📚 Energy Stores and Transfers | 能量储存与转移
Energy is one of the most fundamental ideas in Cambridge IGCSE Science. In physics, energy is defined as the capacity to do work or to cause change. Understanding how energy is stored, transferred, conserved and used efficiently helps you explain almost every process in the physical world, from a falling object to an electric circuit.
能量是剑桥 IGCSE 科学中最基本的概念之一。在物理学中，能量被定义为做功或引起变化的能力。理解能量如何被储存、转移、守恒以及高效利用，能帮助你解释物理世界中几乎所有的过程，从下落的物体到电路中的电流。
1. The Concept of Energy | 能量的概念
Energy is not a substance; it is a property that objects and systems can have. The SI unit of energy is the joule (J). One joule is defined as the work done when a force of one newton moves an object through a distance of one metre in the direction of the force.
能量不是一种物质，而是物体和系统可以具有的一种属性。能量的国际单位是焦耳（J）。一焦耳的定义是：当一牛顿的力使物体沿力的方向移动一米时所做的功。
In everyday life we also use kilojoules (kJ) and megajoules (MJ). It is important to remember that energy can be transferred from one store to another, but it is never created or destroyed.
在日常生活中我们也会使用千焦（kJ）和兆焦（MJ）。要记住能量可以从一种储存形式转移到另一种储存形式，但它永远不会被创造或消灭。
2. Energy Stores | 能量储存类型
In the Cambridge IGCSE Science syllabus, energy is described as being stored in different ways. The main energy stores you need to know are:
在剑桥 IGCSE 科学大纲中，能量被描述为以不同方式储存。你需要掌握的主要能量储存类型包括：
- Chemical energy store
- Kinetic energy store
- Gravitational potential energy store
- Elastic potential energy store
- Thermal energy store
- Nuclear energy store
- Magnetic energy store
- Electrostatic energy store
化学能储存、动能储存、重力势能储存、弹性势能储存、热能储存、核能储存、磁能储存和静电储存。
A moving car has a kinetic energy store because of its motion. A stretched rubber band has an elastic potential energy store. A hot cup of tea has a thermal energy store because its particles are vibrating faster than those in a cooler object.
行驶中的汽车由于运动而具有动能储存。被拉伸的橡皮筋具有弹性势能储存。一杯热茶因为其粒子比温度较低的物体中的粒子振动得更快而具有热能储存。
3. Energy Transfers | 能量转移方式
Energy can be transferred from one store to another by four main pathways: mechanically, electrically, by heating and by radiation. These pathways are useful when you need to describe what happens to energy in a system.
能量可以通过四种主要途径从一种储存形式转移到另一种储存形式：机械做功、电流做功、加热和辐射。当你需要描述系统中能量发生的变化时，这些途径非常有用。
For example, when you lift a book onto a shelf, chemical energy in your muscles is transferred mechanically to the book's gravitational potential energy store. In a torch, chemical energy in the battery is transferred electrically to light energy and thermal energy.
例如，当你把一本书举到书架上时，肌肉中的化学能通过机械做功转移到书的重力势能储存中。在手电筒中，电池中的化学能通过电流做功转移为光能和热能。
Energy transfer summary table
| Pathway | Example |
|---|---|
| Mechanical | A force pushing a box |
| Electrical | Current in a circuit |
| Heating | A pan on a hot stove |
| Radiation | Light from the Sun reaching Earth |
能量转移途径总结表：机械做功、电流做功、加热和辐射。
4. Conservation of Energy | 能量守恒
The principle of conservation of energy states that energy cannot be created or destroyed. It can only be transferred from one store to another, or from one object to another. The total energy of a closed system always remains constant.
能量守恒定律指出，能量不能被创造或消灭。它只能从一种储存形式转移到另一种储存形式，或从一个物体转移到另一个物体。一个封闭系统的总能量始终保持不变。
This principle allows us to use energy calculations even when we cannot see all the transfers happening. For instance, when a pendulum swings, energy repeatedly changes between gravitational potential energy and kinetic energy, but the total energy in the system remains the same if there is no air resistance or friction.
这个定律使我们即使无法观察到所有的能量转移过程，也能进行能量计算。例如，当单摆摆动时，能量在重力势能和动能之间反复转换，但如果没有空气阻力和摩擦力，系统的总能量保持不变。
5. Work Done and Power | 做功与功率
Work is done when a force moves an object. The amount of work done is calculated using the equation:
当一个力使物体移动时，就做了功。做功的多少可以用以下公式计算：
W = F × d
where W is work done in joules (J), F is force in newtons (N), and d is distance moved in metres (m).
