📚 Electricity and Magnetism: Key Points Revision | 电与磁：考点精讲
Electricity and magnetism are fundamental topics in the IGCSE Edexcel Science syllabus. Understanding how charges move, how circuits behave, and how magnetic fields interact with currents is essential for mastering the physics of our modern world. This article covers key concepts, formulas, and applications you need for the exam.
电与磁是 IGCSE Edexcel 科学课程的基础内容。理解电荷如何运动、电路如何工作以及磁场如何与电流相互作用，对于掌握现代物理和应对考试至关重要。本文涵盖了你需要掌握的核心概念、公式及应用。
1. Circuit Basics and Symbols | 电路基础与符号
An electric circuit is a closed loop that provides a complete path for charges to flow. For a current to exist, there must be a source of potential difference (such as a cell or battery) and a conducting pathway with no gaps.
电路是提供电荷流动完整路径的闭合回路。要产生电流，必须有电势差源（如电池）和没有断点的导电路径。
Standard circuit symbols for components like cells, resistors, lamps, switches, ammeters, and voltmeters are used in diagrams. You must be able to draw and interpret these symbols correctly in the exam.
电路中元件（如电池、电阻、灯泡、开关、电流表、电压表）使用标准符号。考试中你必须能正确绘制和识别这些符号。
Key components: cell, battery, fixed resistor, variable resistor, lamp, switch, ammeter, voltmeter, diode, LDR, thermistor.
关键元件：电池、电池组、定值电阻、可变电阻、灯泡、开关、电流表、电压表、二极管、光敏电阻、热敏电阻。
2. Current, Voltage and Resistance | 电流、电压与电阻
Electric current (I) is the rate of flow of charge. It is measured in amperes (A). In a metal conductor, current is due to the movement of free electrons, which flow from negative to positive, opposite to the conventional current direction.
电流（I）是电荷流动的速率，单位为安培（A）。在金属导体中，电流由自由电子移动形成，电子从负极流向正极，与电流的约定方向相反。
Voltage, or potential difference (V), is the energy transferred per unit charge. It is measured in volts (V) and provided by sources like cells. A voltmeter is always connected in parallel across a component.
电压（电势差 V）是每单位电荷转移的能量，单位为伏特（V），由电池等电源提供。电压表必须与被测元件并联。
Resistance (R) opposes the flow of current and is measured in ohms (Ω). The resistance of a wire depends on its length, cross-sectional area, material, and temperature.
电阻（R）阻碍电流流动，单位为欧姆（Ω）。导线的电阻取决于其长度、横截面积、材料和温度。
I = Q / t
电流 = 电荷 ÷ 时间
3. Ohm’s Law | 欧姆定律
Ohm’s Law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant. This relationship gives rise to the formula V = IR.
欧姆定律指出，在温度保持不变的条件下，通过导体的电流与其两端的电势差成正比，由此得到公式 V = IR。
Components that obey Ohm’s Law give a straight-line I–V graph through the origin. A fixed resistor at constant temperature is an ohmic conductor. A filament lamp does not obey Ohm’s Law because its resistance increases as it heats up.
满足欧姆定律的元件，其 I–V 图是一条通过原点的直线。恒定温度下的定值电阻是欧姆导体。白炽灯丝不符合欧姆定律，因为发热后电阻升高。
A diode has a very high resistance in one direction and conducts in the other, showing a non‑linear I–V characteristic.
二极管正向导通、反向截止，呈现出非线性的 I–V 特性曲线。
V = I × R
电压 = 电流 × 电阻
4. Series and Parallel Circuits | 串联和并联电路
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: Rtotal = R1 + R2 + … The supply voltage is shared across components.
串联电路中只有一条电流通路，各处电流相等。总电阻等于各电阻之和：R总 = R1 + R2 + … 电源电压在各元件间分配。
In a parallel circuit, the voltage across each branch is the same. The total current is the sum of the currents in the branches. The combined resistance is less than the smallest individual resistance, given by 1/Rtotal = 1/R1 + 1/R2 + …
并联电路中各支路电压相同。总电流等于各支路电流之和。总电阻小于单个最小电阻，计算公式为 1/R总 = 1/R1 + 1/R2 + …
Potential divider circuits use two resistors in series to produce a smaller fraction of the input voltage, useful for sensor circuits with LDRs or thermistors.
