📚 IGCSE AQA Physics: Electric Current – Key Points | IGCSE AQA 物理：电流 考点精讲
Electric current is one of the most fundamental concepts in electricity. Understanding what it is, how it flows, how to measure it, and how it behaves in series and parallel circuits is essential for success in your IGCSE AQA Physics exam. This article covers every key point, from definitions and formulas to practical measurement and common pitfalls.
电流是电学中最基础的概念之一。理解电流是什么、如何流动、如何测量、以及在串联和并联电路中的行为，对于在 IGCSE AQA 物理考试中取得成功至关重要。本文涵盖从定义、公式到实际测量和常见陷阱的每一个关键考点。
1. What is Electric Current? | 什么是电流？
Electric current is the rate of flow of electric charge. In a conducting material, charges move only when there is a complete circuit and a potential difference (voltage) to push them.
电流是电荷流动的速率。在导电材料中，仅当存在完整回路和推动电荷的电位差（电压）时，电荷才会移动。
More specifically, current measures how much charge passes through a cross-section of a conductor per unit time. If no net flow of charge occurs, there is no current.
更具体地说，电流度量的是单位时间内通过导体横截面的电荷量。如果没有净电荷流动，就没有电流。
In a metal wire, the moving charges are negatively charged electrons, but current can also be carried by positive or negative ions in electrolytes.
在金属导线中，移动的电荷是带负电的电子，但在电解质中，电流也可以由正离子或负离子携带。
2. Charge Carriers in Different Materials | 不同材料中的电荷载体
In solid metallic conductors, the charge carriers are delocalised free electrons that drift through the lattice of positive metal ions.
在固体金属导体中，电荷载体是离域的自由电子，它们在正金属离子晶格中漂移。
In electrolytes (ionic solutions or molten ionic compounds), both positive and negative ions are mobile and contribute to the current. Positive ions move towards the cathode, negative ions towards the anode.
在电解质（离子溶液或熔融离子化合物）中，正负离子均可移动并形成电流。正离子向阴极移动，负离子向阳极移动。
In semiconductors, current is carried by electrons and by ‘holes’ that behave like positive charge carriers, but this is beyond the scope of IGCSE AQA Physics.
在半导体中，电流由电子和相当于正电荷载流子的“空穴”携带，但这超出了 IGCSE AQA 物理的范围。
3. Defining Electric Current Quantitatively | 定量定义电流
Current (I) is defined by the equation:
电流 (I) 由以下方程定义：
I = Q / t
where I is the current in amperes (A), Q is the charge in coulombs (C), and t is the time in seconds (s).
其中 I 是电流，单位为安培 (A)； Q 是电荷，单位为库仑 (C)； t 是时间，单位为秒 (s)。
This relationship tells us that a current of 1 ampere means 1 coulomb of charge passes a point in the circuit every second. Rearranging gives Q = I × t and t = Q / I.
这一关系告诉我们，1 安培的电流意味着每秒钟有 1 库仑的电荷通过电路中的某一点。整理公式可得 Q = I × t 和 t = Q / I。
The charge on a single electron is 1.6 × 10⁻¹⁹ C. To calculate the number of electrons flowing, use total charge divided by this elementary charge.
单个电子的电荷是 1.6 × 10⁻¹⁹ 库仑。要计算流过的电子数，用总电荷除以这个基本电荷即可。
4. The Ampere and Charge Calculations | 安培与电荷计算
The ampere (A) is the SI base unit of electric current. One ampere equals one coulomb per second (1 A = 1 C/s).
安培 (A) 是电流的国际单位制基本单位。1 安培等于每秒 1 库仑 (1 A = 1 C/s)。
For example, a torch bulb carries a steady current of 0.25 A. How much charge flows in 3 minutes?
