📚 IB Physics: Electric Current – Key Points Analysis | IB 物理：电流 考点精讲
Electric current is a cornerstone of IB Physics, linking the microscopic world of charge carriers to the macroscopic behaviour of circuits. A solid grasp of current is essential for tackling topics from simple Ohm’s law problems to complex electromagnetic phenomena. This article unpacks every key point, from definitions and drift velocity to Kirchhoff’s laws and exam tactics, equipping you with both conceptual understanding and problem-solving confidence.
电流是IB物理的基石，它将电荷载流子的微观世界与电路的宏观行为联系起来。牢固掌握电流知识对于从简单的欧姆定律问题到复杂的电磁现象都至关重要。本文解析每一个考点，从定义和漂移速度到基尔霍夫定律和考试技巧，为你提供概念理解和解题信心。
1. Definition and Unit of Current | 电流的定义和单位
Electric current (I) is defined as the rate at which electric charge flows through a given cross-sectional area. Quantitatively, I = Δq / Δt, where Δq is the net charge passing through the area in time interval Δt.
电流（I）定义为电荷流过给定横截面积的速率。定量表达为 I = Δq / Δt，其中 Δq 是在时间间隔 Δt 内通过该面积的净电荷量。
I = Δq / Δt
The SI unit of current is the ampere (A), which is a base unit. One ampere corresponds to a flow of one coulomb of charge per second (1 A = 1 C s-1).
电流的国际单位是安培（A），它是一个基本单位。1安培相当于每秒流过1库仑的电荷（1 A = 1 C s-1）。
Even though current has a direction, it is treated as a scalar quantity in IB Physics. The direction only indicates the sense of flow, not a vector orientation in space.
尽管电流有方向，但在IB物理中它被视为标量。方向仅表示流动的指向，而非空间中的矢量取向。
2. Conventional Current vs Electron Flow | 约定电流方向与电子流动
Historically, before the discovery of the electron, scientists defined current as the flow of positive charge. This is called conventional current, and it moves from the positive terminal to the negative terminal of a power source.
历史上，在发现电子之前，科学家将电流定义为正电荷的流动。这被称为约定电流，它从电源的正极流向负极。
In metallic conductors, the actual mobile charge carriers are free electrons, which move in the opposite direction – from negative to positive. This is electron flow.
在金属导体中，实际移动的电荷载流子是自由电子，它们朝相反方向移动——从负极到正极。这就是电子流动。
IB exams almost always use conventional current, unless explicitly stated otherwise. Circuit symbols like arrows on diodes indicate the direction of conventional current.
IB考试几乎总是使用约定电流，除非特别说明。电路符号（如二极管上的箭头）指示的是约定电流的方向。
Understanding this distinction helps avoid confusion when discussing magnetic force directions or Hall effect, where the sign of charge carriers matters.
理解这一区别有助于在讨论磁力方向或霍尔效应时避免混淆，因为这些场合电荷载流子的符号很重要。
3. Kirchhoff’s Current Law (KCL) | 基尔霍夫电流定律
Kirchhoff’s Current Law arises from the conservation of electric charge. It states that at any junction in an electrical circuit, the total current entering the junction equals the total current leaving it.
基尔霍夫电流定律源于电荷守恒。它指出，在电路的任一节点处，流入节点的总电流等于流出节点的总电流。
Σ Iin = Σ Iout
For a simple junction with three branches, if two currents I₁ and I₂ flow in, and I₃ flows out, KCL gives I₃ = I₁ + I₂. There is no accumulation of charge at the junction.
对于一个简单的三分支节点，如果两个电流 I₁ 和 I₂ 流入，而 I₃ 流出，则 KCL 给出 I₃ = I₁ + I₂。节点处没有电荷累积。
KCL is a powerful tool for analysing complex circuits. Combined with Kirchhoff’s Voltage Law, it enables you to solve for unknown currents in multiple loops.
KCL 是分析复杂电路的有力工具。与基尔霍夫电压定律结合，可以求解多回路中的未知电流。
4. Current in Series and Parallel Circuits | 串联和并联电路中的电流
The behaviour of current differs fundamentally between series and parallel configurations. In a series circuit, there is only one path for charge flow: the current is the same at every point.
