A-Level Physics: Current Concepts and Measurement | A-Level 物理：电流概念与测量

📚 A-Level Physics: Current Concepts and Measurement | A-Level 物理：电流概念与测量

Electric current is one of the most fundamental ideas in physics. It explains how energy is transferred through circuits and how electrical devices operate. In this A-Level revision guide, we will define current, explore its microscopic origin, and show how it is measured correctly in the laboratory and in exam questions.

电流是物理学中最基本的概念之一。它解释能量如何通过电路传递、电器如何工作。在这篇 A-Level 复习指南中，我们将定义电流、探讨其微观本质，并说明如何在实验和考试问题中正确测量电流。

1. What Is Electric Current? | 什么是电流？

Electric current is the rate of flow of electric charge past a point in a circuit. If a charge ΔQ passes through a cross-section in a time Δt, the average current is given by the equation below.

电流是电荷流过电路中某一点的速率。如果在时间 Δt 内通过某一横截面的电荷为 ΔQ，则平均电流由以下公式给出。

I = ΔQ / Δt

For a current that varies with time, we use the instantaneous rate of charge flow.

如果电流随时间变化，我们使用电荷流动的瞬时变化率。

I = dQ / dt

The SI unit of current is the ampere (A). One ampere is equal to one coulomb per second.

电流的国际单位是安培（A）。1 安培等于每秒通过 1 库仑的电荷。

1 A = 1 C s⁻¹

2. Charge Carriers | 电荷载体

Current is not a flow of current itself; it is a flow of charge carriers. The type of charge carrier depends on the material or medium.

电流并不是“电流本身”在流动，而是电荷载体在流动。电荷载体的类型取决于材料或介质。

Medium	Charge Carriers
Metal wire	Free electrons
Electrolyte / solution	Positive and negative ions
Semiconductor	Electrons and holes
Ionised gas / plasma	Electrons and positive ions

Charge is quantised: every charge is an integer multiple of the elementary charge e, where e = 1.6 × 10⁻¹⁹ C. A charge Q delivered by n individual charge carriers each of charge e is therefore Q = ne.

电荷是量子化的：任何电荷都是元电荷 e 的整数倍，其中 e = 1.6 × 10⁻¹⁹ C。因此 n 个电荷量各为 e 的电荷载体携带的总电荷为 Q = ne。

3. Conventional Current vs Electron Flow | 常规电流方向与电子流动方向

By convention, the direction of electric current is the direction in which positive charge would move. This is called conventional current. In most circuits, conventional current flows from the positive terminal of a battery, through the external circuit, and back to the negative terminal.

按照惯例，电流的方向被定义为正电荷移动的方向，这称为常规电流方向。在大多数电路中，常规电流从电池正极出发，流经外部电路，再回到负极。

However, in a metal wire the actual moving charge carriers are electrons, which are negatively charged. Therefore electrons flow in the opposite direction to conventional current.

然而，在金属导线中实际移动的电荷载体是带负电的电子。因此，电子流动方向与常规电流方向相反。

Conventional current: positive to negative
Electron flow: negative to positive

This distinction is important when describing current direction in circuit diagrams and when using the left-hand rule for forces on current-carrying conductors.

在电路图中描述电流方向，以及用左手定则判断通电导线受力方向时，这一区别非常重要。

4. Current and Drift Velocity | 电流与漂移速度

Inside a metal conductor, free electrons move randomly at high speeds due to thermal energy. When an electric field is applied, the electrons also acquire a slow net movement along the wire. This slow average velocity is called the drift velocity, v.

在金属导体内部，自由电子因热运动而以高速做无规则运动。当施加电场时，电子还会沿导线获得一个缓慢的定向运动。这个缓慢的平均速度称为漂移速度 v。

Typical drift velocities in a normal wire are only a fraction of a millimetre per second, even though the electric field itself propagates through the circuit almost at the speed of light.

