📚 Electric Current and Charge: Q = It | 电流与电荷：Q = It
ric current is essential for mastering IGCSE circuit theory. Current is not simply a vague ‘flow of electricity’ – matters in both series and parallel circuits. You will learn how to link ammeter readings to the movement of charge carriers, and you will be able to solve typical Edexcel IGCSE Science problems with confidence.
理解电流是掌握 IGCSE 电路理论的基础。电流并不是一种模糊的“电的流动”——它是电荷通过某一点的精确速率。本文将引导你学习 Q = It 这个公式，如何运用它，以及它在串联和并联电路中的重要性。你将学会如何将电流表读数与载流子的运动联系起来，并能够自信地解答典型的 Edexcel IGCSE 科学题目。
1. What is Electric Current? | 什么是电流？
Electric current is defined as the rate of flow of electric charge. In a metal conductor, the moving charges are free electrons that drift slowly along the wire when a potential difference is applied. The conventional direction of current is taken as the direction in which positive charges would flow, which is opposite to the actual electron flow. This convention is used in all circuit diagrams and calculations.
电流被定义为电荷流动的速率。在金属导体中，移动的电荷是自由电子，当施加电势差时，它们会沿导线缓慢漂移。电流的常规方向被规定为正电荷流动的方向，这与电子的实际流动方向相反。所有的电路图和计算都采用这一惯例。
2. The Equation Q = It | 公式 Q = It
The relationship between charge, current and time is given by the simple equation:
Q = I × t
where Q is the electric charge in coulombs (C), I is the current in amperes (A), and t is the time in seconds (s). This equation tells us that if a steady current of 1 A flows for 1 second, exactly 1 C of charge passes any cross‑section of the circuit.
电荷、电流和时间之间的关系由这个简单的公式表示：
Q = I × t
其中 Q 是以库仑（C）为单位的电荷量，I 是以安培（A）为单位的电流，t 是以秒（s）为单位的时间。这个公式告诉我们，如果 1 A 的恒定电流流过 1 秒，那么就有恰好 1 C 的电荷通过电路的任何横截面。
3. Units in Detail | 详细单位
Charge is measured in coulombs (C). One coulomb is a very large amount of charge – t. A common mistake is to substitute minutes directly, leading to a wrong value of charge.
电荷的单位是库仑（C）。1 库仑是一个非常大的电荷量——单个电子所带的电荷量约为 1.60 × 10⁻¹⁹ C。电流的单位是安培（A），常简称为安。时间必须始终以秒为单位；如果题目给出的时间是分钟或小时，必须先转换为秒再使用 Q = It。一个常见的错误是直接代入分钟数，导致得到的电荷值出错。
4. Measuring Current with an Ammeter | 用电流表测量电流
An ammeter is used to measure the current flowing through a component. It must always be connected in series with the component, so that all the charge flowing through the component also flows through the meter. Ammeters have a very low resistance, so they do not significantly affect the circuit. In the IGCSE exam, you may be asked to sketch a circuit with an ammeter correctly placed.
电流表用来测量流过某个元件的电流。它必须始终与该元件串联，这样所有流过该元件的电荷也会流经电流表。电流表的电阻非常小，因此不会对电路产生显著影响。在 IGCSE 考试中，你可能会被要求画出正确放置电流表的电路图。
5. Charge Conservation and Current in Series | 电荷守恒与串联电流
Electric charge is conserved; it cannot be created or destroyed in a circuit. This means that in a series loop, the current is the same at every point. Whatever charge enters one end of a series section per second must leave the other end per second. Therefore, two ammeters placed anywhere in the same series loop will show identical readings.
电荷是守恒的；它在电路中既不能被创造也不能被消灭。这意味着在串联回路中，各点的电流都相同。每秒进入串联段一端的电荷必定会从另一端流出。因此，放置在同一个串联回路中任意位置的两个电流表会显示相同的读数。
6. Current in Parallel Circuits | 并联电路中的电流
In a parallel circuit, the total current from the source splits at a junction and then recombines. The sum of the currents in the parallel branches equals the total current entering the junction. This is another consequence of charge conservation. If the source provides 3 A and one branch carries 1 A, the other branch must carry 2 A.
