Work, Energy and Power | 功、能量与功率

📚 Work, Energy and Power | 功、能量与功率

In IGCSE Science, understanding work, energy and power is fundamental to explaining how and why things move, heat up or change. This article covers the key definitions, equations and real‑world applications you need for the Edexcel specification, linking concepts from physics to everyday phenomena.

在IGCSE科学课程中，理解功、能量与功率是解释物体如何以及为何运动、升温或变化的基础。本文涵盖Edexcel考试大纲所需的关键定义、方程式和实际应用，并将物理概念与日常现象联系起来。

1. What is Work? | 什么是功？

In physics, work is done when a force moves an object in the direction of the force. If you push a box and it slides across the floor, you are doing work on the box. However, if you push against a wall and the wall does not move, no work is done in the scientific sense, even though you may feel tired.

在物理学中，当一个力使物体沿力的方向移动时，就做了功。如果你推一个箱子，箱子在地板上滑动，你就对箱子做了功。然而，如果你推一堵墙而墙没动，科学意义上就没有做功，尽管你可能会感到累。

Work is a scalar quantity, meaning it only has magnitude and no direction. It is measured in joules (J), the same unit as energy. The amount of work done depends on two things: the size of the force applied and the distance moved in the direction of that force.

功是标量，表示它只有大小没有方向。它以焦耳（J）为单位，与能量的单位相同。做功的多少取决于两个因素：施加的力的大小以及物体沿该力方向移动的距离。

2. The Work Done Equation | 计算做功的方程

The work done by a constant force can be calculated using a simple equation:

恒力所做的功可以用一个简单的方程计算：

Work done (W) = Force (F) × Distance (d)

Or in symbols: W = F × d, where W is in joules (J), F in newtons (N) and d in metres (m). One joule is defined as the work done when a force of one newton moves an object through one metre in the direction of the force.

或者用符号表示：W = F × d，其中W以焦耳（J）为单位，F以牛顿（N）为单位，d以米（m）为单位。1焦耳的定义是：1牛顿的力使物体沿力的方向移动1米所做的功。

If the force is applied at an angle to the direction of motion, you must only consider the component of the force parallel to the displacement. In most IGCSE questions, forces and motion are in the same straight line, so the calculation is straightforward.

如果力与运动方向成一定角度，则只需考虑与位移平行的分力。在大多数IGCSE试题中，力与运动在同一直线上，因此计算很简单。

3. Units of Work and Energy | 功和能量的单位

The joule (J) is the SI unit for both work and energy. Larger amounts of energy are often expressed in kilojoules (kJ), where 1 kJ = 1000 J. Other common energy units include the calorie (cal) and the kilowatt‑hour (kW h), although in IGCSE science the joule is standard.

焦耳（J）是功和能量的国际单位制单位。较大的能量常用千焦（kJ）表示，1 kJ = 1000 J。其他常见的能量单位还有卡路里（cal）和千瓦时（kW h），但在IGCSE科学中标准单位是焦耳。

Energy is defined as the capacity to do work. When work is done, energy is transferred from one store to another. Therefore, any change in energy is also measured in joules.

能量被定义为做功的能力。当做功时，能量从一个储存库转移到另一个储存库。因此，能量的任何变化也以焦耳计量。

4. What is Energy? | 什么是能量？

Energy is a property that must be transferred to an object in order to perform work on, or to heat, the object. It exists in different forms and can be stored in various ways. Energy is never created or destroyed – it can only be transferred, stored or dissipated. This is the principle of conservation of energy.

能量是一种属性，必须传递给物体才能对其做功或使其升温。能量以不同形式存在，能以各种方式储存。能量既不会凭空产生也不会凭空消失——它只能被转移、储存或散失。这就是能量守恒原理。

In Edexcel IGCSE, we often describe energy in terms of stores. Examples include kinetic energy stores, gravitational potential energy stores, thermal (internal) energy stores, chemical energy stores and elastic potential energy stores.

在Edexcel IGCSE中，我们常用能量储存库来描述能量。例子包括动能储存、重力势能储存、内能（热）储存、化学能储存和弹性势能储存。

5. Forms of Energy and Energy Stores | 能量的形式与储存

Energy can appear in several forms, all measured in joules:

能量可以表现为多种形式，均以焦耳计量：

• Kinetic energy: energy of a moving object.
• Gravitational potential energy: energy stored due to an object’s height above the ground.
• Thermal energy: energy associated with the temperature of an object – the total kinetic and potential energy of its particles.
• Chemical energy: energy stored in bonds between atoms, released during chemical reactions.
• Elastic potential energy: energy stored in stretched or compressed objects like springs.
• Nuclear energy: energy stored in the nucleus of an atom, released during fission or fusion.
• Electrical energy: energy transferred by moving charges.
• Light (radiant) energy: energy carried by electromagnetic waves.
• Sound energy: energy carried by vibrating particles in a medium.

