📚 GCSE AQA Science: Electricity and Magnetism – Key Revision Points | GCSE AQA 科学：电与磁 考点精讲
Electricity and magnetism are two fundamental and intimately linked topics in GCSE AQA Science. From the flow of charge in circuits to the generation of electricity via electromagnetic induction, these concepts explain everything from household appliances to the national grid. This revision guide covers the essential knowledge you need for the exams, with clear explanations of static charges, circuit rules, magnetic fields, the motor effect, generators and transformers.
电与磁是 GCSE AQA 科学中两个基础且紧密相连的主题。从电路中的电荷流动到利用电磁感应发电，这些概念解释了从家用电器到全国电网的一切原理。本备考指南涵盖了考试所需的必备知识，对静电、电路规则、磁场、电动机效应、发电机和变压器进行了清晰阐释。
1. Static Electricity and Electric Fields | 静电与电场
Static electricity is the build-up of electric charge on the surface of an insulator. 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.
静电是绝缘体表面积聚的电荷。当两种绝缘材料相互摩擦时，电子会从一种材料转移至另一种材料。获得电子的材料带负电，而失去电子的材料带正电。
A charged object creates an electric field around it. The field is strongest near the charged object and points away from positive charges and towards negative charges. A second charged object placed in this field experiences an electrostatic force – like charges repel and opposite charges attract.
带电物体周围会产生电场。电场在靠近带电物体处最强，电场方向由正电荷指向外，指向负电荷。放置在电场中的第二个带电物体会受到静电力作用——同种电荷相互排斥，异种电荷相互吸引。
Spark discharges occur when a large enough charge builds up and the electric field strength is high enough to ionise the air, allowing charge to flow through it. This is the principle behind lightning and everyday static shocks.
当积聚的电荷足够大、电场强度足以使空气电离时，就会发生火花放电，使电荷通过空气流动。这就是闪电和日常静电电击的原理。
2. Electric Circuits and Charge | 电路与电荷
An electric current is the rate of flow of electric charge. In a metal wire, the current is carried by delocalised electrons. The unit of current is the ampere (A). One ampere is equal to one coulomb of charge passing a point per second. The relationship is given by the equation:
电流是电荷的流动速率。在金属导线中，电流由自由电子携带。电流的单位是安培 (A)。1 安培等于每秒有 1 库仑的电荷通过某一点。关系式如下：
Q = I × t
where Q is the charge in coulombs (C), I is the current in amperes (A), and t is the time in seconds (s).
其中 Q 为电荷量 (C)，I 为电流 (A)，t 为时间 (s)。
For current to flow, there must be a complete circuit made of conducting materials, and a source of potential difference (p.d.), such as a cell or battery. The potential difference is the energy transferred per unit charge.
要使电流流动，必须有一个由导电材料构成的完整回路，以及一个电势差（电压）源，如电池或电源。电势差是每单位电荷转移的能量。
3. Current, Voltage and Resistance | 电流、电压与电阻
Resistance is a measure of how much a component opposes the flow of current. The unit of resistance is the ohm (Ω). Potential difference (V), current (I) and resistance (R) are linked by Ohm’s law, which applies when temperature is constant:
电阻衡量一个元件对电流阻碍作用的大小。电阻的单位是欧姆 (Ω)。电势差 (V)、电流 (I) 和电阻 (R) 之间的关系由欧姆定律给出，该定律在温度恒定条件下适用：
V = I × R
A resistor that obeys Ohm’s law is called an ohmic conductor – its resistance remains constant as long as its temperature does not change. A filament lamp, however, is a non-ohmic conductor because its resistance increases when its temperature rises.
遵循欧姆定律的电阻器称为欧姆导体——只要温度不变，其电阻就保持不变。然而，白炽灯是非欧姆导体，因为它的电阻会随温度升高而增加。
Components such as resistors can be fixed or variable. A variable resistor or rheostat allows the resistance to be changed manually, which alters the current in the circuit.
电阻器等元件可以是定值电阻或可变电阻。可变电阻器或滑线变阻器允许手动改变电阻，从而改变电路中的电流。
4. I-V Characteristics of Components | 元件的 I-V 特性
The current–voltage (I–V) graph of an ohmic resistor at constant temperature is a straight line passing through the origin, showing that current is directly proportional to voltage.
在恒定温度下，欧姆电阻的电流-电压 (I–V) 特性图是一条通过原点的直线，表明电流与电压成正比。
For a filament lamp, the I–V graph curves because the resistance increases as the filament heats up. For a diode, current flows only in one direction; the graph shows a very high resistance in the reverse direction until the forward voltage exceeds a threshold, after which current rises steeply.
对于白炽灯，I–V 图呈曲线，因为灯丝受热时电阻增大。对于二极管，电流只能单向流动；其特性图显示在反向电压下电阻极大，直到正向电压超过阈值后电流才急剧上升。
These characteristics can be investigated by using a circuit with a variable power supply, an ammeter in series and a voltmeter in parallel across the component.