例如，一个手电筒灯泡通过 0.25 安的稳定电流。3 分钟内流过的电荷量是多少？
t = 3 × 60 s = 180 s
Q = I × t = 0.25 A × 180 s = 45 C
Always convert time into seconds before using the formula. A common mistake is to use minutes or hours, which gives a wrong answer.
始终先将时间转换为秒，再代入公式。一个常见错误是使用分钟或小时，导致答案错误。
To find the number of electrons represented by 45 C, divide by 1.6 × 10⁻¹⁹ C: that is approximately 2.8 × 10²⁰ electrons.
要求 45 C 表示的电子数，除以 1.6 × 10⁻¹⁹ C：结果大约是 2.8 × 10²⁰ 个电子。
5. Conventional Current vs Electron Flow | 常规电流与电子流
Historical convention defines the direction of conventional current as the direction in which positive charge would move: from the positive terminal to the negative terminal of a cell or battery.
历史上的惯例将常规电流的方向定义为正电荷移动的方向：即从电池或电源的正极到负极。
In reality, in metal wires, electrons (negative charges) move in the opposite direction: from the negative terminal to the positive terminal. This is called electron flow.
实际上，在金属导线中，电子（负电荷）沿着相反方向移动：从负极到正极。这被称为电子流。
Conventional current: Positive → Negative
Electron flow: Negative → Positive
In IGCSE Physics, when we draw arrows to show current on a circuit diagram, we almost always use conventional current direction. You should be able to recognise both, and the exam may ask you to state the difference.
在 IGCSE 物理中，当我们在电路图上画箭头表示电流时，几乎总是使用常规电流方向。你应该能识别两者，考试可能会要求你说明它们的区别。
6. Measuring Current: The Ammeter | 电流测量：安培表
Current is measured using an ammeter. The ammeter must always be connected in series with the component or section of the circuit where you wish to measure the current.
用安培表测量电流。安培表必须始终与被测元件或电路部分串联连接。
If an ammeter is connected in parallel, its very low resistance will create a short circuit, which can damage the meter and the circuit. The ammeter’s positive terminal (often red) must be connected towards the positive terminal of the cell, i.e. the conventional current enters the positive terminal of the ammeter.
若将安培表并联，其电阻极低，将形成短路，可能损坏电表和电路。安培表的正接线柱（常为红色）必须朝向电池正极连接，即常规电流从安培表的正极流入。
An ideal ammeter has zero resistance so that it does not affect the current it is measuring. Real ammeters have a very small resistance, but we treat them as ideal in exam problems unless told otherwise.
理想的安培表电阻为零，以便不影响所测电流。真实的安培表电阻极小，但在考试题目中，除非另有说明，我们都视其为理想电表。
7. Current in Series Circuits | 串联电路中的电流
In a series circuit, all components are connected one after another, forming a single loop. The current is the same at every point in the circuit.
在串联电路中，所有元件首尾相连，形成一个单一的回路。电路中各点的电流都相同。
Itotal = I₁ = I₂ = I₃ = …
This is because there is only one path for the charge to flow. Charges do not get ‘used up’ or lost; the same number of charges per second pass through every component. An ammeter placed anywhere in the loop will read the same value.
这是因为只有一条电荷流动的通路。电荷不会被“用掉”或消失；每秒通过每个元件的电荷数相同。将安培表置于回路中任何位置，读数都相同。
If you know the current through one resistor in a series circuit, you automatically know the current through all others and through the cell.
如果你知道串联电路中通过一个电阻器的电流，就自动知道了通过其他所有电阻器以及电池的电流。
8. Current in Parallel Circuits | 并联电路中的电流
In a parallel circuit, the path for current splits into two or more branches. The total current leaving the cell splits among the branches and recombines before returning to the cell.
在并联电路中，电流的通路分成两个或多个支路。离开电池的总电流在支路之间分流，返回电池前重新汇合。
Itotal = I₁ + I₂ + I₃ + …
This follows directly from the conservation of charge: the sum of the currents entering a junction equals the sum leaving the junction. The current in each branch is not necessarily equal; it depends on the resistance of that branch.
