📚 Energy Stores, Transfers and Efficiency | 能量储存、转换与效率
This revision guide covers a core topic in Edexcel IGCSE Science: energy stores, transfer pathways, the law of conservation of energy, work and power, efficiency, and practical ways to reduce energy loss. You will need these ideas for physics questions and for the energy resources part of the Science course.
本复习指南涵盖 Edexcel IGCSE 科学的核心主题：能量储存、转移途径、能量守恒定律、做功与功率、效率以及减少能量损失的实际方法。这些概念不仅是物理部分的重要内容，也是科学课程中能源资源部分的基础。
1. Energy Stores and Transfer Pathways | 能量储存与转移途径
In the IGCSE course, energy can be held in eight main stores: chemical, kinetic, gravitational potential, elastic potential, thermal, nuclear, magnetic, and electrostatic. It is useful to describe energy changes in a system using these stores.
在 IGCSE 课程中，能量可以储存在八种主要形式中：化学能、动能、重力势能、弹性势能、热能、核能、磁能和静电势能。用这些储存形式描述系统中的能量变化是非常有用的。
Energy can move from one store to another through four pathways: mechanical working, electrical working, heating, and radiation. Light and sound transfer energy by radiation.
能量可以通过四种途径从一种储存形式转移到另一种：机械做功、电做功、加热和辐射。光和声通过辐射传递能量。
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Mechanical working: a force moves an object, e.g. a person pushing a trolley.
机械做功：力使物体移动，例如人推动手推车。
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Electrical working: a current transfers energy around a circuit, e.g. a fan connected to a battery.
电做功：电流在电路中传递能量，例如连接到电池上的风扇。
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Heating: energy transfers from a hotter region to a cooler region, e.g. a kettle heating water.
加热：能量从较热的区域传到较冷的区域，例如水壶烧水。
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Radiation: energy travels as waves, e.g. light from the Sun reaches Earth.
辐射：能量以波的形式传播，例如太阳光到达地球。
2. Conservation of Energy | 能量守恒
The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another or dissipated to the surroundings. The total energy in a closed system stays the same.
能量守恒定律指出：能量既不能被创造，也不能被消灭，只能从一种储存形式转换为另一种，或者耗散到周围环境中。在封闭系统中，总能量保持不变。
For any event, the total energy before the change equals the total energy after the change, including any energy transferred to the surroundings. For example, when a book falls from a shelf, its gravitational potential energy decreases while its kinetic energy increases.
在任何变化前后，总能量相等，包括转移到周围环境的能量。例如，一本书从书架落下时，其重力势能减少，而动能增加。
Total energy before = Total energy after
变化前总能量 = 变化后总能量
Although energy is conserved, it often becomes less useful. Wasted energy is usually transferred to thermal stores in the surroundings, making it harder to use again.
虽然能量守恒，但它往往会变得不那么有用。被浪费的能量通常转移到周围环境的热能储存中，使其难以再利用。
3. Kinetic Energy and Gravitational Potential Energy | 动能与重力势能
Kinetic energy is the energy stored in a moving object. It depends on the mass and speed of the object. The greater the mass or speed, the greater the kinetic energy.
动能是运动物体所储存的能量。它取决于物体的质量和速度。质量或速度越大，动能越大。
Eₖ = ½ × m × v²
动能 = ½ × 质量 × 速度²
In this equation, Eₖ is kinetic energy in joules (J), m is mass in kilograms (kg), and v is speed in metres per second (m/s). Because the speed is squared, doubling the speed makes the kinetic energy four times larger.
在这个方程中，Eₖ 是动能，单位是焦耳（J）；m 是质量，单位是千克（kg）；v 是速度，单位是米每秒（m/s）。由于速度被平方，速度加倍会使动能增加到原来的四倍。
Example: A 2 kg ball moves at 3 m/s. Its kinetic energy is ½ × 2 × 3² = 9 J.
例题：一个质量为 2 kg 的球以 3 m/s 的速度运动。它的动能为 ½ × 2 × 3² = 9 J。
Gravitational potential energy is the energy stored in an object due to its height above the ground. It depends on mass, gravitational field strength, and height.