• 动能：运动物体的能量。
• 重力势能：物体因离地高度而储存的能量。
• 内能（热能）：与物体温度相关的能量——即其粒子的总动能和势能。
• 化学能：储存在原子间化学键中的能量，在化学反应中释放。
• 弹性势能：储存在被拉伸或压缩的物体（如弹簧）中的能量。
• 核能：储存在原子核中的能量，在裂变或聚变时释放。
• 电能：由电荷移动所传递的能量。
• 光能（辐射能）：电磁波携带的能量。
• 声能：介质中振动粒子携带的能量。

In Edexcel IGCSE, you must be able to identify the main energy stores and describe transfers between them in mechanical systems, heating and electrical appliances.

在Edexcel IGCSE中，你必须能够识别主要的能量储存库，并能描述机械系统、加热过程和电器中的能量转移。

6. Energy Transfers and Conservation of Energy | 能量转移与能量守恒

When a system changes, energy is transferred from one store to another. For instance, when a ball falls, gravitational potential energy is transferred to kinetic energy. If the ball hits the ground, some kinetic energy is transferred to thermal energy of the ball, the floor and the surrounding air, and some to sound energy.

当系统发生变化时，能量会从一个储存库转移到另一个储存库。例如，当球下落时，重力势能转化为动能。如果球撞击地面，一些动能转化为球、地面和周围空气的内能，还有一些转化为声能。

The total energy before and after a transfer is always the same. This is the law of conservation of energy. It means that energy cannot be lost, only turned into less useful forms like dissipated heat. Diagrams such as Sankey diagrams can represent energy transfers and efficiency.

转移发生前后的总能量始终相同。这是能量守恒定律。这意味着能量不会消失，只会转化为不太有用的形式，如散失的热量。像桑基图这样的图表可以表示能量转移和效率。

7. Gravitational Potential Energy (GPE) | 重力势能

Gravitational potential energy is the energy stored in an object because of its position in a gravitational field. Near the Earth’s surface, it can be calculated using:

重力势能是物体由于其在引力场中的位置而储存的能量。在地球表面附近，可以用下式计算：

GPE (Ep) = mass (m) × gravitational field strength (g) × height (h)

Ep = m g h

Where m is in kilograms (kg), g is in N/kg (on Earth, g = 9.8 N/kg, often approximated as 10 N/kg in IGCSE), and h is the vertical height in metres (m). GPE is measured in joules (J).

其中 m 的单位是千克（kg），g 的单位是 N/kg（地球上 g = 9.8 N/kg，IGCSE中常取10 N/kg），h 是垂直高度，单位为米（m）。重力势能的单位是焦耳（J）。

A lifted object gains GPE. When it falls, that GPE is converted into kinetic energy, assuming no air resistance. Understanding this conversion helps solve problems about falling objects, roller coasters and pendulum swings.

被举高的物体获得重力势能。下落时，假设没有空气阻力，重力势能转化为动能。理解这一转化有助于解决有关落体、过山车和摆锤的问题。

8. Kinetic Energy (KE) | 动能

All moving objects have kinetic energy. The kinetic energy of an object depends on its mass and its speed, and is given by:

所有运动的物体都具有动能。物体的动能取决于其质量和速率，并可由下式给出：

Kinetic Energy (Ek) = ½ × mass (m) × speed² (v²)

Ek = ½ m v²

Where m is in kilograms (kg), v is in metres per second (m/s), and Ek is in joules (J). The squared speed means that doubling the speed quadruples the kinetic energy – a vital safety consideration in vehicle collisions.

其中 m 的单位是千克（kg），v 是米每秒（m/s），Ek 是焦耳（J）。速度的平方意味着速度变为两倍时，动能变为原来的四倍——这是车辆碰撞中至关重要的安全考量。

In many IGCSE problems, you will be asked to link GPE and KE by assuming conservation of energy: mgh = ½ m v². Cancel mass, and you can find the speed of a falling object from a given height, or vice versa.

在许多IGCSE问题中，你会被要求通过假设能量守恒来联系重力势能和动能：mgh = ½ m v²。消去质量，你可以从给定高度求出下落物体的速度，反之亦然。

9. What is Power? | 什么是功率？

Power is the rate at which work is done or energy is transferred. A more powerful machine does the same amount of work in less time. Power is a scalar quantity and is measured in watts (W). One watt is equal to one joule per second (1 W = 1 J/s).