串联和并联结构中电流的行为有本质区别。在串联电路中，电荷只有一条通路：各点的电流处处相等。
Itotal = I₁ = I₂ = I₃ = …
In a parallel circuit, the total current from the source splits among the parallel branches. The sum of the branch currents equals the main line current.
在并联电路中，来自电源的总电流在各并联支路中分配。各支路电流之和等于干线电流。
Itotal = I₁ + I₂ + I₃ + …
| Circuit Type | Current Rule | Example |
| Series | Same current everywhere | Three bulbs in series share identical current. |
| Parallel | Current divides; total = sum of branches | Household appliances wired in parallel draw independent currents. |
上表总结了串并联的电流规则。串联时电流恒定，并联时总电流为各支路电流的代数和。这能解释为何串联的灯泡会分担亮度，而并联的灯泡独立工作。
5. Microscopic Motion: Drift Velocity | 微观运动：漂移速度
At the microscopic level, current is produced by the net drift of charge carriers. In a metal, free electrons move randomly at very high speeds (~10⁶ m s-1) but produce no net current without an electric field.
在微观层面，电流由电荷载流子的净漂移产生。在金属中，自由电子以极高的速度随机运动（~10⁶ m s-1），但没有电场时不会产生净电流。
When a potential difference is applied, electrons acquire a small, steady drift velocity vd superimposed on their random motion. This drift is extremely slow, typically of the order of mm s-1.
当施加电位差时，电子在随机运动之上获得一个微小而稳定的漂移速度 vd。这种漂移极其缓慢，数量级通常为 mm s-1。
The current I can be expressed in terms of microscopic quantities:
I = n A vd q
电流 I 可以用微观量表示：I = n A vd q，其中 n 为电荷载流子数密度（每立方米个数），A 为横截面积，vd 为漂移速度，q 为每个载流子的电荷。对于金属，q = e = 1.60 × 10-19 C。
Example: A copper wire of cross-sectional area 1.0 × 10-6 m² carries a current of 2.0 A. Given n = 8.5 × 1028 m-3 and e = 1.60 × 10-19 C, calculate the drift velocity.
示例：一根横截面积为1.0 × 10-6 m² 的铜导线，载流 2.0 A。已知 n = 8.5 × 1028 m-3，e = 1.60 × 10-19 C，计算漂移速度。
Rearranging: vd = I / (n A e) = 2.0 / (8.5 × 1028 × 1.0 × 10-6 × 1.60 × 10-19) ≈ 1.5 × 10-4 m s-1. This tiny value shows that the signal (energy transfer) travels near the speed of light, while individual electrons crawl.
计算：vd = I / (n A e) ≈ 1.5 × 10-4 m s-1。这个微小的值表明，信号（能量传递）以接近光速传播，而单个电子却在缓慢爬行。
6. Current Density | 电流密度
Current density J is defined as the current per unit cross-sectional area and is a vector quantity. It is related to the microscopic model by J = I / A = n vd q.
电流密度 J 定义为单位横截面积上的电流，是一个矢量。它与微观模型的关系为 J = I / A = n vd q。
J = I / A
The unit of current density is A m-2. Using current density can simplify calculations involving non-uniform cross-sections or when describing current as a flow field.
电流密度的单位是 A m-2。在处理不均匀截面或把电流描述为流场时，使用电流密度可以简化计算。
For a given conductor, if the cross-sectional area decreases, the current density increases for the same total current, leading to higher resistive heating in that thinner region.
对于给定导体，若总电流不变，截面积减小时电流密度增大，导致较细区域产生更高的电阻热。
7. Ohm’s Law and Electrical Resistance | 欧姆定律与电阻
Ohm’s law states that for many conductors at constant temperature, the current through the conductor is directly proportional to the potential difference across it: V = I R, where R is the resistance.
欧姆定律指出，对于许多恒温下的导体，通过导体的电流与其两端的电位差成正比：V = I R，其中 R 为电阻。
V = I R
Resistance R is measured in ohms (Ω). A component that obeys Ohm’s law is called an ohmic conductor; its I–V graph is a straight line through the origin. Filament lamps and diodes are non-ohmic because their resistance changes with temperature or voltage.