即使电场本身几乎以光速在电路中传播，普通导线中的典型漂移速度也只有每秒不到一毫米。

Drift velocity is very small because electrons constantly collide with atoms in the metal. These collisions slow the electrons and cause electrical resistance.

漂移速度非常小，因为电子不断与金属中的原子碰撞。这些碰撞会阻碍电子的运动，并导致电阻。

5. The Microscopic Current Equation | 电流的微观表达式

For a wire of cross-sectional area A, containing n free electrons per unit volume, and with electron charge e, the current is related to the drift velocity v by the equation below.

对于横截面积为 A 的导线，如果单位体积内有 n 个自由电子，每个电子电荷为 e，则电流与漂移速度 v 的关系如下。

I = n A v e

This is one of the most important equations in A-Level electric circuits. It shows that increasing the number density of charge carriers, the cross-sectional area, or the drift velocity will increase the current.

这是 A-Level 电路中最重要公式之一。它表明：电荷载体数密度、横截面积或漂移速度增大，都会使电流增大。

For a general charge carrier with charge q, the equation is written as I = nAvq.

对于电荷量为 q 的一般电荷载体，方程写为 I = nAvq。

Symbol	Meaning	SI unit
I	electric current	A
n	number density of charge carriers	m⁻³
A	cross-sectional area	m²
v	drift velocity	m s⁻¹
e	elementary charge	C

6. Measuring Current: The Ammeter | 电流的测量：安培计

Electric current is measured with an ammeter. To measure the current passing through a component, the ammeter must be connected in series with that component.

电流用安培计测量。要测量通过某元件的电流，安培计必须与该元件串联。

This is because the same current is expected to pass through both the component and the ammeter. If an ammeter were connected in parallel, it would provide an extra conducting path and the current in the circuit would change.

这是因为我们期望同一电流通过该元件和安培计。如果安培计并联连接，就会提供一条额外的通路，从而改变电路中的电流。

Ammeter connection: always in series

In the school laboratory, digital multimeters are often used as ammeters. They should be set to the appropriate “A” range, and the circuit should be checked before closing the switch.

在学校实验室中，数字万用表常被用作电流表。使用时应选择合适的“A”量程，并在闭合开关前检查电路。

7. Ideal Ammeter and Practical Considerations | 理想电流表与实际注意事项

For accurate measurements, an ammeter should not significantly change the current it is measuring. Therefore the resistance of an ammeter should be as small as possible.

为了测量准确，电流表不应该显著改变被测电流。因此，电流表的电阻应尽可能小。

An ideal ammeter has zero resistance. In exam questions, unless told otherwise, you may assume that the ammeter has negligible resistance.

理想电流表电阻为零。在考试问题中，除非另有说明，你可以假设电流表的电阻可以忽略不计。

If an ammeter has high resistance, it reduces the circuit current and gives a false reading.

如果电流表电阻很大，它会减小电路电流，给出不准确的读数。
For DC measurements, observe the correct polarity: the positive terminal of the ammeter should be connected to the side at higher potential.

直流测量时要注意极性：电流表正接线柱应接在电势较高的一侧。
Never connect an ammeter directly across a battery without a resistor unless it is designed for very high current; it could be damaged.

除非电流表专为极高电流设计，否则不要把电流表直接跨接在电池两端；否则可能损坏仪表。

8. Direct Current and Alternating Current | 直流电与交流电

A cell or battery provides direct current (DC), in which charge flows in one direction only. The current remains constant if the supply is steady.

电池提供的是直流电（DC），电荷只沿一个方向流动。当电源稳定时，电流大小保持不变。

An alternating current (AC) regularly reverses direction. In the UK and China, the mains supply is AC with a frequency of 50 Hz, meaning it reverses direction 50 times per second.

交流电（AC）会周期性地改变方向。在英国和中国，市电是 50 Hz 的交流电，也就是说电流每秒改变方向 50 次。

In AC, the charge carriers oscillate back and forth rather than moving steadily around the circuit. Alternating current can be transformed to different voltages using transformers, which is why it is used in national power distribution.

