Energy Transfers and Efficiency | 能量转移与效率

📚 Energy Transfers and Efficiency | 能量转移与效率

In Edexcel IGCSE Science, understanding how energy is stored, transferred and conserved lies at the heart of both physics and our everyday experiences. Energy can never be created or destroyed, only shifted from one store to another or moved between objects. This article explores energy stores, transfer pathways, work done, power, efficiency, the Sankey diagram, and the roles of conduction, convection and radiation. We will also practise calculations that are essential for exam success, using real-world examples such as electric motors, falling objects and home insulation.

在爱德思 IGCSE 科学课程中，理解能量如何被储存、转移和守恒是物理乃至日常经验的核心。能量既不能被创造也不能被消灭，只能从一个储能体转移到另一个储能体，或在物体之间移动。本文会探讨能量储存、转移路径、做功、功率、效率、桑基图以及传导、对流和辐射的作用。我们还会练习考试必备的计算，借助电动机、落体和家庭保温等真实例子。

1. Energy Stores | 能量储存

Energy can be held in a system in several distinct stores. The main stores you need to know for the IGCSE are: kinetic energy, gravitational potential energy, elastic potential energy, thermal (internal) energy, chemical energy, magnetic energy, electrostatic energy and nuclear energy. A system simply means the object or group of objects you are focusing on. When a book is lifted onto a shelf, the book–Earth system gains gravitational potential energy; when a moving car brakes, kinetic energy is dissipated as thermal energy in the brakes and surroundings.

能量可以储存在系统中的几种不同储能体中。IGCSE 需要掌握的主要储能体有：动能、重力势能、弹性势能、热（内）能、化学能、磁能、静电势能和核能。系统仅指你关注的一个物体或一组物体。当一本书被抬到书架上时，书–地球系统的重力势能增加；当行驶中的汽车刹车时，动能转化为热能耗散到刹车片和周围环境中。

Kinetic energy: energy of a moving object (depends on mass and speed).
动能：运动物体具有的能量（取决于质量和速度）。
Gravitational potential energy: energy due to an object’s height in a gravitational field.
重力势能：因物体在引力场中的高度而具有的能量。
Thermal energy: sum of kinetic and potential energies of particles in a substance.
热能：物质中粒子动能和势能的总和。
Chemical energy: stored in bonds of fuels, food and batteries.
化学能：储存在燃料、食物和电池的化学键中。
Elastic potential energy: stored when a material is stretched or compressed.
弹性势能：材料被拉伸或压缩时储存的能量。

2. Energy Transfer Pathways | 能量转移途径

Energy can move from one store to another through four transfer pathways: mechanically (by a force doing work), electrically (when a current flows), by heating (because of a temperature difference) and by radiation (e.g. light or sound waves). In any energy description, you should identify the initial store, the pathway and the final store. For example, when a bow is drawn to fire an arrow, chemical energy from the archer’s muscles is transferred mechanically to the elastic potential store of the bow; when the string is released, the elastic energy is transferred mechanically to the kinetic store of the arrow.

能量可以通过四种转移途径从一个储能体转移到另一个：机械做功（力作用过程中传递）、电做功（电流流动时）、加热（因温差）和辐射（例如光波或声波）。在任何能量描述中，你都需要明示初始储能体、转移途径和最终储能体。例如拉弓射箭时，弓箭手肌肉中的化学能通过机械做功转移为弓的弹性势能；释放弓弦时，弹性势能通过机械做功转移为箭的动能。

Mechanical pathway requires a force to move an object.
机械途径需要力使物体移动。
Electrical pathway requires charges to move through a potential difference.
电学途径需要电荷在电势差中移动。
Heating pathway requires a temperature difference between objects.
加热途径需要物体之间存在温度差。
Radiation pathway involves electromagnetic waves or sound travelling.
辐射途径包括电磁波或声音的传播。

3. The Principle of Conservation of Energy | 能量守恒原理

The total energy of a closed system always remains constant. Energy can be transferred usefully, stored or dissipated (spread out to the surroundings making it less useful), but it is never destroyed. When a mobile phone is used, the chemical energy in its battery is transferred electrically to light and sound, and also dissipated as thermal energy that warms the device slightly. The total energy at the start equals the total energy at the end, even if some has become harder to reuse.