功率是做功或能量转移的速率。一台功率更大的机器能够在更短时间内做同样的功。功率是标量，以瓦特（W）为单位。1瓦特等于1焦耳每秒（1 W = 1 J/s）。

The equation linking power, work done (or energy transferred) and time is:

联系功率、所做的功（或转移的能量）和时间的方程式为：

Power (P) = Work done (W) / time (t)

P = W / t

If the energy transferred (ΔE) is used instead of work done, the equation becomes P = E / t. Remember to use seconds for time and joules for energy or work.

如果用转移的能量（ΔE）代替所做的功，方程则变为 P = E / t。记住时间用秒，能量或功用焦耳。

Alternative forms are useful when dealing with electrical appliances: P = I × V (current × voltage). This is part of the electricity topic, but it is another illustration of power in action.

在处理电器时，另一种形式很有用：P = I × V（电流 × 电压）。这是电学章节的一部分，但同样说明了功率的实际应用。

10. Efficiency and Energy Dissipation | 效率与能量耗散

No energy transfer is 100% efficient. Some energy is always dissipated, usually as heat, due to friction, air resistance or electrical resistance. The efficiency of a device or process tells us how much of the total input energy is converted into useful output energy.

没有能量转移是100%高效的。由于摩擦、空气阻力或电阻，总会有一些能量耗散，通常以热的形式散失。设备或过程的效率告诉我们，总输入能量中有多少转化为有用输出能量。

Efficiency can be calculated using:

效率可以通过以下公式计算：

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

Or using power:

或者使用功率：

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

Efficiency has no units; it is often expressed as a percentage. A perfect machine would have an efficiency of 1 (or 100%), but in practice values are always less due to dissipation. Electric heaters are nearly 100% efficient because all the energy transferred electrically is converted to thermal energy, but even they lose some as light or sound.

效率没有单位；通常以百分比表示。完美的机器效率为1（或100%），但实际上由于耗散，数值总是低于100%。电暖器近乎100%效率，因为所有电能都转化为内能，但即便如此仍有少量以光或声的形式散失。

Boosting efficiency reduces wasted energy, which saves fuel and reduces environmental impact. Designing streamlined shapes, using lubricants, and improving insulation are all ways to reduce unwanted energy transfers.

提高效率可以减少浪费的能量，从而节省燃料并降低对环境的影响。设计流线型外形、使用润滑剂和改进隔热都是减少不必要能量转移的方法。

11. Sankey Diagrams | 桑基图

A Sankey diagram is a visual representation of energy transfers. The width of each arrow is proportional to the amount of energy it represents. The input arrow is drawn on the left, and useful and wasted energy arrows branch to the right. The total width of the input equals the total width of all output arrows, illustrating conservation of energy.

桑基图是能量转移的可视化表示。每个箭头的宽度与其代表的能量大小成比例。输入箭头画在左侧，有用能量和浪费能量的箭头向右分支。输入的总宽度等于所有输出箭头的总宽度，这体现了能量守恒。

In an IGCSE exam, you might be asked to interpret a Sankey diagram, calculate efficiency from the proportions, or complete a diagram by drawing arrows of correct relative widths. It is a straightforward way to show how much energy ends up in useful forms and how much is dissipated.

在IGCSE考试中，你可能会被要求解读桑基图、根据比例计算效率，或通过绘制正确相对宽度的箭头来完成图表。这是一种直观展示多少能量最终成为有用形式、多少能量被耗散的方法。

12. Real‑World Applications and Exam Tips | 实际应用与考试技巧

These concepts appear in many everyday situations: the energy changes in a bouncing ball, the power of a light bulb, the fuel efficiency of a car, or the electricity consumption of appliances at home. When tackling exam questions, always start by identifying the initial and final energy stores, state the relevant equation, and show your working with correct units.

这些概念出现在许多日常情景中：弹跳球的能量变化、灯泡的功率、汽车的燃油效率或家用电器的耗电量。在处理考试题目时，始终要先确定初始和最终的能量储存库，写出相关方程，并在解题过程中展示步骤及正确单位。

Remember the key unit relationships: 1 J = 1 N m, 1 W = 1 J/s. In calculations involving GPE, use g = 10 N/kg unless told otherwise. Be careful to square the speed when calculating kinetic energy, and convert all distances to metres and masses to kilograms before substituting into formulas.

记住关键的单位关系：1 J = 1 N m，1 W = 1 J/s。在涉及重力势能的计算中，除非另有说明，使用 g = 10 N/kg。计算动能时注意要将速度平方，并在代入公式前将所有距离转换为米、质量转换为千克。

Practice drawing and reading Sankey diagrams, and always check whether a question asks for efficiency as a decimal or a percentage. With a strong grasp of work, energy and power, you will be well prepared for the mechanics and energy sections of your IGCSE Science paper.

练习绘制和阅读桑基图，并注意题目要求效率是用小数还是百分比表示。牢固掌握了功、能量和功率的知识后，你将为IGCSE科学试卷中的力学和能量部分做好充分准备。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）