电阻 R 的单位是欧姆（Ω）。遵守欧姆定律的组件称为欧姆导体，其 I–V 图为过原点的直线。白炽灯和二极管是非欧姆组件，因为它们的电阻随温度或电压变化。
The resistance of a metal wire is given by R = ρ L / A, where ρ is resistivity. Since resistivity increases with temperature, current causes heating, which can alter the resistance in a circuit.
金属导线的电阻由 R = ρ L / A 给出，其中 ρ 为电阻率。由于电阻率随温度升高而增大，电流引起发热，可能改变电路中的电阻。
8. Measuring Current with Ammeters | 用安培表测量电流
To measure the current through a component, an ammeter must be connected in series with it. An ideal ammeter has zero internal resistance so that it does not alter the current being measured.
要测量通过组件的电流，必须将安培表与该组件串联。理想安培表的内阻为零，这样它就不会改变被测电流。
Practical ammeters have very low resistance. Connecting an ammeter in parallel with a component would create a short circuit (very low resistance path), causing a dangerously large current and potential damage.
实际安培表的电阻非常低。若将安培表并联在组件两端，会形成短路（极低电阻通路），导致危险的巨大电流并可能损坏仪表。
In circuit diagrams, an ammeter is represented by a circle with ‘A’ inside. Always double-check that it is placed in series along the path whose current you wish to find.
在电路图中，安培表用一个内含’A’的圆圈表示。务必检查它是否串联在你想要求取电流的通路中。
9. Power and Current | 功率与电流
The electrical power dissipated or delivered by a circuit element is the product of current and potential difference: P = I V. Combining with Ohm’s law yields alternative forms.
电路元件耗散或提供的电功率是电流与电位差的乘积：P = I V。结合欧姆定律可得到其他形式。
P = I V = I2 R = V2 / R
These equations highlight that for a fixed resistance, doubling the current quadruples the power dissipated (I2 R). This explains why high-current transmission lines use very high voltage to reduce I and hence minimise I2R losses.
这些方程表明，对于固定电阻，电流加倍会使耗散功率变为四倍（I2 R）。这就解释了为何高电流输电线使用极高电压来减小 I，从而最小化 I2R 损耗。
Fuses and circuit breakers rely on the heating effect of current. A fuse has a thin wire that melts when the current exceeds its rated value, protecting the circuit.
保险丝和断路器依赖于电流的热效应。保险丝含有一根细金属丝，当电流超过额定值时熔断，以保护电路。
10. Exam Tips and Common Mistakes | 考试技巧与常见错误
Carefully read whether the question expects conventional current direction or mentions electron flow. In most IB problems, arrows indicate conventional current unless specified.
仔细读题，看题目期望的是约定电流方向还是提及了电子流。在大多数IB问题中，箭头指示约定电流，除非另有说明。
Always check units: convert mm2 to m2 when using current-density or drift-velocity formulas. A common slip is keeping area in mm2 and getting an answer orders of magnitude wrong.
务必检查单位：使用电流密度或漂移速度公式时，将 mm2 转换为 m2。常见的失误是保留 mm2 单位，导致答案出现数量级错误。
Remember that in a series circuit, current is constant; in a parallel circuit, current splits. Do not confuse these rules with those for voltage (which are opposite).
记住，串联电路中电流恒定；并联电路中电流分流。不要将这些规则与电压规则混淆（电压规则与之相反）。
When applying KCL, set up a consistent sign convention, e.g., currents entering as positive and leaving as negative. Sum them to zero: Σ I = 0.
应用 KCL 时，建立一致的符号约定，例如流入为正、流出为负。求和为零：Σ I = 0。
Finally, in a data-analysis question involving an I–V graph, identify whether the component is ohmic from the straight line. The gradient of an I–V graph is 1/R, not R; this distinction often appears in multi-choice questions.
最后，在涉及 I–V 图的数据分析题中，通过直线判断组件是否为欧姆导体。I–V 图的斜率是 1/R，而非 R；这一区别常在选择题中出现。
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