封闭系统的总能量始终保持不变。能量可以有用转移、储存或耗散（散布到周围环境中导致可利用价值降低），但永远不会被消灭。使用手机时，电池中的化学能通过电路转移为光能和声能，同时一部分以热能形式耗散使设备轻微变热。起始时的总能量等于结束时的总能量，即使部分能量已难以再次利用。

Total energy before transfer = Total energy after transfer

转移前总能量 = 转移后总能量

Dissipation explains why processes are never 100% efficient. The ‘wasted’ energy is usually transferred to the thermal store of the surroundings, increasing the internal energy of the air or nearby objects. You need to use the term ‘dissipated’ precisely in exam answers to describe energy that becomes spread out and less useful.

耗散解释了为什么没有任何过程是100%高效的。“浪费”的能量通常会转移到周围环境的热能储能体中，增加空气或附近物体的内能。在考试答题时，你需要准确使用“耗散”一词来描述那些扩散开来且不再可利用的能量。

4. Calculating Kinetic and Potential Energy | 动能和势能的计算

Two equations are fundamental for quantitative problems in the IGCSE specification: kinetic energy and gravitational potential energy. Kinetic energy (KE) depends on mass m (kg) and speed v (m/s). The equation is:

两个方程是IGCSE课程中定量问题的基石：动能和重力势能。动能（KE）取决于质量m（千克）和速度v（米/秒），方程为：

KE = ½ m v²

Gravitational potential energy (GPE) depends on mass m (kg), gravitational field strength g (N/kg) and height h (m). On Earth, g is taken as 9.8 N/kg (though 10 N/kg is often allowed in exams).

重力势能（GPE）取决于质量m（千克）、引力场强度g（牛/千克）和高度h（米）。在地球上，g取9.8牛/千克（考试中常允许使用10牛/千克）。

GPE = m g h

When an object falls, its loss of GPE equals the gain in KE if air resistance is negligible. For instance, a 0.5 kg ball dropped from 8 m: GPE lost = 0.5 × 10 × 8 = 40 J. Its speed just before hitting the ground can be found by setting KE = 40 J, so ½ × 0.5 × v² = 40 → v² = 160 → v ≈ 12.6 m/s.

当物体下落且空气阻力可忽略时，减少的重力势能等于增加的动能。例如一个0.5千克的球从8米高处释放：损失的重力势能=0.5×10×8=40焦。落地前的速度可由KE=40 J求出，即½×0.5×v²=40 → v²=160 → v≈12.6米/秒。

5. Work Done and Energy Transferred | 做功与能量转移

In physics, ‘work done’ means energy transferred when a force moves an object. The work done W (J) equals force F (N) multiplied by distance d (m) moved in the direction of the force.

物理学中，“做功”指力使物体移动时传递的能量。做功W（焦）= 力F（牛）× 沿力方向移动的距离d（米）。

W = F d

If you lift a 20 N weight by 2 m, the work done against gravity is 20 × 2 = 40 J, and the object gains 40 J of gravitational potential energy. When friction is present, some of the work done by an applied force is transferred to thermal energy, warming the surfaces. In machines, the useful work output is always less than the total work input because friction and other losses dissipate energy.

若你把一个20牛的重物提升2米，克服重力做的功为20×2=40焦，物体获得40焦的重力势能。当有摩擦力时，施加力所作的一部分功会转化为热能，让接触面升温。在机器中，有用输出功总是小于总输入功，因为摩擦和其他损耗会耗散能量。

6. Power – The Rate of Energy Transfer | 功率 – 能量转移的速率

Power measures how quickly energy is transferred or work is done. The unit is the watt (W), where 1 W = 1 J/s. Two equivalent equations are used:

功率衡量能量转移或做功的快慢。单位是瓦特（W），1瓦=1焦/秒。两个等效公式为：

P = E / t (where E is energy transferred in J and t is time in s)

P = E / t (E为转移的能量/焦，t为时间/秒)

P = W / t (where W is work done in J)

A motor that lifts a 500 N crate through 6 m in 20 s does work W = 500 × 6 = 3000 J. Its power output is 3000 / 20 = 150 W. If the motor is 70% efficient, the input power must be 150 / 0.70 ≈ 214 W, with 64 W dissipated as heat and sound.

