A-Level Physics: Kirchhoff’s Second Law (Voltage Law) | A-Level 物理：基尔霍夫第二定律（电压定律）

📚 A-Level Physics: Kirchhoff’s Second Law (Voltage Law) | A-Level 物理：基尔霍夫第二定律（电压定律）

Kirchhoff’s second law, also called Kirchhoff’s voltage law (KVL), is one of the two fundamental rules used to analyse electric circuits at A-Level. It states that for any closed loop in a circuit, the algebraic sum of the e.m.f.s equals the algebraic sum of the potential differences around the loop. This law is a direct consequence of the conservation of energy.

基尔霍夫第二定律，又称基尔霍夫电压定律（KVL），是 A-Level 分析电路的两条基本规律之一。它指出：在电路中任意闭合回路内，所有电动势的代数和等于所有电位差（电压降）的代数和。该定律是能量守恒的直接推论。

1. Statement of Kirchhoff’s Second Law | 基尔霍夫第二定律的表述

The formal statement can be written as: the algebraic sum of the e.m.f.s around any closed loop is equal to the algebraic sum of the IR products around the same loop.

基尔霍夫第二定律的正式表述为：沿任意闭合回路，所有电动势的代数和等于所有电阻电压降（IR 乘积）的代数和。

Σε = ΣIR

Here ε represents each e.m.f. in the loop, I is the current through a resistor, and R is the resistance. The word “algebraic” is essential: each term must be given a positive or negative sign depending on the chosen traverse direction.

式中 ε 表示回路中的每一个电动势，I 是通过电阻的电流，R 是电阻。“代数”一词至关重要：每一项都要根据所选的回路绕行方向取正号或负号。

This relationship can also be understood from the idea that the net change in electric potential around a closed loop is zero. A charge that returns to its starting point cannot have gained or lost energy overall.

该关系也可理解为：沿闭合回路绕行一周，电势的总变化量为零。电荷回到起点时，总能量既不能增加也不能减少。

2. The Sign Convention | 符号约定

Before applying Kirchhoff’s second law, you must choose a traverse direction for the loop and an assumed direction for each current. You may choose clockwise or anticlockwise; consistency is more important than correctness at this stage.

在应用基尔霍夫第二定律前，必须为回路选定绕行方向，并为每条电流假设方向。你可以选择顺时针或逆时针；此时一致性比“猜对”更重要。

The following table summarises the sign rules.

下表总结了符号规则。

If a solution produces a negative current, this simply means that the real current flows opposite to the direction you assumed. The magnitude of the current remains correct.

如果解出的电流为负值，只说明实际电流方向与你假设的方向相反，电流大小仍然正确。

3. Applying KVL to a Single Loop | 对单个回路应用基尔霍夫电压定律

Consider a simple circuit with one cell of e.m.f. ε = 6 V and two resistors R₁ = 2 Ω and R₂ = 4 Ω connected in series. Assume the current I flows clockwise. Start at the negative terminal of the cell and move clockwise around the loop.

考虑一个简单电路：电动势 ε = 6 V 的电池与两个电阻 R₁ = 2 Ω、R₂ = 4 Ω 串联。假设电流 I 沿顺时针方向流动。从电池负极出发，沿顺时针

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