📚 Electrical Circuits: Current, Voltage and Resistance | 电路：电流、电压与电阻
In the Cambridge science curriculum, electricity is a core topic that links physical concepts to everyday applications. This article reviews current, voltage, resistance, circuit rules, and energy transfer in a clear and exam-focused way.
在剑桥科学课程中，电学是连接物理概念与日常应用的核心主题。本文以清晰且贴近考试的方式复习电流、电压、电阻、电路规律和能量转移。
1. Circuit Basics | 电路基础
An electric circuit is a closed loop that allows charge to flow continuously. It must include a source of electromotive force (e.m.f.), such as a cell or battery, conducting wires, and usually one or more components such as lamps, resistors, or switches.
电路是允许电荷持续流动的闭合回路。它必须包含电动势源（如电池）、导线，并且通常包含一个或多个元件，例如灯泡、电阻器或开关。
Standard circuit symbols are used in Cambridge exams: a cell is shown as a long and short line, a battery as two or more cells, a resistor as a rectangle, and an ammeter as a circle with A.
剑桥考试中使用标准电路符号：电池用一长一短两条线表示，电池组用两个或多个电池表示，电阻用矩形表示，电流表用带 A 的圆圈表示。
2. Electric Current | 电流
Electric current is the rate of flow of electric charge. In a metal wire, current is carried by free electrons moving from the negative terminal to the positive terminal, but conventional current is defined as flowing from positive to negative.
电流是电荷流动的速率。在金属导线中，电流由自由电子从负极向正极移动形成，但规定电流方向为正电荷从正极到负极的方向。
I = Q ÷ t
The equation is I = Q ÷ t, where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds (s). An ammeter must be connected in series with the component so that the same current passes through it.
公式为 I = Q ÷ t，其中 I 是电流（安培，A），Q 是电荷量（库仑，C），t 是时间（秒，s）。电流表必须与元件串联，使相同的电流通过它。
3. Voltage (Potential Difference) | 电压（电势差）
Voltage, or potential difference, is the energy transferred per unit charge between two points in a circuit. It is measured in volts (V) using a voltmeter connected in parallel.
电压（电势差）是电路中两点之间每单位电荷转移的能量。它用伏特（V）计量，电压表应并联连接。
V = W ÷ Q
V = W ÷ Q, where V is voltage in volts (V), W is work done or energy transferred in joules (J), and Q is charge in coulombs (C).
V = W ÷ Q，其中 V 是电压（伏特，V），W 是做功或转移的能量（焦耳，J），Q 是电荷量（库仑，C）。
4. Resistance | 电阻
Resistance is a measure of how much a component opposes the flow of current. It is measured in ohms (Ω), and it depends on the material, length, thickness and temperature of the conductor.
电阻是元件阻碍电流流动的程度。它的单位是欧姆（Ω），取决于导体的材料、长度、粗细和温度。
For a metal wire, resistance increases with length, decreases with cross-sectional area, and generally increases with temperature.
对于金属导线，电阻随长度增加而增大，随横截面积增大而减小，并且通常随温度升高而增大。
5. Ohm’s Law | 欧姆定律
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. This gives R = V ÷ I.
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。因此有 R = V ÷ I。
R = V ÷ I
R = V ÷ I, where R is resistance in ohms (Ω), V is voltage in volts (V), and I is current in amperes (A). The I-V graph for an ohmic conductor is a straight line through the origin.
R = V ÷ I，其中 R 是电阻（欧姆，Ω），V 是电压（伏特，V），I 是电流（安培，A）。欧姆导体的 I-V 图像是一条过原点的直线。
6. Series Circuits | 串联电路
In a series circuit, components are connected end-to-end in one loop. The current is the same at every point, but the supply voltage is shared between the components.
在串联电路中，元件首尾相连组成一个回路。电路中各点的电流相同，但电源电压在元件之间分配。
- Itotal = I₁ = I₂ = …
- Vtotal = V₁ + V₂ + …
- Rtotal = R₁ + R₂ + …
The total resistance in series is the sum of individual resistances, so adding more resistors increases the total resistance and reduces the current.
