📚 DC Circuits and Kirchhoff's Laws | 直流电路与基尔霍夫定律
Understanding DC circuits is fundamental to IB Physics. This article reviews charge, current, potential difference, Ohm's law, resistivity, series and parallel combinations, Kirchhoff's laws, internal resistance, power, potential dividers, and essential safety concepts. Each topic is paired with clear English–Chinese explanations to support bilingual learning and exam success.
理解直流电路是 IB 物理的基础。本文回顾电荷、电流、电势差、欧姆定律、电阻率、串并联组合、基尔霍夫定律、内阻、功率、分压器以及必要的安全概念。每个主题均配有清晰的中英对照解释，以支持双语学习和考试成功。
1. Charge, Current and Potential Difference | 电荷、电流与电势差
the rate of flow of charge: I = ΔQ / Δt. The unit is the ampere (A), where 1 A = 1 C s⁻¹.
电荷 (Q) 的单位是库仑 (C)。基本电荷 e = 1.60 × 10⁻¹⁹ C。电流 I 是电荷流动的速率：I = ΔQ / Δt。电流的单位是安培 (A)，1 A = 1 C s⁻¹。
ork done per unit charge moving between them. Its unit is the volt (V), and 1 V = 1 J C⁻¹.
两点之间的电势差（电压）是将单位电荷从一点移到另一点所做的功。其单位为伏特 (V)，1 V = 1 J C⁻¹。
2. Ohm's Law and Resistance | 欧姆定律与电阻
For many conductors, especially metallic conductors at constant temperature, the current through the conductor is directly proportional to the potential difference across it. This is Ohm's law.
对于许多导体，特别是恒温下的金属导体，通过导体的电流与导体两端的电势差成正比。这就是欧姆定律。
The resistance R of a component is defined as R = V / I. Its unit is the ohm (Ω).
元件的电阻 R 定义为 R = V / I。电阻的单位是欧姆 (Ω)。
Ohm's law can be written as:
V = I R
欧姆定律可写为：
V = I R
Note that some components, such as filament lamps and diodes, have non-ohmic I-V characteristics.
注意，有些元件（如灯丝灯泡和二极管）具有非欧姆的 I-V 特性。
3. Resistivity | 电阻率
The resistance of a uniform wire depends on its length L, cross-sectional area A, and the material's resistivity ρ (rho). The relationship is:
R = ρ L / A
均匀导线的电阻取决于其长度 L、横截面积 A 以及材料的电阻率 ρ。关系式为：
R = ρ L / A
Resistivity is measured in ohm metres (Ω m). For most metals, resistivity increases with temperature. Superconductors have zero resistivity below a critical temperature.
电阻率的单位是欧姆·米 (Ω m)。对于大多数金属，电阻率随温度升高而增大。超导体在临界温度以下电阻率为零。
4. Series Circuits | 串联电路
In a series circuit, components are connected end-to-end, so the same current passes through each component.
在串联电路中，元件首尾相连，因此每个元件中流过的电流相同。
The total resistance Rtotal is the sum of individual resistances: Rtotal = R₁ + R₂ + R₃ + …
总电阻 Rtotal 等于各电阻之和：Rtotal = R₁ + R₂ + R₃ + …
The supply voltage Vs is divided across the resistors: Vs = V₁ + V₂ + V₃ + …, where Vi = I Ri.
电源电压 Vs 被分配到各个电阻上：Vs = V₁ + V₂ + V₃ + …，其中 Vi = I Ri。
5. Parallel Circuits | 并联电路
In a parallel circuit, components are connected across common points, so the potential difference across each branch is the same.
在并联电路中，元件连接在公共点之间，因此每个支路两端的电势差相同。
The total current from the source equals the sum of the currents in each branch: Itotal = I₁ + I₂ + I₃ + …
从电源流出的总电流等于各支路电流之和：Itotal = I₁ + I₂ + I₃ + …
The combined resistance Rp is found from:
1 / Rp = 1 / R₁ + 1 / R₂ + 1 / R₃ + …
总电阻 Rp 可根据下式求出：
1 / Rp = 1 / R₁ + 1 / R₂ + 1 / R₃ + …
6. Kirchhoff's Current Law (KCL) | 基尔霍夫电流定律
Kirchhoff's first law (the junction rule) states that the algebraic sum of currents entering any junction is zero. Equivalently, the total current entering a junction equals the total current leaving it.
基尔霍夫第一定律（节点规则）指出，流入任何一个节点的电流的代数和为零。等效的说法是，流入节点的总电流等于流出节点的总电流。
s flowing towards the junction taken as positive and those leaving as negative (or vice versa, consistently).
数学表达式为 Σ I = 0，通常规定流向节点的电流为正，离开的为负（或反过来，保持一致）。
This law is a consequence of conservation of
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