分压电路利用两个串联电阻获得输入电压的一部分，常用于含光敏电阻或热敏电阻的传感器电路。
5. Electrical Energy and Power | 电能与电功率
Electrical power (P) is the rate at which energy is transferred. It is measured in watts (W). For any circuit component, power can be calculated using P = IV.
电功率（P）是能量转移的速率，单位为瓦特（W）。对任何电路元件，可用 P = IV 计算。
Using Ohm’s Law, the power formula can also be expressed as P = I²R or P = V²/R. These variations are useful depending on which quantities are known.
结合欧姆定律，电功率还可表示为 P = I²R 或 P = V²/R，视已知量灵活选用。
Energy transferred (E) is power × time: E = IVt. The kilowatt‑hour (kWh) is a practical unit used for household electricity bills.
转移的电能（E）等于功率乘以时间：E = IVt。千瓦时（kWh）是家庭电费账单的实用单位。
P = I V E = I V t
功率 = 电流 × 电压 电能 = 电流 × 电压 × 时间
6. Mains Electricity and Safety | 家庭用电与安全
Mains electricity in homes is an alternating current (a.c.) supply, typically 230 V at a frequency of 50 Hz in the UK. A.c. means the current repeatedly reverses direction.
家庭供电为交流电（a.c.），英国标准为 230 V、50 Hz。交流电意味着电流方向周期性改变。
Plugs and sockets use a three‑wire system: live (brown), neutral (blue), and earth (green/yellow). The earth wire is a safety feature that connects the metal case of an appliance to ground, preventing electric shock if a fault occurs.
插头和插座采用三线制：火线（棕色）、零线（蓝色）和地线（绿/黄）。地线是安全装置，将电器金属外壳接地，防止故障时触电。
Fuses and circuit breakers are protective devices that break the circuit if the current exceeds a safe value. A fuse contains a thin wire that melts; a circuit breaker uses an electromagnet to switch off.
熔断器和断路器是当电流超过安全值时会断开电路的保护装置。熔断器含易熔金属丝；断路器利用电磁开关跳闸。
7. Magnets and Magnetic Fields | 磁铁与磁场
A magnet has two poles: north (N) and south (S). Like poles repel; unlike poles attract. A magnetic field is a region where magnetic materials experience a force. Field lines show the direction from north to south.
磁铁有两个磁极：北极（N）和南极（S）。同极相斥，异极相吸。磁场是磁性材料受力的区域，磁感线从北极指向南极。
Magnetic materials, such as iron, nickel, and cobalt, can be magnetised. Steel retains magnetism well and is used for permanent magnets. Soft iron is easily magnetised and demagnetised, suitable for temporary cores.
铁、镍、钴等磁性材料可被磁化。钢能很好地保持磁性，用于制造永磁体；软铁易磁化也易退磁，适合做临时铁芯。
The Earth itself behaves like a giant magnet with a magnetic field roughly aligned with the geographic axis. This is why a freely suspended magnet aligns north–south.
地球本身就像一个巨大的磁体，其磁场大致与地理轴对齐，因此自由悬挂的磁针会指向南北。
8. Electromagnets | 电磁铁
When a current flows through a wire, it produces a weak magnetic field around the wire. Coiling the wire into a solenoid concentrates the field and creates a strong, uniform field inside.
电流通过导线时，在导线周围产生弱磁场。将导线绕成螺线管可集中磁场，在线圈内部产生强而均匀的磁场。
Inserting a soft iron core into a solenoid greatly strengthens the magnetic field, forming an electromagnet. The field strength can be increased by increasing the current or the number of turns on the coil.
在螺线管中插入软铁芯可极大增强磁场，形成电磁铁。增大电流或增加线圈匝数均可增强磁场强度。
Electromagnets are used in relays, electric bells, loudspeakers, and scrap‑metal lifting cranes. Their advantage is that they can be switched on and off.
电磁铁应用于继电器、电铃、扬声器和废铁起重机等场合，其优点是磁场可被接通和关断。
9. The Motor Effect | 电动机效应
A current‑carrying conductor placed in a magnetic field experiences a force. This is called the motor effect. The force is maximum when the conductor is perpendicular to the field lines.
