📚 Current Electricity | 电流
Electric current is the lifeblood of modern technology. In this GCSE WJEC Physics guide, we break down the core concepts of current electricity, from the movement of charge to circuit behaviour, power ratings and household wiring. Mastering these ideas will not only help you tackle exam questions with confidence but also give you the tools to understand every gadget and appliance around you.
电流是现代技术的命脉。在这份GCSE WJEC物理指南中，我们将深入解析电流的核心概念，从电荷的定向移动、电路行为、功率计算到家用电器的布线。掌握这些知识不仅能让你在考试中从容应对，还能帮助你理解身边每一个电子设备的工作原理。
1. What is Electric Current? | 什么是电流？
An electric current is the rate of flow of electric charge. In metal wires, the moving charges are free electrons that drift away from the negative terminal and towards the positive terminal of a battery or power supply. Current is measured in amperes (A), often shortened to 'amps'. It is a scalar quantity, but in diagrams we show its direction as the flow of positive charge — from positive to negative — following conventional current direction. The actual electron flow is opposite to this convention.
电流是电荷流动的速率。在金属导线中，流动的电荷是自由电子，它们从电池或电源的负极漂移向正极。电流的单位是安培(A)，常简称为“安”。它是一个标量，但在电路图中我们按照惯例把电流方向标示为正电荷的流动方向——从正极到负极。实际的电子流动方向与这一规定相反。
- Electric current = rate of flow of charge
- 电流 = 电荷的流动速率
- Measured in amperes (A) using an ammeter connected in series
- 使用电流表串联测量，单位安培(A)
- Conventional current direction: positive to negative
- 约定电流方向：正极到负极
- Electron flow direction: negative to positive
- 电子流动方向：负极到正极
In WJEC exams, always remember that an ammeter must be placed in series and that conventional current is the basis for circuit analysis, even though we know it is electrons moving.
在WJEC考试中，始终记住电流表必须串联连接，并且电路分析都基于约定电流方向，尽管我们清楚移动的其实是电子。
2. Charge, Current and Time | 电荷、电流和时间
The relationship linking charge (Q), current (I) and time (t) is fundamental. It is expressed as:
电荷(Q)、电流(I)和时间(t)的关系是最基本的，表达式为：
I = Q ÷ t
asses a point in the circuit every second. The charge on one electron is approximately 1.6 × 10⁻¹⁹ C, so a coulomb is a very large number of electrons.
其中I是电流(安培)，Q是电荷(库仑)，t是时间(秒)。这个公式表明1 A的电流意味着每秒钟有1 C的电荷通过电路的某一点。单个电子的电荷量约为1.6×10⁻¹⁹ C，因此1库仑代表了极大量的电子。
sk you to calculate the charge that flows through a lamp in a given time when the current is known.
你可以变换公式得到Q = I × t。一道典型的WJEC题目会要求你根据已知电流计算规定时间内流经灯泡的电荷量。
3. Conductors and Insulators | 导体与绝缘体
Materials can be classified by how easily charge can flow through them. Conductors, like copper and aluminium, have a large number of free electrons that can move when a potential difference is applied. Insulators, such as plastic, rubber and glass, have very few free charge carriers and therefore do not allow current to pass easily. In a circuit, wires are made of conducting metal surrounded by insulating plastic to prevent short circuits and electric shocks.
材料可以根据电荷在其中流动的难易程度来分类。导体，如铜和铝，拥有大量自由电子，在施加电压时能够移动。绝缘体，如塑料、橡胶和玻璃，自由电荷载流子极少，因此不易导电。在电路中，导线由金属导体制成，外包绝缘塑料以防止短路和触电。
Semiconductors, like silicon, have properties between conductors and insulators and are vital in electronics. For WJEC, the key distinction is the availability of free electrons: conductors have many, insulators have negligible numbers.
半导体（如硅）的导电性能介于导体和绝缘体之间，在电子学中至关重要。在WJEC考试中，关键的区分在于自由电子的数量：导体有很多，绝缘体微乎其微。
4. Potential Difference and Electromotive Force | 电压与电动势
Potential difference (p.d.) is the energy transferred per unit charge as charge moves between two points in a circuit. It is measured in volts (V). A voltmeter is always connected in parallel across a component. One volt means one joule of energy is transferred per coulomb of charge.
电压是单位电荷在电路中两点之间移动时转移的能量，单位是伏特(V)。电压表必须并联在元件两端。1伏特表示每库仑电荷转移1焦耳的能量。
Electromotive force (e.m.f.) is the total energy supplied per coulomb by a source such as a battery or power supply. Although also measured in volts, e.m.f. refers to the total energy provided before any is lost in internal resistance. The equation linking energy (E), charge (Q) and voltage (V) is:
电动势是由电池或电源等提供的每库仑总能量。虽然单位也是伏特，但电动势指在考虑内电阻损失之前提供的总能量。相关公式为：
E = Q × V
This equation is central to understanding energy transfers in circuits and appears frequently in WJEC exam papers.