这些特性可以通过使用一个带有可调电源、串联安培表和并联在元件两端伏特表的电路来进行研究。
5. 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 of them. The total resistance is the sum of individual resistances:
在串联电路中，元件依次连接，因此流过所有元件的电流相同。总电阻等于各电阻之和：
Rtotal = R₁ + R₂ + R₃ + …
The total potential difference of the supply is shared across the components, and the sum of the individual p.d.s equals the supply p.d.
电源总电势差在各个元件上分配，每个元件两端电势差之和等于电源电势差。
In a parallel circuit, the components are connected on separate branches. The total current from the supply is the sum of the currents in each branch. The potential difference across each branch is the same as the supply p.d. The total resistance is calculated using:
在并联电路中，元件连接在各自独立的分路上。电源提供的总电流等于各支路电流之和。每条支路两端的电势差相同，且等于电源电势差。总电阻由下式计算：
1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃ + …
If one component fails in a series circuit, the entire circuit is broken; in a parallel circuit, other branches continue to work, which is why household wiring uses parallel connections.
如果串联电路中一个元件损坏，整个电路就会断开；而在并联电路中，其他支路仍可继续工作，这就是家庭电路采用并联连接的原因。
6. Domestic Electricity and Safety | 家庭用电与安全
Mains electricity in the UK is an alternating current (a.c.) supply with a frequency of 50 Hz and a voltage of about 230 V. A.c. means the current repeatedly reverses direction, whereas direct current (d.c.) flows in one direction only, as from a battery.
英国的家庭用电是频率为 50 Hz、电压约为 230 V 的交流电 (a.c.)。交流电意味着电流方向周期性反转，而直流电 (d.c.) 像电池那样只沿一个方向流动。
A standard three-pin plug contains a live wire (brown), a neutral wire (blue) and an earth wire (green and yellow). The live wire carries the alternating potential difference from the supply; the neutral wire completes the circuit. The earth wire is a safety feature that provides a low-resistance path to the ground, preventing the metal casing of an appliance from becoming live if a fault occurs.
标准三脚插头包含火线（棕色）、零线（蓝色）和地线（绿/黄色）。火线从电源传输交变电势差；零线构成回路。地线是一种安全装置，提供一条接地的低电阻通路，防止在故障时电器金属外壳带电。
Fuses and circuit breakers protect circuits by melting or tripping when the current exceeds a safe value, cutting off the supply. The correct fuse rating should be slightly higher than the normal operating current of the appliance.
保险丝和断路器在电流超过安全值时会熔断或跳闸，从而切断电源，保护电路。正确的保险丝额定值应略高于电器的正常工作电流。
Electrical power is the rate at which energy is transferred, and is calculated using:
电功率是能量转移的速率，计算式如下：
P = I × V
where P is power in watts (W). Energy transferred can be found from E = P × t.
其中 P 为功率，单位为瓦特 (W)。转移的能量可由 E = P × t 计算。
7. Magnetism and Magnetic Fields | 磁性及磁场
A magnet has a north-seeking pole and a south-seeking pole. Like poles repel and unlike poles attract. Permanent magnets are made of magnetically hard materials, such as steel, that retain their magnetism, whereas soft magnetic materials, like iron, become temporary magnets when placed in a magnetic field but lose it quickly when removed.
磁铁有北极和南极。同极相斥，异极相吸。永磁体由钢等硬磁性材料制成，能保持磁性；而铁等软磁性材料放入磁场中会暂时被磁化，移除后迅速失去磁性。
A magnetic field is a region around a magnet where a magnetic material experiences a force. Field lines run from north to south outside the magnet. The closer the lines, the stronger the field. A plotting compass can be used to trace field lines; its needle points along the direction of the field.
磁场是磁体周围一个磁性材料会受到作用力的区域。磁体外部的磁感线由北极指向南极。线越密集，磁场越强。可以使用指南针描绘磁感线；其指针指向磁场方向。
The Earth itself has a magnetic field due to its molten iron core. Its north magnetic pole is actually a magnetic south pole, which is why the north pole of a compass needle points roughly towards geographic north.
地球由于其熔融铁核而具有磁场。它的北磁极实际上是一个磁南极，正因为如此，指南针的北极大致指向地理北方。
8. Electromagnetism | 电磁学
When an electric current passes through a straight wire, a circular magnetic field is generated around it. The direction of the field can be found using the right-hand grip rule: if you grip the wire with your right hand with the thumb pointing in the direction of the current, your fingers curl in the direction of the magnetic field.