通电导体在磁场中会受到力的作用，这称为电动机效应。当导体与磁感线垂直时，受力最大。
The direction of the force is given by Fleming’s left‑hand rule: thumb = force, first finger = magnetic field (N to S), second finger = conventional current (positive to negative). The force can be reversed by reversing either the current or the magnetic field.
力的方向用弗莱明左手定则判断：大拇指指向力，食指指向磁场（N 到 S），中指指向电流（正到负）。改变电流或磁场方向即可改变力方向。
In a simple d.c. electric motor, a coil rotates between poles of a magnet. A split‑ring commutator reverses the current every half‑turn, ensuring continuous rotation in the same direction.
在简单的直流电动机中，线圈在磁极间转动。换向器每半圈反转电流方向，确保线圈持续朝同一方向旋转。
Force on a conductor: F = B I L (for a perpendicular field)
导体受力：F = B I L（磁场垂直时）
10. Electromagnetic Induction and Generators | 电磁感应与发电机
Electromagnetic induction occurs when a conductor cuts magnetic field lines, inducing a potential difference across its ends. This can be achieved by moving a magnet into a coil or moving a wire through a field.
当导体切割磁感线时，两端会感应出电势差，这就是电磁感应现象。通过将磁铁插入线圈或让导线在磁场中运动均可实现。
The induced voltage depends on the strength of the magnet, the speed of movement, and the number of turns on the coil. If the movement is reversed, the induced voltage is reversed.
感应电压的大小与磁铁强度、运动速度及线圈匝数有关。运动方向反转，电压极性也随之反转。
An a.c. generator (alternator) uses slip rings and brushes to produce alternating current. A d.c. generator uses a split‑ring commutator to produce direct current.
交流发电机（交流发电机）用滑环和电刷产生交流电；直流发电机用换向器产生直流电。
Lenz’s Law states that the direction of an induced current opposes the change that caused it. This is a consequence of energy conservation.
楞次定律指出，感应电流的方向总是反抗引起它的变化，这是能量守恒的体现。
11. Transformers | 变压器
A transformer consists of two coils of wire wound on a common soft iron core. An alternating current in the primary coil produces a changing magnetic field that induces a voltage in the secondary coil.
变压器由绕在同一软铁芯上的两个线圈组成。初级线圈中的交变电流产生变化磁场，在次级线圈中感应出电压。
Transformers only work with a.c., because a steady d.c. does not produce a changing magnetic field. The ratio of the voltages is equal to the ratio of the number of turns on the coils.
变压器仅适用于交流电，因为稳定的直流电不产生变化的磁场。线圈两端电压比等于线圈匝数比。
Vp / Vs = Np / Ns
初级电压 ÷ 次级电压 = 初级匝数 ÷ 次级匝数
Assuming 100% efficiency, input power equals output power: Ip Vp = Is Vs. Step‑up transformers increase voltage and decrease current; step‑down transformers do the opposite.
假定效率 100%，输入功率等于输出功率：IpVp = IsVs。升压变压器提高电压、降低电流；降压变压器则相反。
12. The National Grid | 国家电网
The National Grid is a network of cables and transformers that distributes electrical energy from power stations to consumers. Electricity is transmitted at very high voltages (e.g. 400 kV) to reduce energy loss as heat in the cables.
国家电网是由电缆和变压器组成的输电网络，将电能从发电站输送到用户。为减少线路热量损耗，采用超高电压（如 400 kV）输送电能。
High voltage means low current for the same power (P = IV). Lower current reduces I²R losses in the transmission lines. Step‑up transformers are used at power stations, and step‑down transformers near homes reduce the voltage to safe levels.
相同功率下（P = IV），电压越高电流越小。小电流降低了输电线的 I²R 损耗。发电站使用升压变压器，家庭附近则用降压变压器将电压降至安全水平。
Understanding the efficiency of the Grid and the role of transformers helps explain why a.c. is preferred for large‑scale power distribution.
理解电网的效率和变压器的作用，有助于解释为何大型输配电系统普遍采用交流电。
Published by TutorHao | Science Revision Series | aleveler.com
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