这个公式对理解电路中的能量转移至关重要，也是WJEC考卷上的常客。
5. Resistance and Ohm's Law | 电阻与欧姆定律
Resistance is a measure of how much a component opposes the flow of electric current. It is defined as the ratio of potential difference across a component to the current flowing through it, and its unit is the ohm (Ω).
电阻是衡量元件对电流阻碍作用大小的物理量，定义为元件两端电压与通过电流的比值，单位是欧姆(Ω)。
R = V ÷ I
For many metal conductors at constant temperature, the current is directly proportional to the potential difference. This is known as Ohm's Law. A component that obeys this law is called an ohmic conductor, and its I-V graph is a straight line passing through the origin.
在温度不变的情况下，许多金属导体的电流与电压成正比，这就是欧姆定律。符合这一规律的元件被称为欧姆导体，其I-V图是一条过原点的直线。
It is critical to remember that resistance remains constant only if temperature is kept constant. Resistance can change if the material heats up. In WJEC, you must be able to state Ohm's Law and identify the condition ‘at constant temperature’.
必须记住，只有在温度恒定的情况下电阻才保持不变。当材料升温时电阻可能改变。在WJEC中，你需要能陈述欧姆定律并指明“温度恒定”这一条件。
6. Factors Affecting Resistance | 影响电阻的因素
The resistance of a wire depends on several factors which are often tested in the required practical activity:
导线的电阻取决于多个因素，这些因素也常常是必做实验活动的考查内容：
- Length: The longer the wire, the greater its resistance (direct proportion)
- 长度: 导线越长，电阻越大（正比关系）
- Cross-sectional area: The thicker the wire, the lower its resistance (inverse proportion)
- 横截面积: 导线越粗，电阻越小（反比关系）
- Material: Different materials have different resistivities; nichrome has a higher resistivity than copper
- 材料: 不同材料的电阻率不同；镍铬合金的电阻率比铜高
- Temperature: For a metal conductor, resistance increases with temperature because the vibrating ions impede electron flow more
- 温度: 对金属导体而言，温度升高电阻增大，因为离子振动加剧阻碍了电子流动
In a WJEC practical, you might investigate how resistance changes with the length of a wire using an ammeter, voltmeter, and a variable power supply. The relationship R ∝ L can be plotted to verify direct proportionality.
在WJEC实验中，你可能需要使用电流表、电压表和可调电源探究导线电阻随长度的变化。可以绘制R ∝ L图来验证正比关系。
7. I-V Characteristics of Components | 元件的电流-电压特性
The way current varies with voltage is unique for different components. WJEC expects you to recall and sketch the I-V graphs for the following:
不同元件的电流随电压变化的方式各具特色。WJEC要求你回忆并画出下列元件的I-V特性图：
| Component | I-V Graph Shape | Key Feature |
| Fixed resistor (ohmic conductor) | Straight line through origin | Constant resistance; obeys Ohm's Law |
| Filament lamp | S-shaped curve (steep increase then flattening) | Resistance increases as temperature rises |
| Diode | Very high resistance in reverse, sudden rise in forward bias above ~0.7 V | Conducts in one direction only |
For the filament lamp, the resistance increase is evident because the graph curves: as current goes up, the metal filament heats up and the vibrating ions scatter electrons more, raising resistance. For the diode, current is virtually zero until the forward voltage exceeds the threshold, then it rises sharply.
对于白炽灯，电阻增大的表现是曲线弯曲：随着电流升高，灯丝温度上升，离子振动加剧，电子散射增多，电阻增大。对于二极管，正向电压超过阈值（约0.7 V）之前电流几乎为零，然后急剧上升。
8. Series and Parallel Circuits | 串联与并联电路
Understanding how current and potential difference behave in series and parallel circuits is a core WJEC topic.
理解电流和电压在串联和并联电路中的表现是WJEC的核心内容。
In a series circuit:
在串联电路中：
- The current is the same at all points (I₁ = I₂ = I₃)
- 各处电流相等 (I₁ = I₂ = I₃)
- The total potential difference is shared across components: Vₜₒₜₐₗ = V₁ + V₂ + …
- 总电压在各元件上分配：Vₜₒₜₐₗ = V₁ + V₂ + …
- The total resistance is the sum of individual resistances: Rₜₒₜₐₗ = R₁ + R₂ + …
- 总电阻等于各电阻之和：Rₜₒₜₐₗ = R₁ + R₂ + …
In a parallel circuit:
在并联电路中：
- nt from the source equals the sum of the currents in separate branches: Iₜₒₜₐₗ = I₁ + I₂ + …
- 干路总电流等于各支路电流之和：Iₜₒₜₐₗ = I₁ + I₂ + …
- The potential difference across each branch is the same as the source voltage
- 各支路两端电压与电源电压相等
- Adding more branches decreases the total resistance, so current from the source increases
- 增加支路会减小总电阻，因此干路电流增大
I × R and P = I × V to analyse voltage splits, current distribution and power dissipation.
