📚 Electric Charge, Electric Fields and Electric Current | 电荷、电场与电流的产生
This article covers the fundamental concepts of electric charge, electric fields and the generation of electric current, tailored for the Edexcel IGCSE Physics syllabus. We will explore how charges behave, how fields are mapped, and how current flows in circuits.
本文面向 Edexcel IGCSE 物理考纲，系统讲解电荷、电场与电流产生的基本概念。我们将探讨电荷如何相互作用、电场如何描绘，以及电流如何在电路中流动。
1. Types of Electric Charge | 电荷的种类
There are two types of electric charge: positive charge and negative charge. Protons carry a positive charge, while electrons carry a negative charge. In a neutral atom, the number of protons equals the number of electrons.
电荷分为两种：正电荷与负电荷。质子带正电，电子带负电。在电中性的原子中，质子数与电子数相等。
Like charges repel each other, whereas unlike charges attract each other. This is the fundamental rule of electrostatics.
同种电荷相互排斥，异种电荷相互吸引。这是静电学的基本规律。
harge is the coulomb (C). The charge of a single electron is approximately −1.6 × 10⁻¹⁹ C.
电荷的单位是库仑（C）。单个电子所带的电荷量约为 −1.6 × 10⁻¹⁹ C。
2. Charging by Friction, Conduction and Induction | 摩擦起电、接触起电与感应起电
Friction: When two different insulating materials are rubbed together, electrons are transferred from one material to the other. The material that loses electrons becomes positively charged, and the one that gains electrons becomes negatively charged.
摩擦起电：当两种不同的绝缘体相互摩擦时，电子会从一个物体转移到另一个物体。失去电子的物体带正电，得到电子的物体带负电。
For example, rubbing a polythene rod with a duster transfers electrons from the duster to the rod, giving the rod a negative charge.
例如，用布摩擦聚乙烯棒时，电子从布转移到棒上，使棒带负电。
Conduction: A charged object touches a neutral conductor, and charge is transferred directly. For example, a negatively charged rod touching a metal sphere causes some electrons to move onto the sphere.
接触起电：带电体接触中性导体时，电荷直接转移。例如，带负电的棒接触金属球时，部分电子会转移到球上。
Induction: A charged object is brought near a conductor without touching it. This redistributes the charges in the conductor, causing one side to become positive and the other negative. If the conductor is earthed while the charged object is near, a net opposite charge remains after removing the charged object.
感应起电：将带电体靠近导体但不接触，导体内电荷重新分布，使一端带正电、另一端带负电。若在带电体靠近时接地，移走带电体后导体将带上异种净电荷。
3. Electric Field Representation | 电场的表示
An electric field is a region around a charged object where another charge experiences a force. Electric fields are represented by field lines or lines of force.
电场是带电体周围的一个区域，在该区域内其他电荷会受到力的作用。电场用电场线（电力线）来表示。
- Field lines always point from positive charge to negative charge.
- Lines never cross each other.
- The spacing of lines indicates field strength: closer lines mean a stronger field.
- For a point charge, the field lines radiate outward for a positive charge and inward for a negative charge.
- 电场线总是从正电荷指向负电荷。
- 电场线永不相交。
- 电场线的疏密表示电场强弱：线越密，电场越强。
- 对于点电荷，正电荷的电场线向外辐射，负电荷的电场线向内汇聚。
Between two parallel charged plates, the electric field is uniform. The field lines are parallel and equally spaced.
在两块平行带电板之间，电场是匀强电场。电场线平行且间距相等。
4. Electric Field Strength | 电场强度
Electric field strength (E) is defined as the force (F) experienced per unit positive charge (q) placed at that point.
电场强度（E）定义为某点处单位正电荷（q）所受的力（F）。
E = F / q
The unit of electric field strength is newtons per coulomb (N/C), which is equivalent to volts per metre (V/m).
电场强度的单位是牛每库（N/C），也等价于伏每米（V/m）。
For a uniform field between two parallel plates separated by distance d and with potential difference V, the field strength is also given by:
对于相距为 d、电势差为 V 的两平行板之间的匀强电场，电场强度也可表示为：
E = V / d
This shows that a higher voltage or a smaller gap results in a stronger electric field.
这表明电压越大或板间距越小，电场越强。
5. Electric Current | 电流
Electric current is the rate of flow of electric charge. When free electrons in a conductor drift in a particular direction, a net charge flows, creating a current.
电流是电荷流动的速率。当导体中的自由电子沿某一方向定向移动时，净电荷发生流动，便形成电流。
The amount of charge (Q) passing a point in a time (t) is related to the current (I):
在时间（t）内通过某一点的电荷量（Q）与电流（I）的关系为：
I = Q / t
Current is measured in amperes (A); 1 A = 1 C/s. Charge is measured in coulombs (C).
电流的单位是安培（A）；1 A = 1 C/s。电荷量的单位是库仑（C）。
For a current to flow, there must be a complete circuit and a source of energy, such as a battery or power supply, to push the charges around.
要使电流流动，必须有闭合回路以及能量来源（如电池或电源），以推动电荷移动。
6. Conventional Current vs Electron Flow | 传统电流与电子流动
In a metal conductor, electric current is due to the movement of free electrons. Electrons are negatively charged, so they flow from the negative terminal toward the positive terminal of a battery.
在金属导体中，电流是由自由电子的运动引起的。电子带负电，因此它们从电池的负极流向正极。
However, by convention, electric current is defined as the direction in which positive charge would flow. This is called conventional current and it flows from the positive terminal to the negative terminal in an external circuit.
