Faraday’s Law Exam Mastery for A2 Physics | A2 物理：法拉第定律考点精讲

📚 Faraday’s Law Exam Mastery for A2 Physics | A2 物理：法拉第定律考点精讲

Faraday’s law of electromagnetic induction is a cornerstone of A2 Physics, linking changing magnetic fields to induced voltages. This article covers every essential concept, equation, and exam trick you need, from magnetic flux to eddy currents and AC generators. Each point is paired in English and Chinese to reinforce your learning.

法拉第电磁感应定律是 A2 物理的基石，将变化的磁场与感应电压联系起来。本文涵盖你所需要掌握的每一个核心概念、公式和考试技巧，从磁通量到涡流、交流发电机。每个要点均配有中英文对照，以强化学习效果。

1. Magnetic Flux | 磁通量

Magnetic flux Φ through a surface area A is defined as Φ = B A cosθ, where B is the magnetic flux density, and θ is the angle between the magnetic field lines and the normal to the surface. The SI unit is the weber (Wb); 1 Wb = 1 T m².

穿过面积 A 的磁通量 Φ 定义为 Φ = B A cosθ，其中 B 是磁感应强度，θ 是磁场方向与表面法线之间的夹角。国际单位是韦伯 (Wb)，1 Wb = 1 T m²。

When the field is perpendicular to the area (θ = 0°), flux is maximum: Φ = B A. When the field is parallel to the surface (θ = 90°), flux is zero. Always identify the correct angle in an exam — it’s measured from the normal, not the plane.

当磁场垂直于面积时 (θ = 0°)，磁通量最大：Φ = B A。当磁场平行于表面时 (θ = 90°)，磁通量为零。考试中务必确认正确的角度——角度从法线量起，而不是从平面。

2. Flux Linkage | 磁链

For a coil of N turns, the total flux threading the coil is called flux linkage and is given by NΦ. If the coil is wound closely, the same flux Φ passes through each turn. Then flux linkage = N B A cosθ.

对于匝数为 N 的线圈，穿过线圈的总磁通量称为磁链（或磁通匝连数），表示为 NΦ。如果线圈绕制紧密，每匝都通过相同的磁通量 Φ，则磁链 = N B A cosθ。

When Φ changes, the flux linkage changes by N times the change in flux, which is crucial for calculating induced emf. Many students forget to multiply by N — always check if you are dealing with a single loop or a multi-turn coil.

当 Φ 变化时，磁链的变化量是磁通量变化的 N 倍，这在计算感应电动势时至关重要。许多学生忘记乘以 N ——务必核实你处理的是单匝回路还是多匝线圈。

3. Faraday’s Law of Induction | 法拉第电磁感应定律

Faraday’s law states: The magnitude of the induced electromotive force (emf) in a circuit is equal to the rate of change of magnetic flux linkage through the circuit.

法拉第定律指出：回路中感应电动势的大小等于穿过该回路的磁链变化率。

ε = -N (dΦ/dt) or ε = – Δ(NΦ)/Δt

In symbols: induced emf ε (in volts) is proportional to the rate of change of flux. The negative sign indicates the direction (Lenz’s law). The average emf can be written as ε_avg = -N ΔΦ/Δt.

符号表示：感应电动势 ε（伏特）与磁通量的变化率成正比。负号表示方向（楞次定律）。平均电动势可写作 ε_avg = -N ΔΦ/Δt。

Key point: A changing magnetic flux is required; a steady flux produces zero emf. It is the rate of change, not the amount of flux, that determines the induced voltage. This is a common misconception that examiners love to test.

关键点：必须有变化的磁通量；恒定的磁通量产生零电动势。起决定作用的是磁通量的变化率，而不是磁通量的大小。这是一个常见的误解，考官们很喜欢考察这一点。

4. Lenz’s Law and the Negative Sign | 楞次定律与负号

Lenz’s law states: The direction of the induced current is such that it opposes the change in magnetic flux that produced it. This is a consequence of energy conservation. If the induced current aided the flux change, energy would be created from nothing.

楞次定律指出：感应电流的方向总是使其本身产生的磁通量反抗引起感应电流的磁通量变化。这是能量守恒的必然结果。如果感应电流助长磁通量的变化，就会无中生有地创造能量。

The negative sign in Faraday’s law, ε = -N dΦ/dt, mathematically encodes Lenz’s law. When flux is increasing

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