这直接源自电荷守恒：流入节点的电流之和等于流出节点的电流之和。每条支路中的电流不一定相等；它取决于该支路的电阻。
Adding more branches in parallel provides additional paths for charge, so the total current drawn from the cell increases, while the cell’s voltage remains constant.
并联更多的支路为电荷提供了额外通路，因此从电池获取的总电流增大，而电池电压保持不变。
9. Kirchhoff’s Current Law for Parallel Circuits | 并联电路的基尔霍夫电流定律
The behaviour of current in parallel circuits is a direct application of Kirchhoff’s First Law (the junction rule): the total current entering a junction equals the total current leaving it.
并联电路的电流行为是基尔霍夫第一定律（节点定则）的直接应用：流入节点的总电流等于流出节点的总电流。
In IGCSE exams, you may be asked to predict missing ammeter readings in a parallel network. Simply write an equation based on Itotal = I₁ + I₂. If three branches draw 0.2 A, 0.3 A and 0.5 A, the total current is 1.0 A.
在 IGCSE 考试中，可能要求你预测并联网络中缺失的安培表读数。只需根据 Itotal = I₁ + I₂ 列方程。若三个支路分别流过 0.2 安、0.3 安和 0.5 安，则总电流为 1.0 安。
This law also applies to series-parallel combinations. Identify the junctions, label currents, and apply the conservation rule to solve for unknowns.
该定律同样适用于串并联组合电路。找出节点，标出电流，并运用守恒规则求解未知量。
10. Common Exam Pitfalls and Tips | 常见考试陷阱与建议
Mistake 1: Drawing the ammeter in parallel. Always ensure the ammeter is connected in series. If a circuit diagram shows an ammeter incorrectly placed in parallel, the ammeter could be damaged and the component will not function properly.
错误 1：将安培表并联。务必确保安培表串联。若电路图中安培表错误地并联，电表可能损坏，元件也无法正常工作。
Mistake 2: Forgetting to convert time to seconds. ‘5 minutes’ must become 300 s before using I = Q/t or Q = I × t.
错误 2：忘记将时间换算成秒。在代入 I = Q/t 或 Q = I × t 之前，“5 分钟”必须变为 300 秒。
Mistake 3: Confusing conventional current direction with electron flow. Read the question carefully: if it asks which way electrons move, the answer is from negative to positive. If it asks the direction of current, use the conventional direction (positive to negative) unless specified otherwise.
错误 3：混淆常规电流方向与电子流方向。仔细读题：若问电子如何运动，答案是从负极到正极。若问电流方向，除非特别说明，否则使用常规方向（正极到负极）。
Mistake 4: Assuming parallel branch currents are equal. They only divide equally if the branch resistances are identical. With different resistances, the branch with lower resistance carries more current.
错误 4：假设并联支路电流相等。仅当支路电阻相同时，电流才平分。电阻不同时，电阻较小的支路电流更大。
Use the following table to summarise the key rules for current:
使用下表总结电流的关键规则：
| Property | 性质 | Series Circuit | 串联电路 | Parallel Circuit | 并联电路 |
|---|---|---|
| Current rule | 电流规则 | Same everywhere | 处处相等 | Splits (Itotal = I₁ + I₂) | 分流 (总 = 支路之和) |
| Ammeter connection | 安培表连接 | In series with any point | 串联在任意位置 | In series with the branch or total path | 串联于支路或干路 |
| Effect of adding components | 增加元件的影响 | Current decreases (resistance increases) | 电流减小（电阻增大） | Total current increases (more paths) | 总电流增大（更多通路） |
Practice past paper questions on current calculations and circuit analysis until you can apply the rules quickly and confidently.
练习有关电流计算和电路分析的历年真题，直到你能快速、自信地运用这些规则为止。
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