在并联电路中，来自电源的总电流会在分支点分流，然后再合并。各并联支路电流之和等于流入分支点的总电流。这是电荷守恒的另一个推论。如果电源提供 3 A 电流，而一条支路的电流为 1 A，那么另一条支路必然流过 2 A。
7. Rearranging Q = It | 公式 Q = It 的变形
The equation can be rearranged to make I or t the subject:
I = Q ÷ t
t = Q ÷ I
Use these forms when you know the charge and time and need to find the current, or when you are given the charge and current and need to find how long a process lasts. Always check that the units are consistent before calculating.
该公式可以变形，以 I 或 t 为主语：
I = Q ÷ t
t = Q ÷ I
当你知道电荷和时间需要求电流时，或者给出电荷和电流需要求持续时间时，就使用这些形式。计算前务必确认单位一致。
8. Worked Example 1: Simple Battery and Bulb | 例题1：简单的电池和灯泡
A torch bulb carries a steady current of 0.4 A. Calculate the charge that passes through the bulb in 3 minutes.
Q = I × t = 0.4 × 180 = 72 C. So 72 coulombs of charge flow through the bulb in that time.
一个手电筒灯泡中流过 0.4 A 的恒定电流。计算在 3 分钟内通过灯泡的电荷量。
先转换时间：3 分钟 = 3 × 60 = 180 s。然后应用 Q = I × t = 0.4 × 180 = 72 C。因此，在这段时间内有 72 库仑的电荷流经灯泡。
9. Worked Example 2: Finding Time from Charge and Current | 例题2：由电荷和电流求时间
A smartphone charger delivers a current of 1.5 A. How long must it run to transfer 5400 C of charge to the battery?
Use t = Q ÷ I = 5400 ÷ 1.5 = 3600 s. This is 3600 ÷ 60 = 60 minutes, or exactly 1 hour.
一个智能手机充电器输出 1.5 A 的电流。它需要运行多长时间才能向电池输送 5400 C 的电荷？
使用 t = Q ÷ I = 5400 ÷ 1.5 = 3600 s。这相当于 3600 ÷ 60 = 60 分钟，恰好是 1 小时。
10. Linking Current to the Number of Electrons | 电流与电子数的关联
scopic world. Since the charge on one electron is −1.60 × 10⁻¹⁹ C, the number of electrons n in a given charge Q is:
n = Q ÷ e
where e ays directly examined at IGCSE level, but it deepens your understanding of what current means.
库仑是一个宏观单位，但它可以与微观世界联系起来。由于一个电子的电荷为 −1.60 × 10⁻¹⁹ C，给定电荷量 Q 中的电子数 n 为：
n = Q ÷ e
其中 e = 1.60 × 10⁻¹⁹ C。例如，72 C 的电荷中包含 72 ÷ (1.60 × 10⁻¹⁹) = 4.5 × 10²⁰ 个电子。这个知识点在 IGCSE 阶段不一定会直接考查，但它能加深你对电流含义的理解。
11. Common Misconceptions | 常见误区
Current is ‘used up’ by components: False. Current is the same before and after a component in series. Energy is transferred, not charge or current.
元件会“消耗”电流：错误。在串联电路中，元件前后的电流是相同的。被转移的是能量，而不是电荷或电流。
Electrons travel at near the speed of light: False. The drift velocity of electrons is very small (fractions of a millimetre per second), but the electrical signal travels close to light speed because the electric field is established almost instantly throughout the wire.
电子以接近光速运动：错误。电子的漂移速度非常小（每秒不足一毫米），但电信号几乎以光速传播，因为电场几乎瞬间在整个导线中建立起来。
Ammeters can be placed anywhere, even in parallel: False. An ammeter in parallel will draw a very large current (because of its low resistance) and may be damaged.
电流表可以放在任何位置，甚至是并联：错误。并联的电流表会因电阻很小而流过非常大的电流，可能会损坏。
12. Summary and Key Points | 总结与要点
| Main equation | Q = I × t |
| Units | Charge (C), current (A), time (s) |
| Series current rule | Current is the same everywhere in a series loop |
| Parallel current rule | Total current = sum of branch currents |
| Charge conservation | Charge cannot be created or destroyed in a circuit |
掌握 Q = It 是解决 IGCSE 电路中定量问题的基础。始终确保以秒为单位的时间，记住串联和并联电路中电流的规律。通过练习转换单位和公式变形，你就能在考试中快速准确地解答这类问题。
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