在解决电路问题时，运用这些规则并结合V = I × R和P = I × V公式，分析电压分配、电流分布和功率消耗。
9. Electrical Power and Energy | 电功率与电能
Power is the rate at which energy is transferred or work is done. In electrical terms, power (P) is measured in watts (W). The fundamental equations are:
功率是能量转移或做功的速率。在电学中，功率(P)的单位是瓦特(W)。基本公式为：
P = I × V
P = I² × R
E = P × t
Where E is energy in joules (J) and t is time in seconds (s). In household electricity, energy is often measured in kilowatt-hours (kWh) using the formula:
其中E为能量(焦耳J)，t为时间(秒)。在家用电器中，能量常用于千瓦时(kWh)计量，公式为：
Energy transferred (kWh) = Power (kW) × time (h)
tt-hours or calculate the cost of running an appliance. Remember that 1 kWh = 3.6 × 10⁶ J.
WJEC考题经常要求你在焦耳和千瓦时之间进行换算，或计算电器的运行成本。记住1 kWh = 3.6 × 10⁶ J。
10. Domestic Electricity and Safety | 家庭用电与安全
The UK mains supply provides an alternating current with a potential difference of about 230 V and a frequency of 50 Hz. Most household circuits use three wires:
英国市电提供约230 V、频率50 Hz的交流电。大多数家用电路使用三根导线：
- Live wire (brown): carries the alternating potential difference from the supply; the most dangerous
- 火线（棕色）: 从电源输送交变电压；最危险
- Neutral wire (blue): completes the circuit and is normally at or near earth potential
- 零线（蓝色）: 构成回路，通常处于或接近地电位
- Earth wire (green/yellow): a safety wire that provides a low-resistance path to ground, preventing the metal casing from becoming live
- 地线（绿/黄）: 安全线，提供低电阻接地通道，防止金属外壳带电
Fuses and circuit breakers protect circuits from overcurrent. A fuse contains a thin wire that melts if the current exceeds the rated value. A circuit breaker trips a switch open using an electromagnet or bimetallic strip. For WJEC, you need to explain why appliances with metal cases must be earthed and describe the role of double insulation in appliances with plastic cases.
保险丝和断路器用于防止电路过流。保险丝含有一段细导线，当电流超过额定值时会熔断。断路器则利用电磁铁或双金属片跳闸断开电路。在WJEC中，你需要解释为何金属外壳的电器必须接地，并描述塑料外壳电器中双重绝缘的作用。
11. Energy Transfers in Circuits | 电路中的能量转移
ct. This is the principle behind electric heaters, kettles and filament bulbs. The energy transfer can be calculated using E = I × V × t or E = I² × R × t. In a resistor, the temperature rise depends on the current, resistance and time.
当电流通过电阻时，电能会被转移到元件的热能储存中，常产生热效应。这就是电暖器、电热水壶和白炽灯的原理。能量转移可以用E = I × V × t 或 E = I² × R × t 计算。在电阻器中，温升取决于电流、电阻和时间。
Energy is transferred usefully in devices like lamps (light) and motors (kinetic energy), but it can also be wasted as heat. Efficiency can be calculated with:
在灯泡（光）和电动机（动能）中，能量被有效转移，但也可能以热量形式浪费。效率可计算如下：
Efficiency = Useful energy output ÷ Total energy input (× 100%)
Understanding energy transfers helps explain why some circuits or components become warm and why high-power devices need thicker wires or heat sinks.
理解能量转移有助于解释为什么某些电路或元件会变热，以及为什么大功率设备需要更粗的导线或散热片。
12. Exam Tips for WJEC | WJEC考试技巧
Success in the WJEC Current Electricity topic relies on precision and practice. Keep these pointers in mind:
要在WJEC电流这一专题中取得成功，需要精确和练习。请牢记以下要点：
- Always use correct units and convert to standard units: amps, ohms, volts, coulombs, seconds
- 始终使用正确单位并转换为标准单位：安培、欧姆、伏特、库仑、秒
- Show full working when rearranging equations; a step-by-step approach earns method marks
- 变换公式时写出完整步骤；分步推导可得到方法分
- Label axes clearly on I-V graphs and indicate the shape for each component
- 绘制I-V图时标清坐标轴，并画出各元件的曲线形状
- Use the phrase 'at constant temperature' when stating Ohm's Law
- 陈述欧姆定律时必须使用“在温度不变的情况下”
- Explain the difference between p.d. and e.m.f. using energy per coulomb
- 用每库仑能量解释电压与电动势的区别
- In circuit calculations, first identify whether components are in series or parallel, then apply the relevant rules
- 在电路计算中，首先判断元件是串联还是并联，再运用相应规则
- Practice converting between joules and kilowatt-hours, and read the number of fuses or rating plates carefully
- 练习焦耳与千瓦时的换算，仔细阅读保险丝额定值或铭牌信息
Mastering these fundamentals will give you the confidence to tackle both structured questions and practical-based exam items. Regular past paper practice is the best way to sharpen your skills.
掌握这些基础知识将使你有信心应对结构化试题以及基于实验的考题。定期练习历年真题是提高应试能力的最佳途径。
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