然而，按照惯例，电流的方向被定义为正电荷流动的方向。这称为“传统电流方向”，在外电路中它从正极流向负极。
| Direction | Conventional current | Electron flow |
| Outside the battery | + to − | − to + |
| Inside the battery | − to + | + to − |
Remember: conventional current is the opposite direction to electron flow in a metallic conductor.
切记：在金属导体中，传统电流方向与电子流动方向相反。
7. Potential Difference and Electromotive Force | 电势差与电动势
it charge between two points in a circuit. It is measured in volts (V), where 1 V = 1 J/C.
电势差（电压）是电路中两点之间每单位电荷所转移的能量。其单位是伏特（V），1 V = 1 J/C。
V = W / Q
where V is potential difference, W is energy transferred and Q is charge.
其中 V 是电势差，W 是转移的能量，Q 是电荷量。
Electromotive force (e.m.f.) is the total energy supplied per unit charge by a source such as a battery. It is also measured in volts. The p.d. across a component is the energy transferred by the component, while e.m.f. is the energy supplied by the source.
电动势（e.m.f.）是电源（如电池）向每单位电荷提供的总能量，单位也是伏特。元件两端的电势差是元件消耗的能量，而电动势是电源提供的能量。
8. Resistance and Ohm's Law | 电阻与欧姆定律
Resistance is the opposition to the flow of electric current. It is defined as the ratio of potential difference across a component to the current through it.
电阻是对电流流动的阻碍作用。它定义为元件两端电势差与通过它的电流之比。
R = V / I
The unit of resistance is the ohm (Ω). 1 Ω = 1 V/A.
电阻的单位是欧姆（Ω）。1 Ω = 1 V/A。
Ohm's law states that, provided temperature and other physical conditions remain constant, the current through an ohmic conductor is directly proportional to the potential difference across it.
欧姆定律指出：在温度和其他物理条件保持恒定时，通过欧姆导体的电流与它两端的电势差成正比。
Ohmic conductors, such as metal wires at constant temperature, obey Ohm's law. Non-ohmic components, such as filament lamps and diodes, do not follow this linear relationship.
欧姆导体（如恒温下的金属导线）遵循欧姆定律。非欧姆元件（如白炽灯丝和二极管）不满足这种线性关系。
9. Current-Voltage Characteristics | 电流-电压特性
The current-voltage (I–V) graph shows how the current through a component varies with the potential difference across it.
电流-电压（I–V）图表示通过元件的电流随其两端电势差变化的规律。
- For a fixed resistor: a straight line through the origin, showing that I is proportional to V.
- For a filament lamp: the line curves, because as temperature rises, resistance increases and the graph becomes less steep.
- For a diode: current flows easily in one direction (forward bias) but very little in the reverse direction (reverse bias).
- 定值电阻：过原点的直线，说明 I 与 V 成正比。
- 白炽灯：图线弯曲，因为温度升高导致电阻增加，斜率变小。
- 二极管：正向偏置时电流容易通过，反向偏置时电流几乎为零。
For a semiconductor diode, the forward threshold voltage is about 0.6–0.7 V for a silicon diode.
对于半导体二极管，硅管的正向导通阈值电压约为 0.6–0.7 V。
10. Drift Velocity and Charge Carriers | 漂移速度与载流子
In a conductor, free electrons move randomly at high speeds due to thermal energy. When an electric field is applied, they also drift slowly in a particular direction. This average net velocity is called the drift velocity.
在导体中，自由电子因热运动而高速随机移动。当外加电场时，它们还会沿某一方向缓慢漂移。这个平均净速度称为漂移速度。
The current can be expressed in terms of charge carriers:
电流可以用载流子表示：
I = n A v q
where n is the number of free charge carriers per unit volume, A is the cross-sectional area, v is the drift velocity, and q is the charge of each carrier.
其中 n 是单位体积内自由载流子数，A 是导体的横截面积，v 是漂移速度，q 是每个载流子所带的电荷量。
For a given current, a thinner conductor or a material with fewer charge carriers will require a higher drift velocity.
对于给定电流，导体越细或载流子浓度越小，所需的漂移速度就越大。
11. Applications and Hazards of Electrostatics | 静电的应用与危害
Static electricity has many practical uses. For example, electrostatic precipitators remove smoke particles from industrial exhaust gases, and photocopiers use electrostatic charge to attract toner particles to paper.
静电有许多实际应用。例如，静电除尘器可从工业废气中除去烟尘颗粒，复印机利用静电吸引碳粉到纸上。
Static charge can also be dangerous. In hospitals, static sparks can ignite flammable anaesthetic gases. When refuelling aircraft, static charges must be earthed to prevent sparks that could ignite fuel fumes.
静电也可能造成危险。在医院中，静电火花可能引燃易燃的麻醉气体。在为飞机加油时，必须接地以释放静电，防止火花引燃燃油蒸气。
Storing large amounts of separated charge creates high voltages, which is why electrostatic equipment must be carefully designed with safety features.
大量分离电荷会产生高电压，因此静电设备必须谨慎设计并配备安全措施。
12. Summary | 总结
Electric charge is the foundation of all electrical phenomena. Charges create electric fields, which exert forces on other charges. Electric current arises from the flow of charge, and it is driven by potential difference. Resistance and Ohm's law explain how much current flows for a given voltage. Understanding these ideas is essential for IGCSE Physics and for further study of electricity and magnetism.
电荷是一切电现象的基础。电荷产生电场，电场对其他电荷施力。电流源于电荷的流动，并由电势差驱动。电阻与欧姆定律解释了在给定电压下电流的大小。理解这些概念对于 IGCSE 物理以及后续学习电学和磁学至关重要。
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