Energy Stores, Transfers and Changes | 能量储存、转化与变化

📚 Energy Stores, Transfers and Changes | 能量储存、转化与变化

For the Edexcel IGCSE Science course, understanding how energy behaves is essential across all three sciences. The idea of energy connects physics, chemistry and biology: it powers machines, drives chemical reactions and sustains life. This revision article walks through the key ideas you need for the exam.

在 Edexcel IGCSE 科学课程中，理解能量的行为是贯通物理、化学和生物三科的核心。能量驱动机械、推动化学反应并维持生命。本文系统梳理考试所需的关键知识点，帮助你高效复习。

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1. The Law of Conservation of Energy | 能量守恒定律

Energy cannot be created or destroyed; it can only be transferred from one store to another, or transformed from one form to another. In a closed system, the total amount of energy stays constant. This is the most important foundation of all energy calculations.

能量既不能被创造，也不能被消灭；它只能从一个储存装置转移到另一个储存装置，或从一种形式转化为另一种形式。在封闭系统中，总能量保持不变。这是所有能量计算最重要的基础。

A swinging pendulum is a classic example. At the top of its swing, all the energy is stored as gravitational potential energy. At the bottom, this has been transferred to kinetic energy. Energy continuously swaps between the two stores as the pendulum moves.

摆动的钟摆是经典例子。在摆动最高点时，所有能量以重力势能的形式储存；在最低点时，能量已转化为动能。能量在两种储存形式之间不断切换。

In real life, energy is often dissipated (wasted) to the surroundings as thermal energy due to friction, air resistance or sound. The ‘useful’ energy is less than the input, but the total energy is always conserved.

在现实中，能量常因摩擦、空气阻力或声音而耗散（浪费）到周围环境。有用能量少于输入能量，但总能量始终守恒。

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2. Energy Stores | 能量储存形式

The Edexcel specification expects you to recognise and describe the main energy stores. A store is a way of holding energy in a system before it is transferred.

Edexcel 考纲要求你识别并描述主要的能量储存形式。所谓储存，是指能量在转移之前以某种方式被系统保存在其中的状态。

Energy Store | 能量储存	Explanation | 说明
Kinetic | 动能	Energy of a moving object. A faster or heavier object has more kinetic energy. | 运动物体具有的能量。物体越快或质量越大，动能越大。
Gravitational potential | 重力势能	Energy stored in an object due to its height above the ground. | 物体因高于地面一定高度而储存的能量。
Elastic potential | 弹性势能	Energy stored when an object is stretched or compressed, such as a spring or rubber band. | 物体被拉伸或压缩时储存的能量，如弹簧或橡皮筋。
Chemical | 化学能	Energy stored in chemical bonds. Examples: food, fuels, batteries. | 储存在化学键中的能量。例如食物、燃料和电池。
Thermal (internal) | 热能（内能）	Energy due to the temperature of an object. Hotter objects store more thermal energy. | 物体因温度而具有的能量。物体越热，储存的热能越多。
Nuclear | 核能	Energy stored in the nucleus of an atom, released in nuclear fission or fusion. | 储存在原子核中的能量，通过核裂变或核聚变释放。
Magnetic | 磁能	Energy stored when magnetic objects attract or repel each other. | 磁体相互吸引或排斥时储存的能量。
Electrostatic | 静电势能	Energy stored when charged particles attract or repel each other. | 带电粒子相互吸引或排斥时储存的能量。

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3. Energy Transfer Pathways | 能量传递途径

When energy moves from one store to another, it travels along a pathway. The four main pathways you need to name are heating, mechanical working, electrical working and radiation.

当能量从一个储存形式转移到另一个储存形式时，它沿着某种途径传递。你需要掌握四种主要途径：加热、机械做功、电做功和辐射。

• Heating | 加热：Energy transfers from a hotter object to a colder one, e.g. a kettle heating water.

  能量从较热的物体传递给较冷的物体，例如水壶加热水。

• Mechanical working | 机械做功：A force moves an object over a distance, e.g. pushing a trolley or rubbing hands together.

  力使物体移动一段距离，例如推手推车或摩擦双手。

• Electrical working | 电做功：Electric current carries energy to a component, e.g. current flowing through a lamp.