串联总电阻等于各电阻之和，因此增加电阻会增大总电阻并减小电流。
7. Parallel Circuits | 并联电路
In a parallel circuit, components are connected on separate branches. The voltage across each branch is the same as the supply voltage, but the total current is the sum of the branch currents.
在并联电路中，元件连接在不同的支路上。各支路两端的电压与电源电压相同，但总电流等于各支路电流之和。
- Vtotal = V₁ = V₂ = …
- Itotal = I₁ + I₂ + …
- 1/Rtotal = 1/R₁ + 1/R₂ + …
The total resistance in parallel is less than the smallest individual resistance, which is why adding more branches increases the total current.
并联总电阻小于最小的单个电阻，因此增加支路会增大总电流。
8. Combining Resistors | 电阻组合
For two resistors in series, Rtotal = R₁ + R₂. For two resistors in parallel, the product-over-sum rule can be used: Rtotal = (R₁ × R₂) ÷ (R₁ + R₂).
两个电阻串联时，R总 = R₁ + R₂。两个电阻并联时，可以用乘积除以和：R总 = (R₁ × R₂) ÷ (R₁ + R₂)。
For example, a 6 Ω resistor in parallel with a 3 Ω resistor gives Rtotal = (6 × 3) ÷ (6 + 3) = 2 Ω.
例如，一个 6 Ω 电阻与一个 3 Ω 电阻并联，得到 R总 = (6 × 3) ÷ (6 + 3) = 2 Ω。
9. Electrical Energy and Power | 电能与功率
Power is the rate at which electrical energy is transferred. It is calculated using P = I × V, where P is power in watts (W), I is current in amperes, and V is voltage in volts.
功率是电能转移的速率。计算公式为 P = I × V，其中 P 是功率（瓦特，W），I 是电流（安培），V 是电压（伏特）。
P = I × V and E = P × t
Energy transferred is E = P × t, where E is energy in joules (J), P is power in watts, and t is time in seconds.
转移的能量为 E = P × t，其中 E 是能量（焦耳，J），P 是功率（瓦特），t 是时间（秒）。
10. Safety and Measurement | 安全与测量
To measure current, an ammeter is connected in series. To measure voltage, a voltmeter is connected in parallel. Fuses and circuit breakers protect circuits from excessive current.
测量电流时，电流表串联在电路中。测量电压时，电压表并联在元件两端。保险丝和断路器用于保护电路免受过大电流的损害。
In the laboratory and at home, keep electrical devices away from water, use insulated wires, and avoid overloading sockets to reduce the risk of fire and electric shock.
在实验室和家中，应使电器远离水，使用绝缘导线，并避免插座过载，以降低火灾和触电风险。
11. Common Misconceptions | 常见误区
A common misconception is that current is ‘used up’ in a circuit. In fact, current is conserved at junctions, while energy is transferred to components.
常见误区是认为电流在电路中被“用完”。实际上，电流在节点处守恒，而能量被传递给了元件。
Another misconception is that voltage flows through a circuit. Voltage does not flow; it is a difference in energy per charge between two points.
另一个误区是认为电压在电路中流动。电压并不流动；它是两点之间每单位电荷的能量差。
12. Exam Tips | 考试提示
In Cambridge science exams, always state units with numerical answers, use standard symbols in circuit diagrams, and show full working when calculating current, voltage, resistance or power.
在剑桥科学考试中，数值答案必须带单位，电路图使用标准符号，并在计算电流、电压、电阻或功率时写出完整步骤。
Practise rearranging equations such as V = I × R, Q = I × t, and P = V × I, and remember that series circuits share voltage while parallel circuits share current.
练习变换公式，例如 V = I × R、Q = I × t 和 P = V × I，并记住串联电路分压、并联电路分流。
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