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Electric Circuits and Electrical Quantities | 电路与电学量

2026-08-21 13:17 · 本文部分内容由AI辅助创作

📚 Electric Circuits and Electrical Quantities | 电路与电学量

In the Cambridge IGCSE Science syllabus, electric circuits and electrical quantities form a core topic in Physics. This revision article covers current, voltage, resistance, Ohm's law, series and parallel circuits, electrical power, circuit symbols, safety, and common exam techniques. Understanding these ideas helps you analyse real circuits and answer calculation questions accurately.

在剑桥 IGCSE 科学课程中，电路与电学量是物理部分的核心主题。本篇复习文章涵盖电流、电压、电阻、欧姆定律、串联与并联电路、电功率、电路符号、安全用电以及常见考试技巧。理解这些概念有助于你分析实际电路并准确解答计算题。

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 through the lattice of positive ions. Charge (Q) is measured in coulombs (C), and current (I) is measured in amperes (A).

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在正离子晶格中移动形成。电荷（Q）的单位是库仑（C），电流（I）的单位是安培（A）。

The relationship between charge, current and time is given by:

电荷、电流和时间之间的关系如下：

Q = I × t

Where Q is charge in coulombs, I is current in amperes, and t is time in seconds. An ammeter is connected in series to measure the current flowing through a component.

其中 Q 是电荷（库仑），I 是电流（安培），t 是时间（秒）。安培表串联在电路中，用于测量流过元件的电流。

Conventional current is defined as the flow of positive charge, so its direction is opposite to the movement of electrons. This convention is used in circuit diagrams and equations throughout Cambridge IGCSE Science.

传统电流方向定义为正电荷的流动方向，因此与电子的实际移动方向相反。在剑桥 IGCSE 科学的电路图和公式中均使用这一传统方向。

2. Voltage, EMF and Potential Difference | 电压、电动势与电势差

Voltage, or 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). The electromotive force (e.m.f.) of a source is the electrical energy supplied to each coulomb of charge passing through the source.

电压（又称电势差，p.d.）是单位电荷在电路中两点之间移动时转移的能量。电压的单位是伏特（V）。电源的电动势（e.m.f.）是每库仑电荷通过电源时获得的电能。

One volt is defined as one joule of energy transferred per coulomb of charge:

一伏特定义为每库仑电荷转移一焦耳的能量：

V = W ÷ Q

A voltmeter is always connected in parallel across the component to measure the potential difference across it. In circuit diagrams, voltmeters have a very high resistance so they draw negligible current.

伏特表总是并联在元件两端来测量其电势差。在电路图中，伏特表的电阻非常大，因此流过的电流可以忽略不计。

When measuring e.m.f., the voltmeter is placed across the terminals of a cell or battery when no current flows. This open-circuit voltage is slightly higher than the terminal voltage when a current is drawn because the source itself has internal resistance.

测量电动势时，将伏特表连接在电池或电池组两端，且电路中无电流。这种开路电压略高于有电流时的端电压，因为电源本身具有内阻。

3. Resistance and Ohm's Law | 电阻与欧姆定律

Resistance is the opposition to the flow of electric current. It is measured in ohms (Ω). The resistance of a conductor depends on its material, length, cross-sectional area, and temperature.

电阻是对电流流动的阻碍作用，单位是欧姆（Ω）。导体的电阻取决于其材料、长度、横截面积和温度。

Ohm's law states that, for a metallic conductor at constant temperature, the current through it is directly proportional to the potential difference across it. The relationship is written as:

欧姆定律指出，对于温度恒定的金属导体，通过它的电流与它两端的电势差成正比。其关系式为：

V = I × R

Where V is voltage in volts, I is current in amperes, and R is resistance in ohms. A resistor is a component that obeys Ohm's law, and its I-V graph is a straight line through the origin.

其中 V 是电压（伏特），I 是电流（安培），R 是电阻（欧姆）。电阻器是遵守欧姆定律的元件，其 I-V 图是一条经过原点的直线。

For a given material, resistance increases with length and decreases with cross-sectional area. Resistance also increases with temperature for most metallic conductors, which is why filament lamps do not follow Ohm's law at high temperatures.

对于同一种材料，电阻随长度增加而增大，随横截面积增大而减小。大多数金属导体的电阻随温度升高而增大，这就是白炽灯在高温下不遵循欧姆定律的原因。

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4. Series Circuits | 串联电路

In a series circuit, components are connected one after another in a single loop. The same current passes through every component. However, the total potential difference from the source is shared between the components.

在串联电路中，元件依次连接在一个回路中。相同的电流通过每个元件，但电源的总电势差在元件之间分配。

The total resistance in a series circuit is the sum of the individual resistances:

串联电路的总电阻等于各电阻之和：

Rtotal = R₁ + R₂ + R₃

This means adding more resistors in series increases the total resistance and decreases the current, assuming the supply voltage stays constant.

这意味着在串联电路中增加更多电阻会增大总电阻并减小电流，前提是电源电压保持不变。

Key features of series circuits are summarised below:

串联电路的主要特点总结如下：

If one component in a series circuit fails, the whole circuit stops working. This property is used in simple torch circuits and string lights, although modern decorative lamps often use parallel connections to avoid total failure.

如果串联电路中某个元件损坏，整个电路都会停止工作。这一特性被应用在简单手电筒电路和某些串联灯串中，不过现代装饰灯常采用并联连接以避免全部熄灭。

5. Parallel Circuits | 并联电路

In a parallel circuit, components are connected in separate branches across the same two points of the supply. The potential difference across each branch is the same as the supply voltage. The total current from the source is the sum of the currents in each branch.

在并联电路中，元件连接在电源相同两点之间的不同支路上。各支路两端的电势差与电源电压相同。电源输出的总电流等于各支路电流之和。

The total resistance of a parallel circuit is less than the resistance of the smallest individual resistor. It is calculated using:

并联电路的总电阻小于其中最小的单个电阻。其计算公式为：

1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃

Adding more branches in parallel decreases the total resistance and increases the total current drawn from the supply.

在并联电路中增加更多支路会减小总电阻，并增大从电源吸取的总电流。

Key features of parallel circuits are summarised below:

并联电路的主要特点总结如下：