flows for one second.
电荷的单位是库仑 (C)，电流的单位是安培 (A)，时间的单位是秒 (s)。一库仑是一安培电流在一秒内通过某一点的电荷量。
- An ammeter must be connected in series to measure current.
- 安培计必须串联在电路中以测量电流。
2. Voltage, EMF and Potential Difference | 电压、电动势与电势差
Voltage (potential difference) measures the energy transferred per unit charge between two points in a circuit. The electromotive force (EMF) of a cell is the energy supplied per unit charge to drive charge around a complete circuit.
电压（电势差）测量电路中两点之间每单位电荷转移的能量。电池的电动势 (EMF) 是驱动电荷绕完整电路运动时每单位电荷获得的能量。
V = W / Q
The unit is the volt (V), where one volt is one joule per coulomb. A voltmeter must be connected in parallel with the component being measured because it needs to compare the potential on either side.
单位是伏特 (V)，一伏特等于一焦耳每库仑。电压表必须与被测元件并联，因为它需要比较元件两端的电势。
- EMF is measured across the terminals of a cell when no current flows.
- 电动势是在没有电流流过时电池两端的电压。
3. Resistance and Ohm's Law | 电阻与欧姆定律
Resistance is the opposition to current flow and is measured in ohms (Ω). Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across it.
电阻是对电流的阻碍，单位为欧姆 (Ω)。欧姆定律指出，在温度不变时，通过电阻的电流与其两端的电势差成正比。
V = I × R
The resistance of a wire depends on its length, cross-sectional area, and material. Longer wires have higher resistance, while thicker wires have lower resistance because there is more space for electrons to move.
导线的电阻取决于长度、横截面积和材料。导线越长电阻越大，导线越粗电阻越小，因为电子有更多的运动空间。
- For a metallic conductor, resistance increases as temperature rises.
- 对于金属导体，温度升高时电阻增大。
4. Series and Parallel Circuits | 串联与并联电路
In a series circuit, components are connected one after another so the same current flows through all parts. The total resistance is the sum of individual resistances, and the supply voltage is shared between components.
在串联电路中，元件依次连接，因此所有部分通过相同的电流。总电阻等于各电阻之和，电源电压在各元件之间分配。
Rₜ = R₁ + R₂ + R₃
In a parallel circuit, components are connected side by side, so the voltage across each branch is the same. The total current is the sum of branch currents, and the total resistance is smaller than the smallest individual resistance.
在并联电路中，元件并排连接，因此各支路两端的电压相同。总电流等于各支路电流之和，总电阻小于最小的单个电阻。
1/Rₜ = 1/R₁ + 1/R₂
| Series | Parallel |
| Same current everywhere | Same voltage across branches |
| Voltage is divided | Current is divided |
| Total resistance adds up | Total resistance decreases |
串联：各处电流相同，电压分配，总电阻相加。并联：各支路电压相同，电流分配，总电阻减小。
5. Electrical Energy and Power | 电能与电功率
Electrical power is the rate at which electrical energy is transferred. It is calculated by multiplying current and voltage, and its unit is the watt (W).
电功率是电能转移的速率。它等于电流与电压的乘积，单位是瓦特 (W)。
P = V × I
The energy transferred by an appliance depends on its power and the time for which it is used. This is measured in joules (J) or kilowatt-hours (kWh) for domestic electricity meters.
用电器转移的能量取决于其功率和使用时间。能量单位为焦耳 (J)，家用电表常用千瓦时 (kWh)。
E = P × t = V × I × t
- 1 kWh is the energy used by a 1 kW device running for 1 hour, equal to 3.6 × 10⁶ J.
- 1 千瓦时是功率为 1 kW 的设备运行 1 小时所消耗的能量，等于 3.6 × 10⁶ 焦耳。
6. Magnets and Magnetic Fields | 磁铁与磁场
A magnet has a north pole and a south pole. Like poles repel, unlike poles attract. The magnetic field is the region around a magnet where a magnetic force can be detected, and field lines run from the north pole to the south pole outside the magnet.