一台电动机在20秒内将500牛的板条箱提升6米，做功W=500×6=3000焦。其输出功率为3000/20=150瓦。若电动机效率为70%，则输入功率必为150/0.70≈214瓦，其中64瓦以热和声音形式耗散。

7. Efficiency – How Much Energy Is Useful | 效率 – 有多少能量是有用的

Efficiency compares the useful output energy or power with the total input energy or power. It can be expressed as a ratio or a percentage:

效率将有用的输出能量或功率与总输入能量或功率进行比较。可以表示为比值或百分比：

Efficiency = Useful energy output / Total energy input

效率 = 有用输出能量 / 总输入能量

Efficiency (%) = (Useful output / Total input) × 100%

No real device can reach 100% efficiency because some energy is always dissipated, usually as thermal energy due to friction, air resistance or electrical resistance. Low-energy light bulbs (LEDs) are much more efficient than filament bulbs because a much higher proportion of the electrical energy is transferred usefully to light rather than to wasteful heat.

没有任何真实设备能达到100%的效率，因为总有一部分能量被耗散，通常是因摩擦、空气阻力或电阻所产生的热能。LED低能耗灯泡比白炽灯效率高得多，因为它将大部分电能转化为有用的光，而不是浪费的热。

8. Sankey Diagrams | 桑基图

Sankey diagrams are visual representations of energy transfers in a system. The width of each arrow is proportional to the amount of energy it represents. The input energy is drawn as a thick arrow on the left, which then splits into the useful energy output (straight ahead) and one or more wasted energy arrows that curve downward or upward. The total width of the output arrows must equal the width of the input arrow, reflecting the conservation of energy.

桑基图是系统中能量转移的可视化表示。每条箭头的宽度与其代表的能量大小成正比。输入能量在左侧画作一条粗箭头，然后分成向前直行的有用输出能量以及一或多个向下或向上弯曲的浪费能量箭头。输出箭头的总宽度必须等于输入箭头的宽度，以体现能量守恒。

For a typical car engine: about 25% of the chemical energy in petrol is usefully transferred to kinetic energy of the car, while about 75% is dissipated as thermal energy (heated exhaust gases, engine coolant and friction). The Sankey diagram would show a wide input arrow, a narrower straight arrow for kinetic energy, and a much wider wasted thermal arrow branching off.

就典型汽车发动机而言：汽油中约25%的化学能有用转化为汽车的动能，而约75%以热能形式耗散（加热废气、发动机冷却液和摩擦）。桑基图中会显示一条宽阔的输入箭头、一条较窄的向前直行动能箭头，以及一条宽阔得多的分支废热箭头。

9. Reducing Unwanted Energy Transfers – Insulation | 减少不必要能量转移 – 隔热

In homes and machines, we often want to minimise dissipation so that less fuel or electricity is needed. Thermal insulation reduces the rate of heat transfer by conduction, convection and radiation. Loft insulation (fibreglass) traps air in small pockets; air is a poor conductor, so conduction is reduced. Cavity wall insulation fills the gap between inner and outer walls, reducing convection currents and conduction. Double-glazed windows use a thin layer of trapped gas (often argon) and low-emissivity coatings to cut conduction and radiation losses. Draught excluders stop warm air from leaving and cold air from entering by convection.

在家庭和机器中，我们常希望减少耗散以降低燃料或电力消耗。隔热保温可以降低传导、对流和辐射引起的传热速率。阁楼隔热层（玻璃纤维）将空气困在小囊中；空气导热性差，因而降低传导。空心墙隔热填充内外墙之间的空隙，减弱对流和传导。双层玻璃窗利用薄层被困气体（通常为氩气）和低辐射涂层来减少传导和辐射损失。门缝挡风条可阻止暖空气通过对流散失及冷空气进入。

10. Thermal Energy Transfer – Conduction, Convection and Radiation | 热能传递 – 传导、对流和辐射

Conduction is the transfer of thermal energy through a solid without any movement of the material as a whole. Vibrating particles pass energy to neighbouring particles; in metals, free electrons also diffuse rapidly, making metals excellent conductors. Convection occurs in fluids (liquids and gases) where warmer, less dense regions rise and cooler, denser regions sink, forming a convection current. Radiational energy transfer does not require a medium – all objects emit and absorb infrared radiation; dark, matt surfaces are better emitters and absorbers than light, shiny surfaces.