重力势能是物体由于距地面一定高度而储存的能量。它取决于质量、重力场强度和高度。
Eₚ = m × g × h
重力势能 = 质量 × 重力场强度 × 高度
Here, Eₚ is gravitational potential energy in joules (J), m is mass in kilograms (kg), g is gravitational field strength in N/kg, and h is height in metres (m). On Earth, g is approximately 10 N/kg.
其中，Eₚ 是重力势能，单位是焦耳（J）；m 是质量，单位是千克（kg）；g 是重力场强度，单位是 N/kg；h 是高度，单位是米（m）。在地球上，g 约等于 10 N/kg。
4. Work Done and Power | 做功与功率
Work is done when a force causes an object to move in the direction of the force. The amount of work done is equal to the energy transferred.
当力使物体沿力的方向移动时，就做了功。做功的多少等于转移的能量。
W = F × d
功 = 力 × 距离
W is work done in joules (J), F is force in newtons (N), and d is distance moved in metres (m). If the object does not move, no work is done.
W 是功，单位是焦耳（J）；F 是力，单位是牛顿（N）；d 是物体沿力的方向移动的距离，单位是米（m）。如果物体没有移动，就没有做功。
Power is the rate at which energy is transferred or work is done. It tells us how fast energy is used.
功率是能量转移或做功的速率。它表示能量使用的快慢。
P = W ÷ t
功率 = 功 ÷ 时间
P is power in watts (W), W is work done or energy transferred in joules (J), and t is time in seconds (s). One watt is equal to one joule per second.
P 是功率，单位是瓦特（W）；W 是做功或转移的能量，单位是焦耳（J）；t 是时间，单位是秒（s）。1 瓦特等于 1 焦耳每秒。
For a constant force and velocity, power can also be calculated using the equation P = F × v.
当力和速度恒定时，功率也可以用方程 P = F × v 计算。
5. Energy Transfers in Systems | 系统中的能量转换
In a system, energy transfers often involve useful and wasted outputs. For example, a car engine transfers chemical energy from fuel into kinetic energy, but some energy is transferred to the surroundings as thermal energy and sound.
在一个系统中，能量转换通常包括有用输出和浪费输出。例如，汽车发动机将燃料中的化学能转化为动能，但部分能量以热能和声音的形式转移到周围环境中。
A filament lamp is a simple example. Electrical energy is transferred usefully to light energy, but much more is transferred as thermal energy to the surroundings.
白炽灯是一个简单的例子。电能被有用转化为光能，但更多的电能以热能的形式转移到周围环境中。
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Useful output: energy that is transferred to the intended store in the intended form.
有用输出：以预期形式转移到预期储存中的能量。
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Wasted output: energy that is transferred to an unintended store, usually thermal energy in the surroundings.
浪费输出：转移到非预期储存中的能量，通常是周围环境中的热能。
When drawing energy transfer diagrams, always show the initial energy store, the final energy store, and any energy that is dissipated to the surroundings.
在画能量转换图时，一定要标出初始能量储存、最终能量储存，以及耗散到周围环境中的能量。
6. Efficiency | 效率
Efficiency measures how well energy is transferred usefully. It is the proportion of input energy that becomes useful output energy.
效率衡量能量被有效利用的程度。它是有用输出能量占输入能量的比例。
Efficiency = (useful output energy ÷ total input energy) × 100%
效率 =（有用输出能量 ÷ 总输入能量）× 100%
Efficiency can also be calculated using power:
效率也可以用功率来计算：
Efficiency = (useful output power ÷ total input power) × 100%
效率 =（有用输出功率 ÷ 总输入功率）× 100%
Example: An electric motor receives 200 J of electrical energy and transfers 150 J to useful kinetic energy. Its efficiency is (150 ÷ 200) × 100% = 75%.
例题：一台电动机输入 200 J 电能，其中有 150 J 转换为有用动能。它的效率为 (150 ÷ 200) × 100% = 75%。
No machine can be 100% efficient because some energy is always transferred to the surroundings, usually as thermal energy. Efficiency has no unit and is often quoted as a percentage.