磁铁有北极和南极。同名磁极相互排斥，异名磁极相互吸引。磁场是磁铁周围能够检测到磁力的区域，磁感线在磁铁外部从北极指向南极。
Magnetic field lines never cross. The closer the lines are together, the stronger the magnetic field. Soft iron can be temporarily magnetised, while steel retains magnetism for longer.
磁感线永不相交。磁感线越密集，磁场越强。软铁可以被暂时磁化，而钢能较长时间保持磁性。
- A uniform magnetic field has parallel, equally spaced field lines.
- 匀强磁场的磁感线平行且等间距。
7. Electromagnets and the Motor Effect | 电磁铁与电动机效应
An electromagnet is created by passing a current through a coil of wire, often wound around a soft iron core. Its magnetic field can be switched on and off, and its strength increases with current, number of turns, and the presence of an iron core.
电磁铁由电流通过线圈产生，线圈通常绕在软铁芯上。它的磁场可以通断，且随电流、线圈匝数和铁芯的存在而增强。
The motor effect occurs when a current-carrying conductor is placed in a magnetic field and experiences a force. The direction of this force can be predicted using Fleming's left-hand rule.
电动机效应是指载流导体置于磁场中时受到力的作用。这个力的方向可以用弗莱明左手定则来判断。
- In Fleming's left-hand rule, the first finger shows field direction, the second finger shows current, and the thumb shows force.
- 在弗莱明左手定则中，食指表示磁场方向，中指表示电流方向，拇指表示力的方向。
8. Electromagnetic Induction | 电磁感应
Electromagnetic induction occurs when a conductor moves through a magnetic field or when a magnetic field changes around a conductor, causing an induced voltage. If the conductor is part of a complete circuit, an induced current flows.
当导体在磁场中运动或导体周围的磁场发生变化时，就会产生电磁感应，从而产生感应电压。如果导体构成闭合回路的一部分，就会有感应电流流动。
The size of the induced voltage increases when the magnetic field is stronger, the movement is faster, or the coil has more turns. This principle is used in generators and dynamos.
磁场越强、运动越快或线圈匝数越多，感应电压越大。这一原理应用于发电机和直流发电机。
- A coil rotating in a magnetic field produces an alternating current (AC).
- 线圈在磁场中旋转会产生交流电 (AC)。
9. Transformers and Power Transmission | 变压器与电力输送
A transformer changes the size of an alternating voltage. It consists of a primary coil and a secondary coil wound around a laminated soft iron core, and it only works with alternating current because a changing magnetic field is needed.
变压器改变交流电压的大小。它由绕在叠片软铁芯上的初级线圈和次级线圈组成，并且只能用于交流电，因为需要变化的磁场。
Vₚ / Vₛ = Nₚ / Nₛ
A step-up transformer increases voltage and decreases current, which is used in power transmission to reduce energy loss as heat in cables. A step-down transformer reduces voltage to safe levels for homes and schools.
升压变压器升高电压并降低电流，用于电力输送以减少电缆中的热能损失。降压变压器将电压降低到家庭和学校的安全水平。
Vₚ × Iₚ = Vₛ × Iₛ (ideal)
- Transformers do not work with direct current (DC) because the magnetic field would not change continuously.
- 变压器不能使用直流电 (DC)，因为磁场不会持续变化。
10. Mains Electricity and Electrical Safety | 市电与用电安全
Mains electricity uses alternating current at a frequency of 50 Hz or 60 Hz depending on the country. In many regions the domestic supply is about 230 V, and the live wire alternates between positive and negative potential relative to the neutral wire.
市电使用交流电，频率根据国家为 50 Hz 或 60 Hz。许多地区家庭供电电压约为 230 V，火线相对于零线在正负电位之间交替变化。
Fuses, circuit breakers, and earthing protect users from electric shock and fire. A fuse melts and breaks the circuit if the current is too high, while earthing prevents the metal case of an appliance from becoming live if a fault occurs.
保险丝、断路器和
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