传导是热能在固体中传递而材料整体不发生移动的过程。振动的粒子将能量传递给相邻粒子；在金属中，自由电子也能迅速扩散，使金属成为优良导体。对流发生在流体（液体和气体）中：较暖、密度较低的区域上升，较冷、密度较高的区域下沉，形成对流循环。辐射能量转移不需要介质——所有物体都会发出和吸收红外辐射；深色、粗糙表面的发射和吸收能力强于浅色、光亮表面。

Example: A metal spoon in hot soup gets hot by conduction.
例子：放在热汤中的金属勺子通过传导变热。
Example: Hot air rises from a radiator, heating a room by convection.
例子：暖气片旁的热空气上升，通过对流加热房间。
Example: The Sun’s energy reaching Earth travels by radiation through the vacuum of space.
例子：太阳的能量以辐射形式穿越真空到达地球。

11. Gravitational, Elastic and Kinetic Energy Chains | 引力、弹性和动能链

Many IGCSE questions ask you to describe energy changes in a sequence. Consider a bouncing ball: when dropped, GPE → KE. Upon hitting the ground, the ball compresses and KE → elastic potential energy (EPE) + a little thermal energy. As it rebounds, EPE → KE + GPE, but it will not return to its original height because some energy has been dissipated. The bouncing height decreases each cycle. Another classic: a pendulum. At its highest points, the bob has maximum GPE; at its lowest, maximum KE. In an ideal (frictionless) pendulum, the sum of KE and GPE remains constant.

许多IGCSE考题要求你描述一个序列中的能量变化。以弹跳球为例：下落时，重力势能→动能。撞击地面时，球被压缩，动能→弹性势能（EPE）+少量热能。反弹时，弹性势能→动能+重力势能，但它不会回到初始高度，因为一部分能量已被耗散。弹跳高度逐次降低。另一个经典是摆锤：在最高点处，摆锤重力势能最大；在最低点处，动能最大。在理想（无摩擦）摆锤中，动能与重力势能之和保持不变。

12. Exam-Style Calculation Practice | 考试型计算练习

Let us work through a multi-step problem: A crane lifts a 1200 kg concrete block through 15 m at a constant speed in 30 s. Calculate (a) the work done, (b) the power output and (c) the energy dissipated if the crane’s motor is 80% efficient. Take g = 10 N/kg.

让我们演练一个多步骤问题：一架起重机在30秒内将1200千克的混凝土块匀速提升15米。计算(a)做功，(b)输出功率和(c)若起重机电动机效率为80%时的能量耗散。取g=10牛/千克。

Weight = mg = 1200 × 10 = 12 000 N. Work done = force × distance = 12 000 × 15 = 180 000 J. Output power P = W / t = 180 000 / 30 = 6000 W (6 kW). Useful output energy = 180 000 J; total input energy = useful output / efficiency = 180 000 / 0.80 = 225 000 J. Energy dissipated = total input − useful output = 225 000 − 180 000 = 45 000 J. This wasted energy heats the motor and surroundings.

重量=mg=1200×10=12 000牛。做功=力×距离=12 000×15=180 000焦。输出功率P=W/t=180 000/30=6000瓦（6千瓦）。有用输出能量=180 000焦；总输入能量=有用输出/效率=180 000/0.80=225 000焦。耗散能量=总输入−有用输出=225 000−180 000=45 000焦。这些浪费的能量使电动机和周围环境升温。

Always show working, use correct units and state the formula before substituting values. This maximises marks even if a calculation error is made.

务必展示计算过程，使用正确单位，在代入数值前先写出公式。这样即便算错也能最大化得分。

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Energy Transfers and Efficiency | 能量转移与效率