Energy Changes | 能量变化

📚 Energy Changes | 能量变化

Energy is a central idea in CIE A-Level Physics. The principle of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed between different stores. This article reviews key concepts, equations and exam-style applications of energy changes.

能量是 CIE A-Level 物理的核心概念。能量守恒定律指出，能量不能被创造或消灭，只能在不同的储存库之间转移或转化。本文回顾能量变化的关键概念、公式和考试常见应用。

1. Work and Energy Transfer | 功与能量转移

In mechanics, work is done whenever a force moves its point of application. Work is a scalar quantity, but it can be positive, negative or zero depending on the direction of the force relative to the displacement.

在力学中，只要力使其作用点发生移动，就做了功。功是标量，但根据力相对于位移的方向，功可以是正值、负值或零。

For a constant force F acting at an angle θ to a displacement s, the work done W is:

对于与位移 s 成 θ 角的恒力 F，所做的功 W 为：

W = Fs cos θ

If θ = 0°, the force does maximum positive work; if θ = 90°, cos θ = 0 and no work is done; if θ = 180°, the force does negative work, meaning energy is taken from the system.

若 θ = 0°，力做最大正功；若 θ = 90°，cos θ = 0，不做功；若 θ = 180°，力做负功，表示能量从系统中被取走。

The SI

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