  电流将能量传递给用电器，例如电流流过灯泡。

• Radiation | 辐射：Energy travels as waves, e.g. light from the Sun or sound from a speaker.

  能量以波的形式传播，例如来自太阳的光或扬声器发出的声音。

In exam questions, connect the pathway to the energy store. For example, a falling ball: gravitational potential store → kinetic store, via mechanical working due to gravity.

在考试答题时，将途径与能量储存联系起来。例如，一个下落的球：重力势能储存 → 动能储存，通过重力做功实现。

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4. Work Done and Power | 做功与功率

When a force moves an object, energy is transferred. The amount of energy transferred by a force is called work done. Both work done and energy are measured in joules (J).

当力使物体移动时，能量发生转移。力所传递的能量称为做功。做功和能量的单位都是焦耳（J）。

work done = force × distance moved | 功 = 力 × 距离
W = F × d

For example, if you push a box with a force of 50 N over a distance of 10 m, the work done is 50 × 10 = 500 J. Remember that the distance must be in metres when using this formula.

例如，你用 50 N 的力推动箱子移动 10 m，做功为 50 × 10 = 500 J。注意，使用此公式时距离单位必须是米。

Power is the rate of energy transfer, measured in watts (W). One watt equals one joule per second.

功率是能量传递的速率，单位是瓦特（W）。1 瓦特等于每秒传递 1 焦耳能量。

power = energy transferred ÷ time taken | 功率 = 传递的能量 ÷ 所用时间
P = E ÷ t

If 500 J is transferred in 20 seconds, the power is 500 ÷ 20 = 25 W. Always check that time is in seconds.

若在 20 秒内传递了 500 J 能量，则功率为 500 ÷ 20 = 25 W。务必确认时间单位是秒。

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5. Kinetic and Gravitational Potential Energy | 动能与重力势能

Two calculation formulas appear frequently in IGCSE papers: kinetic energy (KE) and gravitational potential energy (GPE). You must learn them exactly, including the squares.

两个计算公式在 IGCSE 试卷中频繁出现：动能（KE）和重力势能（GPE）。你必须准确记忆它们，包括公式中的平方。

KE = ½ × m × v² | 动能 = ½ × 质量 × 速度²

GPE = m × g × h | 重力势能 = 质量 × g × 高度

Here m is mass in kilograms, v is speed in metres per second, g is gravitational field strength (take 10 N/kg for Earth) and h is height in metres.

其中 m 是质量（单位 kg），v 是速度（单位 m/s），g 是重力场强度（地球上取 10 N/kg），h 是高度（单位 m）。

Worked example: a car of mass 800 kg travels at 20 m/s.

例题：一辆质量为 800 kg 的汽车以 20 m/s 行驶。

KE = ½ × 800 × 20² = ½ × 800 × 400 = 160,000 J = 160 kJ

Worked example: a 2 kg book is lifted 3 m onto a shelf.

例题：一本 2 kg 的书被提升到 3 m 高的书架上。

GPE = 2 × 10 × 3 = 60 J

Examiners often ask you to apply conservation of energy: when an object falls freely, the lost GPE equals the gained KE (ignoring air resistance). Use this to find speed just before hitting the ground.

考官常要求你运用能量守恒：物体自由下落时，减少的重力势能等于增加的动能（忽略空气阻力）。据此可求出物体落地前的速度。

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6. Energy Efficiency | 能量效率

Machines never convert all input energy into useful output energy. Some energy is always wasted, usually as thermal energy. Efficiency tells us how good a device is at using energy usefully.

机器永远无法将所有输入能量转化为有用输出能量，总有一部分被浪费，通常以热能形式散失。效率反映装置利用能量的好坏程度。

efficiency = (useful output energy ÷ total input energy) × 100% | 效率 =（有用输出能量 ÷ 总输入能量）× 100%

For example, if an electric motor receives 300 J of electrical energy and delivers 150 J of useful kinetic energy, its efficiency is 150 ÷ 300 × 100% = 50%.

例如，一台电动机输入 300 J 电能，输出 150 J 有用动能，其效率为 150 ÷ 300 × 100% = 50%。

Efficiency can also be calculated using power: efficiency = useful power output ÷ total power input. A Sankey diagram shows the energy input splitting into useful and wasted energy arrows; the width of each arrow is proportional to the amount of energy.

