IGCSE Physics: Electric Current Explained | IGCSE 物理：电流考点精讲

📚 IGCSE Physics: Electric Current Explained | IGCSE 物理：电流考点精讲

Electric current is one of the most fundamental concepts in IGCSE Physics. It describes the flow of electric charge around a circuit and forms the basis for understanding how all electrical devices work. In this revision guide, we will break down everything you need to know about current — from its definition and formula to how it behaves in series and parallel circuits.

电流是 IGCSE 物理中最基础的概念之一。它描述了电荷在电路中的流动，是理解所有电器工作原理的基础。在本复习指南中，我们将详细讲解关于电流你需要掌握的全部内容——从定义和公式，到电流在串联和并联电路中的表现。

1. What is Electric Current? | 什么是电流？

Electric current is the rate of flow of electric charge. In a metal wire, it consists of a flow of negatively charged electrons moving from the negative terminal to the positive terminal of a power supply. In other conductors, such as electrolytes, current may also be carried by positive ions moving in the opposite direction.

电流是电荷流动的速率。在金属导线中，电流由带负电的电子从电源的负极流向正极而形成。在其他导体（如电解质）中，电流也可能由朝相反方向移动的正离子承载。

Current is a scalar quantity, but it is often associated with a direction when we describe it in a circuit. It is measured using an ammeter and its SI unit is the ampere (A).

电流是一个标量，但在电路中描述它时通常会指明方向。电流使用电流表测量，其国际单位是安培 (A)。

2. The Unit of Current: Ampere | 电流的单位：安培

The SI unit of electric current is the ampere, often shortened to ‘amp’ and symbolised by the letter A. One ampere is defined as one coulomb of charge passing a point in a circuit per second.

电流的国际单位是安培，常简称为“安”，用字母 A 表示。1 安培的定义是每秒有 1 库仑的电荷通过电路中的某一点。

Typical current values in common devices: a small LED may draw about 0.02 A (20 mA), a light bulb around 0.3–0.5 A, and an electric heater can draw 10 A or more. In IGCSE problems, you will often work with milliamperes (mA), where 1 A = 1000 mA.

常见电器中的典型电流值：一个小型 LED 约流过 0.02 A (20 mA)，一个灯泡约 0.3–0.5 A，电暖器则可能流过 10 A 或更大。在 IGCSE 题目中，你会经常用到毫安 (mA)，1 A = 1000 mA。

3. Formula: I = Q / t | 公式：I = Q / t

The relationship between current (I), charge (Q) and time (t) is given by the equation:

电流 (I)、电荷量 (Q) 和时间 (t) 之间的关系由以下公式给出：

I = Q / t

where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds (s). This equation can be rearranged to Q = I × t to calculate the total charge transferred.

其中 I 为电流，单位安培 (A)；Q 为电荷量，单位库仑 (C)；t 为时间，单位秒 (s)。此公式可变形为 Q = I × t 以计算转移的总电荷量。

4. Charge (Q) and its Unit: Coulomb | 电荷量 (Q) 及其单位：库仑

Electric charge, denoted by Q, is a fundamental property of matter carried by protons (positive) and electrons (negative). In circuit calculations, one electron carries a charge of approximately 1.6 × 10⁻¹⁹ C, so a large number of electrons is needed to make up 1 C of charge.

电荷量用符号 Q 表示，是物质的一种基本属性，由质子（带正电）和电子（带负电）携带。在电路计算中，一个电子所带的电荷量约为 1.6 × 10⁻¹⁹ C，因此需要大量电子才能积累 1 C 的电荷。

The unit of charge is the coulomb (C). If a current of 1 A flows for 1 s, the charge transferred is exactly 1 C. In IGCSE questions, you may need to convert time from minutes to seconds before using the formula.

电荷的单位是库仑 (C)。如果 1 A 的电流流动 1 s，转移的电荷量就是 1 C。在 IGCSE 题目中，你可能需要先将时间从分钟转换为秒再代入公式。

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

Conventional current is defined as the direction in which positive charges would flow — from the positive terminal to the negative terminal of a battery. This convention was established before the discovery of the electron and is still used in circuit diagrams and analysis.

常规电流的方向被定义为正电荷流动的方向——从电池正极流向负极。这一约定在电子被发现之前就已确立，至今仍用于电路图和分析中。

In reality, in a metal wire, it is negatively charged electrons that move, and they flow from the negative terminal to the positive terminal. This is called electron flow. Students must be careful: when marking arrow directions on diagrams, follow the conventional current direction unless told otherwise.

实际上，在金属导线中移动的是带负电的电子，它们从负极流向正极。这称为电子流动。学生必须注意：在电路图中标记箭头方向时，除非有特别说明，否则应遵循常规电流方向。

6. Measuring Current with an Ammeter | 用电流表测量电流

Current is measured using an ammeter, which must be connected in series with the component you wish to measure. This means the ammeter becomes part of the single loop of the circuit, so all the current flowing through the component also passes through the ammeter.

电流用电流表测量，电流表必须与待测元件串联。这意味着电流表成为电路单回路的一部分，因此流过该元件的所有电流也都流过电流表。

A common mistake is connecting an ammeter in parallel. This can create a very low-resistance path, causing a large current to flow (short circuit) and potentially damaging the meter. In circuit diagrams, the symbol for an ammeter is a circle with an ‘A’ inside.

