📚 Electric Current - WJEC IGCSE Physics | 电流 - WJEC IGCSE 物理
e definition of current and the equation Q = It to circuit rules and common exam pitfalls.
电流是电学中的基础概念。理解电流是什么、它在电路中如何表现以及如何正确测量，对于在 WJEC IGCSE 物理中取得好成绩至关重要。本文将涵盖你需要掌握的所有关键考点，从电流的定义和公式 Q = It，到电路规律和常见考试陷阱。
1. What is Electric Current? | 什么是电流？
Electric current is the flow of electric charge. In a metal wire, this charge is carried by free electrons moving through the metal lattice. When a battery or power supply is connected, it provides a potential difference that pushes these charges around the circuit. The greater the flow of charge per second, the larger the current.
电流是电荷的流动。在金属导线中，电荷由在金属晶格中移动的自由电子携带。当接上电池或电源时，电源提供的电势差推动电荷在电路中移动。每秒流过的电荷越多，电流就越大。
2. Current as the Rate of Flow of Charge | 电流是电荷流动的速率
Current is defined as the rate of flow of electric charge. It tells us how much charge passes a given point in a circuit each second. Think of it like water flowing in a pipe: the amount of water passing a point per second represents the water current. In electricity, it is the number of coulombs of charge moving past a point per second.
电流被定义为电荷流动的速率。它告诉我们每秒钟有多少电荷通过电路中的某一点。可以把它想象成管道中的水流：每秒钟流过某一点的水量代表水流。在电学中，则是每秒钟流过某一点的电荷库仑数。
3. The Equation I = Q/t | 公式 I = Q/t
The relationship between current, charge and time is given by the formula:
电流、电荷和时间之间的关系由以下公式给出：
I = Q / t
and rearranged forms:
及其变形公式：
Q = I × t
t = Q / I
where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds (s). Always make sure time is in seconds before substituting into the equation.
其中 I 是电流，单位安培（A）；Q 是电荷，单位库仑（C）；t 是时间，单位秒（s）。代入公式前务必确保时间单位为秒。
4. The Unit Ampere | 单位安培
) or microamperes (μA). It is vital to convert to amperes when using the equation I = Q/t.
1 安培（1 A）的定义是每秒钟流过 1 库仑电荷：1 A = 1 C/s。小电流通常以毫安（mA）或微安（μA）为单位。在使用公式 I = Q/t 时，必须将电流换算成安培。
Common conversions:
常见换算：
1 A = 1000 mA
1 安培 = 1000 毫安
1 mA = 1000 μA
1 毫安 = 1000 微安
So 1 A = 1 000 000 μA
所以 1 A = 1 000 000 μA
5. Measuring Current with an Ammeter | 用安培表测量电流
To measure the current flowing through a component, an ammeter must be connected in series with that component. This means it becomes part of the same single path of the circuit. The positive terminal of the ammeter should be connected towards the positive side of the power supply, and the negative terminal towards the negative side.
要测量流过某个元件的电流，必须将安培表与该元件串联连接。这意味着安培表成为电路中同一条路径的一部分。安培表的正接线柱应接在靠近电源正极一侧，负接线柱接在靠近负极一侧。
A common mistake students make is connecting the ammeter in parallel. This creates a very low‑resistance path, drawing a dangerously large current that can blow the fuse or damage the ammeter.
学生常犯的一个错误是将安培表并联连接。这会形成一条极低电阻的通路，产生非常大的电流，可能烧断保险丝或损坏安培表。
In circuit diagrams, an ammeter is represented by a circle with a letter 'A' inside. The reading on the ammeter tells you the current at that point in the circuit.
在电路图中，安培表用一个内含字母“A”的圆圈表示。安培表的读数表示该点处的电流大小。
6. Current in Series Circuits | 串联电路中的电流
In a series circuit, there is only one path for the charge to flow. This means the current is the same at all points in the circuit. No matter where you place an ammeter in a series loop, it will give the same reading.
在串联电路中，电荷只有一条流动路径。这意味着电路中各点的电流都相同。无论将安培表放在串联回路的哪个位置，读数都相同。
Mathematically:
数学表达式：
I₁ = I₂ = I₃
The components in series share the same current, but the potential difference across each may be different if their resistances are different.
串联电路中的各元件电流相同，但如果各元件电阻不同，每个元件两端的电压可能不同。
7. Current in Parallel Circuits | 并联电路中的电流
In a parallel circuit, the current splits at junctions. The total current leaving the source is equal to the sum of the currents in the separate branches. This is a consequence of the conservation of charge — charge cannot disappear or build up at a junction.
在并联电路中，电流在节点处分流。离开电源的总电流等于各支路电流之和。这是电荷守恒的结果——电荷不会在节点处消失或积累。
The relationship is:
关系式为：
Iₜ = I₁ + I₂ + I₃
where Iₜ is the total current from the source, and I₁, I₂, I₃ are the branch currents. The current in each branch depends on the resistance of that branch; branches with lower resistance carry a larger share of the current.
其中 Iₜ 是电源提供的总电流，I₁、I₂、I₃ 是各支路电流。每条支路的电流取决于该支路的电阻；电阻越小的支路分得的电流越大。
8. Conventional Current vs Electron Flow | 常规电流与电子流
Historically, current was defined as the flow of positive charge, from the positive terminal of a battery to the negative terminal. This is called conventional current. In reality, in a metal wire, it is negatively charged electrons that move, and they flow from negative to positive. This is electron flow.