没有任何机械的效率可以达到 100%，因为总会有部分能量转移到周围环境中，通常以热能形式散失。效率没有单位，通常用百分数表示。
7. Sankey Diagrams | 桑基图
A Sankey diagram is a flow diagram in which the width of each arrow represents the amount of energy. It is used to show where input energy goes.
桑基图是一种流程图，其中每条箭头的宽度代表能量的大小。它用于展示输入能量的去向。
In a Sankey diagram, the thick incoming arrow on the left shows the total input energy. This arrow splits into a useful energy arrow and one or more wasted energy arrows.
在桑基图中，左侧较粗的箭头表示总输入能量。该箭头分流为有用能量箭头和一条或多条浪费能量箭头。
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Useful output arrow: points to the right and represents the energy that is transferred usefully.
有用输出箭头：指向右侧，表示被有效利用的能量。
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Wasted output arrows: usually slope downwards, showing energy transferred to the surroundings.
浪费输出箭头：通常向下倾斜，表示转移到周围环境的能量。
To read a Sankey diagram, measure the width of the useful arrow relative to the total incoming width. This gives the efficiency.
读桑基图时，比较有用箭头宽度与总输入箭头宽度，即可得到效率。
8. Reducing Thermal Energy Loss in the Home | 减少家庭热能损失
Heat loss from a house happens mainly by conduction through walls, roofs and windows, by convection through gaps, and by radiation from hot surfaces. Reducing these loses helps save energy and money.
房屋的热量损失主要通过墙壁、屋顶和窗户的传导、通过缝隙的对流，以及热表面的辐射发生。减少这些损失有助于节省能源和金钱。
Common methods include loft insulation, cavity wall insulation, double glazing, draught excluders and reflective foil behind radiators.
常见方法包括阁楼保温、空心墙保温、双层玻璃、防风条以及散热器背后的反射箔。
| Method | 方法 | How it reduces loss | 如何减少损失 |
| Loft insulation | 阁楼保温 | Traps air in fibres, reducing conduction and convection | 纤维中 trapped 空气，减少传导和对流 |
| Cavity wall insulation | 空心墙保温 | Fills the gap with insulating material, reducing heat transfer through walls | 用保温材料填充间隙，减少通过墙壁的热量传递 |
| Double glazing | 双层玻璃 | Air or inert gas between two panes reduces conduction | 两层玻璃之间的空气或惰性气体减少传导 |
| Draught excluders | 防风条 | Prevents convection of warm air through gaps | 防止暖空气通过缝隙对流 |
| Reflective foil | 反射箔 | Reflects thermal radiation back into the room | 将热辐射反射回房间 |
9. Energy Resources | 能源资源
Energy resources are divided into non-renewable and renewable categories. Fossil fuels and nuclear fuels are non-renewable because they are used up faster than they are formed. Renewable sources are those that will not run out.
能源资源分为不可再生能源和可再生能源。化石燃料和核燃料属于不可再生能源，因为它们的消耗速度远大于形成速度。可再生能源是不会耗尽的能源。
| Type | 类型 | Examples | 例子 | Advantages | 优点 | Disadvantages | 缺点 |
| Non-renewable | 不可再生 | Coal, oil, natural gas, nuclear | 煤、石油、天然气、核燃料 | Reliable and energy-dense | 可靠且能量密度高 | Produces pollution and greenhouse gases; will run out | 产生污染和温室气体；最终会耗尽 |
| Renewable | 可再生 | Solar, wind, hydroelectric, geothermal, tidal, biomass | 太阳能、风能、水力发电、地热能、潮汐能、生物质能 | Clean and sustainable | 清洁且可持续 | Less reliable; weather dependent; may be expensive to build | 可靠性较低；依赖天气；建设成本可能较高 |
In exam questions, compare resources using criteria such as reliability, pollution, land use, and running costs.
在考试题中，要使用可靠性、污染、土地占用和运行成本等标准来比较能源资源。
10. Key Equations Summary | 关键方程总结
The following table summarises the equations you should be able to use in Edexcel IGCSE Science.
下表总结了你在 Edexcel IGCSE 科学中应当会用的方程。
| Quantity | 物理量 | Equation | 方程 | 更多咨询请联系16621398022（同微信）
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