当电流通过一条直导线时，会在其周围产生环形磁场。磁场的方向可以用右手螺旋定则判断：用右手握住导线，拇指指向电流方向，手指的弯曲方向即为磁场方向。
A solenoid is a long coil of insulated wire. The magnetic field inside a current-carrying solenoid is strong and uniform – similar to the field of a bar magnet. An electromagnet is made by placing a soft iron core inside a solenoid; the iron greatly increases the magnetic field strength and can be demagnetised when the current is switched off, making it very useful for devices like electric bells, relays and lifting magnets in scrapyards.
螺线管是一个长的绝缘导线线圈。通有电流的螺线管内部磁场强而均匀——类似于条形磁铁的磁场。电磁铁是将软铁芯放入螺线管中制成的；铁芯会极大地增强磁场强度，且断电后便会消磁，因此非常适用于电铃、继电器和废料场的起重电磁铁等设备。
9. The Motor Effect | 电动机效应
A current-carrying conductor placed in an external magnetic field experiences a force, provided the conductor is not parallel to the field. This is called the motor effect. The direction of the force is given by Fleming’s left-hand rule: thumb – force (motion), first finger – field (N to S), second finger – current (+ to –).
载流导体处于外部磁场中时，只要导体不与磁场平行，就会受到力的作用。这就是电动机效应。力的方向可用弗莱明左手定则判断：拇指——受力（运动）方向，食指——磁场方向 (N→S)，中指——电流方向 (+→–)。
The magnitude of the force can be calculated using:
力的大小可用下式计算：
F = B × I × L
where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A), and L is the length of conductor within the field in metres (m).
其中 F 为力 (N)，B 为磁通量密度 (T)，I 为电流 (A)，L 为处在磁场中的导体长度 (m)。
A d.c. electric motor makes use of the motor effect. A coil of wire is placed in a magnetic field and a current is passed through it. The forces on opposite sides of the coil create a turning effect that rotates the coil. A split-ring commutator reverses the current every half turn, ensuring the coil continues to rotate in the same direction.
直流电动机利用了电动机效应。线圈置于磁场中并通有电流。线圈两边受到方向相反的力，产生使线圈旋转的转矩。换向器每隔半圈使电流反向，确保线圈持续沿同一方向旋转。
10. Electromagnetic Induction | 电磁感应
If a conductor cuts across magnetic field lines, or if the magnetic field through a coil changes, a potential difference is induced across the ends of the conductor. This is called electromagnetic induction. If the conductor is part of a complete circuit, the induced p.d. causes a current to flow.
如果导体切割磁感线，或者穿过线圈的磁场发生变化，则导体两端会产生感应电势差。这就是电磁感应。若导体是完整回路的一部分，感应电势差便会驱动电流流动。
The generator effect is used in alternators and dynamos. An alternator produces alternating current (a.c.) by rotating a coil in a magnetic field and using slip rings and brushes to maintain connections. A dynamo uses a split-ring commutator to produce direct current (d.c.).
发电机效应被应用于交流发电机和直流发电机。交流发电机通过旋转线圈切割磁场，并利用滑环和电刷维持连接，从而产生交流电 (a.c.)。直流发电机则使用换向器产生直流电 (d.c.)。
The size of the induced p.d. can be increased by moving the magnet or coil more quickly, using a stronger magnet, or adding more turns to the coil.
增大感应电势差的方法包括：加快磁铁或线圈的运动速度、使用更强的磁铁、或增加线圈的匝数。
11. Transformers and the National Grid | 变压器与全国电网
A transformer is a device that changes the size of an alternating voltage. It consists of two separate coils wound on a soft iron core. An alternating current in the primary coil creates a changing magnetic field in the core, which induces an alternating voltage in the secondary coil. The ratio of the voltages is directly proportional to the ratio of the numbers of turns:
变压器是一种改变交流电压大小的装置。它由两个绕在软铁芯上的独立线圈组成。初级线圈中的交流电会在铁芯中产生变化的磁场，从而在次级线圈中感应出交流电压。电压之比与线圈匝数之比成正比：
Vp / Vs = Np / Ns
where Vp and Vs are the primary and secondary voltages, and Np and Ns are the numbers of turns on the primary and secondary coils respectively. Step-up transformers increase voltage and are used at power stations to minimise energy loss during transmission; step-down transformers decrease voltage to safe levels for homes.
其中 Vp 和 Vs 分别为初级和次级电压，Np 和 Ns 分别为初级和次级线圈的匝数。升压变压器提高电压，用于发电站以减少输电过程中的能量损失；降压变压器则将电压降至家庭安全用电水平。
The National Grid is a network of high-voltage transmission cables and transformers that distributes electricity across the country. High voltages are used because, for a given power, a higher voltage means a lower current, which reduces energy wasted as heat in the cables (P = I²R). This makes electricity transmission much more efficient.
全国电网是由高压输电线缆和变压器组成的网络，将电力分配到全国各地。使用高压输电是因为在相同功率下，电压越高电流越小，这能减少因电缆发热而浪费的能量 (P = I²R)，从而大大提高输电效率。
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