其中 W 表示所做的功，单位是焦耳（J）；F 表示力，单位是牛顿（N）；d 表示移动的距离，单位是米（m）。
Power is the rate at which energy is transferred or the rate at which work is done. Power is measured in watts (W). One watt is one joule per second.
功率是能量转移的速率或做功的速率。功率的单位是瓦特（W）。一瓦特等于每秒一焦耳。
P = E / t
where P is power in watts (W), E is energy transferred in joules (J), and t is time in seconds (s).
其中 P 表示功率，单位是瓦特（W）；E 表示转移的能量，单位是焦耳（J）；t 表示时间，单位是秒（s）。
6. Calculating Energy Stores | 计算能量储存
You need to be able to calculate kinetic energy, gravitational potential energy and elastic potential energy. These equations are often tested in Cambridge IGCSE Science papers.
你需要能够计算动能、重力势能和弹性势能。这些公式在剑桥 IGCSE 科学考试中经常出现。
Kinetic energy is the energy stored by a moving object. It depends on the mass and speed of the object:
动能是运动物体储存的能量。它取决于物体的质量和速度：
Eₖ = ½ m v²
where Eₖ is kinetic energy in joules (J), m is mass in kilograms (kg), and v is speed in metres per second (m/s).
其中 Eₖ 表示动能，单位是焦耳（J）；m 表示质量，单位是千克（kg）；v 表示速度，单位是米每秒（m/s）。
Gravitational potential energy is stored by an object raised above the ground. It is calculated using:
重力势能是物体被举高到地面上方时所储存的能量。其计算公式为：
ΔEₚ = m g Δh
where ΔEₚ is change in gravitational potential energy in joules (J), m is mass in kilograms (kg), g is gravitational field strength in newtons per kilogram (N/kg), and Δh is change in height in metres (m).
其中 ΔEₚ 表示重力势能的变化量，单位是焦耳（J）；m 表示质量，单位是千克（kg）；g 表示重力场强度，单位是牛顿每千克（N/kg）；Δh 表示高度的变化量，单位是米（m）。
Elastic potential energy is stored in a stretched or compressed spring. The equation is:
弹性势能储存在被拉伸或压缩的弹簧中。其公式为：
E = ½ k x²
where E is elastic potential energy in joules (J), k is the spring constant in newtons per metre (N/m), and x is the extension or compression in metres (m).
其中 E 表示弹性势能，单位是焦耳（J）；k 表示弹簧常量，单位是牛顿每米（N/m）；x 表示伸长量或压缩量，单位是米（m）。
7. Energy Efficiency | 能量效率
Energy efficiency describes how much of the energy input to a system is transferred to useful energy output. The efficiency can be calculated using the formula:
能量效率描述了一个系统输入的能量中有多少被转移为有用的能量输出。效率可以用以下公式计算：
Efficiency = (useful energy output / total energy input) × 100%
where useful energy output and total energy input are both measured in joules (J).
其中有用能量输出和总能量输入都以焦耳（J）为单位。
No real machine is 100% efficient because some energy is always transferred to the surroundings as thermal energy. For example, an electric motor may have an efficiency of 80%, meaning 80% of the electrical energy is converted into useful kinetic energy, while 20% is lost as heat.
没有任何真实机器的效率能达到 100%，因为总有一部分能量以热能的形式转移到周围环境中。例如，一台电动机的效率可能为 80%，这意味着 80% 的电能被转换成有用的动能，而 20% 以热量形式散失。
8. Sankey Diagrams | 桑基图
A Sankey diagram is a visual way to show energy transfers and efficiency. The width of each arrow in the diagram is proportional to the amount of energy it represents. Useful energy output is shown as an arrow pointing forward, while wasted energy is shown branching downwards or upwards.
桑基图是一种直观展示能量转移和效率的方法。图中每条箭头的宽度与其所代表的能量大小成比例。有用的能量输出用向前的箭头表示，而被浪费的能量则用向下或向上分支的箭头表示。
When drawing a Sankey diagram, always label the energy stores and pathways, and keep the input arrow width equal to the sum of the output arrow widths. This shows that energy is conserved.
绘制桑基图时，一定要标注能量储存类型和转移途径，并保持输入箭头的宽度等于所有输出箭头宽度之和。这体现了能量守恒。
9. Renewable and Non-renewable Energy Resources | 可再生与不可再生能源
Energy resources are used to generate electricity and provide heating and transport. They can be divided into renewable and non-renewable resources. Non-renewable resources include fossil fuels such as coal, oil and natural gas, as well as nuclear fuel. They are finite and will eventually run out.