一个常见错误是将电流表并联连接。这会造成一条电阻极低的路径，导致大电流流过（短路），可能损坏电表。在电路图中，电流表的符号是一个内部写有“A”的圆圈。

7. Current in Series Circuits | 串联电路中的电流

In a series circuit, there is only one path for the current to take. Therefore, the current is the same at all points in the circuit. If you place an ammeter before a resistor, between resistors, or after a resistor, you will read exactly the same value.

在串联电路中，电流只有一条通路。因此，电路中各点的电流都相同。无论将电流表放在电阻器之前、电阻器之间还是电阻器之后，读数都完全相同。

Mathematically: I_total = I₁ = I₂ = I₃ = … This property is independent of the resistance of individual components; the current remains constant along the entire series loop.

数学表达式为：I_total = I₁ = I₂ = I₃ = … 这一性质与各个元件的电阻大小无关；整个串联回路的电流保持恒定。

8. 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 flows through the parallel branches. The sum of the currents in the separate branches equals the total current leaving the source.

在并联电路中，来自电源的总电流在节点处分流，分别流过各并联支路。各支路电流的总和等于离开电源的总电流。

The rule is: I_total = I₁ + I₂ + I₃ + … The current in each branch depends on the resistance of that branch: the branch with lower resistance carries a larger share of the total current. At the recombining junction, the branch currents add back up to the original total current.

遵循环则为：I_total = I₁ + I₂ + I₃ + … 每条支路的电流取决于该支路的电阻：电阻较小的支路承载较大的总电流份额。在汇合节点处，支路电流相加回到最初的总电流。

9. Factors Affecting Current: Resistance and Voltage | 影响电流的因素：电阻与电压

According to Ohm’s Law, the current flowing through a conductor is directly proportional to the voltage (potential difference) across it, provided temperature and other physical conditions remain constant. The relationship is I = V / R.

根据欧姆定律，在温度和其它物理条件保持不变的情况下，流过导体的电流与其两端电压（电势差）成正比。关系式为 I = V / R。

Therefore, for a given resistance, increasing the voltage increases the current. For a fixed voltage, a larger resistance reduces the current. Factors that affect resistance include the length, cross-sectional area, material (resistivity) and temperature of the conductor.

因此，对于给定的电阻，增大电压会增大电流。对于固定的电压，电阻越大会使电流越小。影响电阻的因素包括导体的长度、横截面积、材料（电阻率）和温度。

10. Ohm’s Law and Current | 欧姆定律与电流

Ohm’s Law is often expressed as V = I × R, where V is potential difference in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). This law applies to ohmic conductors, such as a metal wire at constant temperature, for which a graph of V against I is a straight line through the origin.

欧姆定律常表示为 V = I × R，其中 V 为电势差，单位伏特 (V)；I 为电流，单位安培 (A)；R 为电阻，单位欧姆 (Ω)。该定律适用于欧姆导体，例如恒温下的金属导线，其 V-I 图像是一条过原点的直线。

For non-ohmic components, such as diodes or filament lamps, the current is not directly proportional to voltage. The resistance changes with temperature or due to the component’s special properties.

对于非欧姆元件，例如二极管或白炽灯，电流与电压不成正比。电阻会随温度或因元件的特殊性质而改变。

11. Worked Example Calculations | 计算例题

Example 1: A charge of 60 C passes through a lamp in 2 minutes. Calculate the current.
Solution: t = 2 × 60 = 120 s, I = Q / t = 60 / 120 = 0.5 A.

例题 1： 60 C 的电荷在 2 分钟内通过一盏灯。计算电流。
解：t = 2 × 60 = 120 s，I = Q / t = 60 / 120 = 0.5 A。

Example 2: A current of 200 mA flows for 5 minutes. How much charge is transferred?
Solution: I = 200 mA = 0.2 A, t = 5 × 60 = 300 s, Q = I × t = 0.2 × 300 = 60 C.

例题 2： 200 mA 的电流持续流动 5 分钟。转移了多少电荷？
解：I = 200 mA = 0.2 A，t = 5 × 60 = 300 s，Q = I × t = 0.2 × 300 = 60 C。

Example 3: A 12 V battery is connected to a 6 Ω resistor. Find the current.
Solution: I = V / R = 12 / 6 = 2 A.

例题 3： 一个 12 V 的电池连接到一个 6 Ω 的电阻器。求电流。
解：I = V / R = 12 / 6 = 2 A。

12. Common Exam Mistakes and Tips | 常见考试错误与提示

Forgetting to convert time to seconds: always make sure t is in seconds before using Q = I t.

忘记将时间换算为秒：在使用 Q = I t 之前，务必确保 t 以秒为单位。
Confusing series and parallel current rules: remember series current is constant; parallel current splits.

混淆串联与并联的电流规律：记住串联电流处处相等；并联电流分流。
Incorrect ammeter connection: an ammeter is always in series, never in parallel.

电流表连接错误：电流表始终串联，切勿并联。
Direction confusion: stick to conventional current (positive to negative) unless the question specifies electron flow.

方向混淆：除非题目明确电子流动方向，否则一律使用常规电流方向（正到负）。
Misusing Ohm’s Law: the law I = V / R only applies directly to ohmic conductors and fixed resistance.

误用欧姆定律：公式 I = V / R 仅直接适用于欧姆导体和恒定电阻。

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IGCSE Physics: Electric Current Explained | IGCSE 物理：电流考点精讲