在交流电中，电荷载体来回振动，而不是沿电路稳定移动。交流电可以通过变压器变换为不同的电压，因此被用于国家电力分配系统。

9. Current in Series and Parallel Circuits | 串并联电路中的电流

The conservation of charge leads directly to important rules about current in circuits.

电荷守恒直接给出了电路中电流的重要规则。

When components are connected in series, the same current flows through every component. An ammeter placed anywhere on the same single path records the same reading.

当元件串联时，通过每个元件的电流相同。在单一条通路上任何位置放置电流表，读数都相同。

When branches meet at a junction, current is conserved. The total current entering a junction equals the total current leaving the junction. This is Kirchhoff’s first law.

当支路在结点处汇合时，电流守恒。流入结点的总电流等于流出结点的总电流。这就是基尔霍夫第一定律。

I_in = I_out

This law is often tested with questions that ask you to calculate an unknown current from given ammeter readings.

考试中常会要求你根据给定的电流表读数计算未知电流，这时就用到基尔霍夫第一定律。

10. Worked Example: Charge and Drift Velocity | 例题：电荷量与漂移速度

Question 1: A current of 2.4 A flows through a wire for 5 minutes. Calculate the total charge that passes through the wire.

题目 1：一根导线中通过 2.4 A 的电流，持续 5 分钟。求通过导线的总电荷量。

Use Q = I × t. Remember that time must be converted to seconds.

使用 Q = I × t。注意时间要换算为秒。

Q = 2.4 × 300 = 720 C

The charge passing through the wire is 720 coulombs.

通过导线的电荷量为 720 库仑。

Question 2: A copper wire has cross-sectional area 1.5 × 10⁻⁶ m² and free electron density n = 8.5 × 10²⁸ m⁻³. The current is 0.20 A. Calculate the drift velocity of the electrons, using e = 1.6 × 10⁻¹⁹ C.

题目 2：铜导线横截面积为 1.5 × 10⁻⁶ m²，自由电子数密度 n = 8.5 × 10²⁸ m⁻³。电流为 0.20 A。取 e = 1.6 × 10⁻¹⁹ C，求电子的漂移速度。

Rearrange I = nAve to make v the subject.

将 I = nAve 变形，得到 v 的表达式。

v = I / (n A e)

Keep the calculation clear by evaluating the denominator first.

先计算分母，可以使运算更清晰。

n A e = 8.5 × 10²⁸ × 1.5 × 10⁻⁶ × 1.6 × 10⁻¹⁹ = 2.04 × 10⁴

v = 0.20 / (2.04 × 10⁴) = 9.8 × 10⁻⁶ m s⁻¹

The drift velocity is approximately 9.8 × 10⁻⁶ m s⁻¹, which is about one hundredth of a millimetre per second.

漂移速度约为 9.8 × 10⁻⁶ m s⁻¹，也就是大约每秒百分之几毫米。

11. Common Mistakes and Exam Tips | 常见错误与考试要点

Even after studying the theory, many students lose marks on current questions because of small but avoidable errors.

即使学完了理论，很多学生仍然会因细小但可以避免的错误而在电流题目中失分。

Do not use seconds and minutes in the same calculation without conversion. Always convert time to seconds when using Q = I × t.

不要在同一计算中混用秒和分钟。使用 Q = I × t 时，一定要把时间换算为秒。
Do not connect an ammeter in parallel. It must be in series with the component.

不要把电流表并联接入电路。它必须与元件串联。
Do not say that current is “used up” by a bulb or resistor. Charge carriers are conserved.

不要说电流被灯泡或电阻“消耗掉了”。电荷载体是守恒的。
Remember that conventional current is from positive to negative, even though electrons move from negative to positive.

记住：常规电流方向是从正到负，即使实际电子是从负到正移动。
When using I = nAve, keep units consistent and quote the correct unit for drift velocity.

使用 I = nAve 时，保持单位一致，并写出漂移速度的正确单位。

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A-Level Physics: Current Concepts and Measurement | A-Level 物理：电流概念与测量