历史上，电流被定义为正电荷的流动方向，即从电池的正极流向负极。这被称为常规电流。实际上，在金属导线中移动的是带负电的电子，它们是从负极流向正极。这被称为电子流。
For all circuit calculations and circuit diagrams, WJEC uses conventional current direction (positive to negative). However, you must be aware that the actual charge carriers in metals are electrons moving in the opposite direction. An exam question might ask you to compare the two directions.
在所有电路计算和电路图中，WJEC 采用常规电流方向（从正到负）。但是，你必须知道金属中实际的载流子是电子，移动方向相反。考题可能会要求你比较这两种方向。
9. Charge Conservation and the Junction Rule | 电荷守恒与节点定律
The rule for parallel circuits is a direct application of the conservation of charge. The total charge entering a junction per second must equal the total charge leaving it per second. This is often called Kirchhoff's first law unction rule. At any junction: the sum of currents entering = the sum of currents leaving.
并联电路的规律直接应用了电荷守恒定律。每秒钟进入节点的总电荷必须等于离开节点的总电荷。这通常被称为基尔霍夫第一定律或节点电流定律。在任何节点：流入的电流之和 = 流出的电流之和。
This principle helps you analyse more complex circuits. If you know some branch currents, you can always find the missing one by simple addition or subtraction.
这一原理能帮助你分析更复杂的电路。如果已知某些支路的电流，你总能通过简单的加减运算求出未知的支路电流。
10. Worked Examples | 典型例题
Example 1: A charge of 60 C passes a point in a circuit in 12 seconds. Calculate the current.
例题 1： 在 12 秒内有 60 C 的电荷通过电路中某一点。计算电流。
Solution / 解答：
I = Q / t = 60 C / 12 s = 5 A
Example 2: A current of 0.5 A flows for 2 minutes. How much charge passes?
例题 2： 0.5 A 的电流持续流动 2 分钟。有多少电荷通过？
Convert 2 minutes to seconds: 2 × 60 = 120 s / 将 2 分钟换算为秒：2 × 60 = 120 s
Q = I × t = 0.5 × 120 = 60 C
Example 3: In a parallel circuit, the total current from the battery is 4 A. One branch carries 1.5 A. What is the current in the other branch?
例题 3： 在一个并联电路中，电池提供的总电流为 4 A。一条支路的电流为 1.5 A。另一支路的电流是多少？
Iₜ = I₁ + I₂ ⇒ 4 A = 1.5 A + I₂ ⇒ I₂ = 2.5 A
Example 4: An ammeter reads 250 mA. Express this current in amperes and use it to find the charge flowing in 10 s.
例题 4： 安培表读数为 250 mA。将电流换算为安培，并求 10 秒内流过的电荷。
I = 250 / 1000 = 0.25 A / I = 250 / 1000 = 0.25 A
Q = 0.25 × 10 = 2.5 C
11. Common Mistakes and Exam Tips | 常见错误与考试技巧
1. Ammeter in parallel: Always double-check that the ammeter is drawn in series, not parallel, with the component. In practical questions, connecting an ammeter in parallel can lead to damaged equipment.
1. 安培表并联连接： 务必反复检查安培表是否与元件串联而非并联绘制。在实验题中，将安培表并联连接可能导致设备损坏。
2. Unit conversions: Many students forget to convert minutes to seconds, or milliamperes to amperes. Underline the unit of time and do the conversion first.
2. 单位换算： 许多学生忘记将分钟换算为秒，或毫安换算为安培。先圈出时间单位并首先进行换算。
3. Conventional vs electron: When asked for the direction of electric current, give the conventional direction (positive to negative) unless the question specifically asks for electron flow.
3. 常规电流与电子流： 当被问及电流方向时，除非题目明确要求电子流方向，否则请回答常规电流方向（从正到负）。
4. Current in series: Even if the components have different resistances, the current through them is identical. Do not assume that a component with higher resistance has a smaller current in a series circuit — that applies to voltage, not current.
4. 串联电路中的电流： 即使各元件电阻不同，流过它们的电流也完全相同。不要误以为在串联电路中电阻大的元件电流就小——那是针对电压而言，而非电流。
5. Junction sums: in = total out. If you are given most of the currents, you can easily find an unknown one.
5. 节点电流求和： 始终运用电荷守恒：总流入 = 总流出。如果已知大多数电流，就能轻松求出未知电流。
6. Significant figures and rounding: WJEC generally expects final answers to be given to 2 or 3 significant figures. State the formula first, then substitute numbers, then give the answer with a unit.
6. 有效数字与保留小数： WJEC 通常要求最终答案保留 2 或 3 位有效数字。先写出公式，再代入数值，最后给出带单位的答案。
7. Equation practice: d to rearrange the equation under timed conditions, so practise until it is second nature.
7. 公式练习： 熟练掌握 I = Q/t 的三种形式至关重要。你可能需要在计时条件下对公式进行变形，所以要练到驾轻就熟。
By mastering these core ideas, you will be able to tackle any current-related question in the WJEC IGCSE Physics examination with confidence.
掌握这些核心概念，你就能自信地应对 WJEC IGCSE 物理考试中任何与电流相关的题目。
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