📚 Electric Current | 电流
Electric current is one of the most fundamental concepts in physics. It describes the rate at which electric charge flows past a point in a circuit. Understanding current is essential for grasping how circuits work, from simple lamps to complex electronic devices. In the A-Level CIE Physics syllabus, current is linked to charge carriers, potential difference, resistance, power, and circuit laws.
电流是物理学中最基本的概念之一，描述电荷流过电路中某一点的速率。理解电流是掌握电路工作原理的基础——从简单的灯泡到复杂的电子设备。在 A-Level CIE 物理课程中，电流与载流子、电势差、电阻、功率以及电路定律紧密相连。
1. What is Electric Current? | 电流的定义
Electric current (I onal area. The SI unit of current is the ampere (A), where 1 A = 1 coulomb per second (C s−1). If an amount of charge ΔQ passes a point in a time interval Δt, the average current is given by:
电流（I）定义为通过某一横截面的净电荷流动速率。电流的国际单位是安培（A），1 A = 1 库仑每秒（C s−1）。如果在时间间隔 Δt 内有电荷量 ΔQ 通过某一点，则平均电流为：
I = ΔQ / Δt
For a steady direct current (d.c.), the current is constant, so I = Q / t. The direction of conventional current is taken as the direction in which positive charge would flow. In metallic wires, the charge carriers are free electrons, which move in the opposite direction to the conventional current.
对于稳定的直流电（d.c.），电流是恒定的，因此 I = Q / t。约定电流的方向取为正电荷流动的方向。在金属导线中，电荷载流子是自由电子，它们沿与约定电流相反的方向运动。
2. Charge Carriers and Conventional Current | 载流子与约定电流方向
In different materials, the moving charges that constitute current can be electrons, positive ions, negative ions, or even 'holes' in semiconductors. The table below summarises the charge carriers in common conductors:
在不同的材料中，构成电流的移动电荷可以是电子、正离子、负离子，甚至半导体中的“空穴”。下表总结了常见导体中的电荷载流子：
| Material | Charge Carriers |
|---|---|
| Metals (Cu, Ag) | Free electrons (negative) |
| Electrolytes | Positive and negative ions |
| Semiconductors | Electrons and holes |
| Plasmas | Electrons and positive ions |
Conventional current always flows from the positive terminal to the negative terminal of a battery, whereas electrons drift from negative to positive. In circuit diagrams and when applying Kirchhoff's laws, we use conventional current direction.
约定电流总是从电池的正极流向负极，而电子是从负极漂移到正极。在电路图和基尔霍夫定律中，我们使用约定电流方向。
3. Measuring Current: Ammeter and Galvanometer | 电流的测量：电流计与检流计
Current is measured using an ammeter, which must be connected in series with the component of interest so that the same charge per second flows through both. An ideal ammeter has zero resistance to avoid changing the circuit's behaviour. A moving-coil galvanometer can detect tiny currents and be converted into an ammeter by adding a low-resistance shunt in parallel.
电流用电流表测量，电流表必须与待测元件串联，使每秒流过的电荷量相同。理想电流表的内阻为零，以免改变电路特性。动圈式检流计可以检测微小电流，通过并联一个低电阻分流器可将其改装成电流表。
When measuring current, always break the circuit and insert the ammeter in the path. Digital multimeters often combine ammeter, voltmeter and ohmmeter functions. In CIE practicals, students learn to position an ammeter correctly and to interpret its readings.
测量电流时，务必断开电路，将电流表接入回路。数字万用表通常集成了电流表、电压表和欧姆表的功能。在 CIE 实验考试中，学生要学会正确放置电流表并读取读数。
4. Current in a Metal: Drift Velocity and I = nAvq | 金属中的电流：漂移速度
In a metal, free electrons move randomly at high speeds, but when an electric field is applied, they acquire a net slow drift in the direction opposite to the field. The current I can be expressed in terms of the number density n of charge carriers, the cross-sectional area A of the conductor, the drift velocity v, and the charge q of each carrier:
在金属中，自由电子以很高的速度做无规则运动，但当施加电场时，它们会获得一个与电场方向相反的缓慢净漂移。电流 I 可以用载流
Published by TutorHao | A-Level Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图