效率也可以用功率计算：效率 = 有用输出功率 ÷ 总输入功率。桑基图（Sankey diagram）将输入能量箭头分为有用能量和浪费能量两部分；箭头的宽度与能量大小成正比。

To improve efficiency in a real system: lubricate moving parts to reduce friction, insulate to reduce thermal loss, or use a more efficient transformer.

提高系统效率的方法包括：给运动部件加润滑油以减少摩擦，加保温层以减少热能损失，或使用更高效的变压器。

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7. Exothermic and Endothermic Reactions | 放热反应与吸热反应

In chemistry, energy changes accompany chemical reactions. An exothermic reaction releases energy to the surroundings, usually as heat, light or sound. An endothermic reaction takes in energy from the surroundings.

在化学中，化学反应伴随能量变化。放热反应向周围环境释放能量，通常以热、光或声音的形式；吸热反应则从环境中吸收能量。

Type | 类型	Direction of energy | 能量方向	Examples | 实例
Exothermic | 放热	Energy transfers to surroundings; temperature rises | 能量传给环境；温度升高	Combustion, neutralisation, displacement, respiration | 燃烧、中和、置换、呼吸作用
Endothermic | 吸热	Energy is absorbed from surroundings; temperature falls | 从环境吸收能量；温度降低	Thermal decomposition, electrolysis, photosynthesis | 热分解、电解、光合作用

In the laboratory, you can classify a reaction by measuring the temperature change with a thermometer. A temperature rise indicates an exothermic reaction; a temperature fall indicates an endothermic reaction.

实验中可用温度计测量温度变化来判断反应类型。温度升高说明是放热反应；温度降低说明是吸热反应。

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8. Bond Energy Calculations | 键能计算

Chemical reactions involve breaking bonds in reactants and forming bonds in products. Energy is needed to break bonds (endothermic), and energy is released when bonds form (exothermic). The overall energy change is the difference between these two values.

化学反应涉及断裂反应物中的化学键和形成产物中的化学键。断键需要吸收能量（吸热），成键会释放能量（放热）。总能量变化等于两者之差。

ΔH = total energy to break bonds − total energy released by forming bonds | ΔH = 断键吸收总能量 − 成键释放总能量

If the answer is negative, the reaction is exothermic; if positive, it is endothermic. Bond energies are measured in kJ/mol.

若结果为负值，则为放热反应；若为正值，则为吸热反应。键能的单位是 kJ/mol。

Worked example: hydrogen reacts with chlorine. Given bond energies H-H = 436 kJ/mol, Cl-Cl = 243 kJ/mol and H-Cl = 432 kJ/mol:

例题：氢与氯反应。已知键能 H-H = 436 kJ/mol，Cl-Cl = 243 kJ/mol，H-Cl = 432 kJ/mol：

H₂ + Cl₂ → 2HCl
Energy to break bonds = 436 + 243 = 679 kJ
Energy released forming bonds = 2 × 432 = 864 kJ
ΔH = 679 − 864 = −185 kJ/mol → exothermic

Always count the number of bonds correctly. In H₂ there is one H-H bond, and in 2HCl there are two H-Cl bonds formed. A table of bond energies is usually provided for you.

务必准确数出化学键的数目。H₂ 中有一个 H-H 键，2HCl 中形成两个 H-Cl 键。键能表通常在题目中提供。

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9. Energy in Photosynthesis and Respiration | 光合作用与呼吸作用中的能量

Energy changes also link chemistry and biology. Photosynthesis is an endothermic process: plants absorb light energy to build glucose. Respiration is an exothermic process: glucose is broken down to release energy for life processes.

能量变化同样贯穿化学与生物。光合作用是吸热过程：植物吸收光能合成葡萄糖；呼吸作用是放热过程：葡萄糖被分解以释放维持生命活动所需的能量。

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy absorbed)

光合作用：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂（吸收光能）

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy released)

呼吸作用：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O（释放能量）

In exam questions, be able to state that the energy released in respiration is used for muscle contraction, keeping body temperature constant and building larger molecules from smaller ones.

考试中需能说明：呼吸作用释放的能量用于肌肉收缩、维持体温恒定以及由

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