能源资源用于发电、供热和交通运输。它们可以分为可再生能源和不可再生能源。不可再生能源包括煤、石油和天然气等化石燃料，以及核燃料。它们的储量是有限的，最终会枯竭。
Renewable resources include solar, wind, hydroelectric, wave, tidal, geothermal and biomass energy. These resources are replenished naturally and generally produce less carbon dioxide than burning fossil fuels, though they may have other environmental impacts.
可再生能源包括太阳能、风能、水力发电、波浪能、潮汐能、地热能和生物质能。这些资源能够自然再生，与燃烧化石燃料相比通常产生更少的二氧化碳，但它们可能还有其他环境影响。
For the Cambridge IGCSE Science exam, you should be able to compare advantages and disadvantages of different energy resources, including reliability, cost, environmental impact and energy output.
在剑桥 IGCSE 科学考试中，你应该能够比较不同能源资源的优缺点，包括可靠性、成本、环境影响和能量输出。
10. Examples of Energy Transfers in Everyday Life | 日常生活中的能量转移实例
Let us look at some common examples. In a battery-powered torch, chemical energy in the battery is transferred electrically to the bulb, where it is converted into light energy and thermal energy. The useful energy is light, while thermal energy is wasted.
让我们看一些常见的例子。在使用电池的手电筒中，电池中的化学能通过电流做功转移到灯泡，在灯泡中转化为光能和热能。有用的能量是光，而热能被浪费了。
In a hydroelectric power station, water stored in a high reservoir has a large gravitational potential energy store. As the water falls, this energy is transferred to kinetic energy. The moving water turns a turbine, which converts kinetic energy into electrical energy in a generator.
在水力发电站中，储存在高处水库中的水具有很大的重力势能储存。当水流下时，这种能量转移为动能。流动的水推动涡轮机，涡轮机将动能转化为发电机中的电能。
In a moving car, chemical energy from petrol or diesel is converted into thermal energy in the engine, which is then transferred mechanically into kinetic energy of the car. However, much of the energy is wasted as heat and sound, so the car is not very efficient.
在行驶的汽车中，汽油或柴油中的化学能在发动机中转化为热能，然后通过机械做功转移为汽车的动能。然而，大部分能量以热和声音的形式被浪费，因此汽车的效率并不高。
11. Energy Dissipation | 能量耗散
When energy is transferred, some of it usually becomes less useful because it spreads out into the surroundings. This process is called energy dissipation. The energy is not destroyed, but it is no longer available to do useful work.
当能量发生转移时，其中一部分通常会因为扩散到周围环境中而变得不那么有用。这个过程称为能量耗散。能量并没有被消灭，但已无法再用来做有用的功。
For example, when a ball bounces on the floor, each bounce is lower than the previous one. The kinetic energy and gravitational potential energy gradually decrease because energy is dissipated as thermal energy in the floor and the ball, and as sound energy to the air.
例如，当一个球在地板上弹跳时，每次弹起的高度都比上一次低。动能和重力势能逐渐减少，因为能量以地板和球中的热能以及空气中的声能形式耗散掉了。
Reducing unwanted energy transfers is an important part of engineering design. Lubricants reduce friction between moving parts, and thermal insulation reduces heat loss from buildings.
减少不必要的能量转移是工程设计中的一个重要部分。润滑剂可以减少运动部件之间的摩擦，保温材料可以减少建筑物的热量损失。
12. Exam Tips for Cambridge IGCSE Science | 剑桥科学考试技巧
When answering questions on energy, always identify the energy stores involved at the start and the end of the process. Then state the pathway by which energy is transferred. This helps you gain full marks on descriptive questions.
回答能量相关问题时，一定要先找出过程开始和结束时涉及的能量储存类型。然后说明能量转移的途径。这样可以帮助你在描述题上获得满分。
Remember to include the correct units in calculations. Use m for mass in kilograms, v for speed in metres per second, h for height in metres, and g for gravitational field strength in newtons per kilogram. Write down the formula first, substitute the values, and then show your answer.
计算时一定要写对单位。质量用千克（kg），速度用米每秒（m/s），高度用米（m），重力场强度用牛顿每千克（N/kg）。先写出公式，再代入数值，最后展示答案。
For efficiency questions, make sure the useful energy output is never greater than the total energy input. The efficiency must be between 0 and 100%. If your answer is outside this range, check your working.
对于效率问题，确保有用能量输出永远不会大于总能量输入。效率必须在 0 到 100% 之间。如果你的答案超出这个范围，请检查你的计算过程。
Finally, practise drawing Sankey diagrams with accurate widths and clear labels. These diagrams are often used in exams to test your understanding of energy conservation and efficiency.
最后，练习绘制宽度准确、标注清晰的桑基图。这些图在考试中经常用来测试你对能量守恒和效率的理解。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图