📚 Energy Transfers and Efficiency | 能量转移与效率
In IGCSE Science, one of the most important ideas is that energy can be transferred from one store to another, but it cannot be created or destroyed. Understanding how energy moves through systems, and how much of it ends up where we want it, is a key skill for exams and for real-world problem solving.
在 IGCSE 科学课程中，最重要的概念之一就是能量可以从一种储存形式转移到另一种储存形式，但它既不能被创造，也不能被消灭。理解能量如何在系统中移动，以及其中有多少最终到达我们想要的地方，是应对考试和解决现实问题的一项关键技能。
1. Energy Stores and Transfers | 能量储存与转移
Energy can be stored in different ways. The main stores you need to know are: kinetic (movement), gravitational potential (height), elastic potential (stretched or squashed), thermal (heat), chemical (bonds), nuclear, magnetic, and electrostatic. When something happens, energy is transferred from one store to another or from one object to another.
能量可以以不同的方式储存。你需要掌握的主要储存形式包括：动能（运动）、重力势能（高度）、弹性势能（拉伸或压缩）、热能（热量）、化学能（化学键）、核能、磁能和静电能。当某件事发生时，能量会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。
For example, when you lift a book, chemical energy in your muscles is transferred to the book’s gravitational potential energy. When you drop it, the gravitational potential energy is transferred to kinetic energy as it falls, and then to thermal energy and sound when it hits the floor.
例如，当你举起一本书时，肌肉中的化学能转移到书的重力势能中。当你松手时，随着书下落，重力势能转化为动能，然后在书撞击地面时转化为热能和声能。
- Kinetic energy = ½ × mass × speed² | 动能 = ½ × 质量 × 速度²
- Gravitational potential energy = mass × gravitational field strength × height | 重力势能 = 质量 × 重力场强度 × 高度
- Elastic potential energy = ½ × spring constant × extension² | 弹性势能 = ½ × 劲度系数 × 伸长量²
Eₖ = ½ m v² Eₚ = m g h Eₑ = ½ k e²
The symbol Δ means “change in”. When calculating energy changes, always identify which store is increasing and which is decreasing.
符号 Δ 表示“变化量”。在计算能量变化时，一定要先判断哪种储存形式在增加、哪种在减少。
2. Conservation of Energy | 能量守恒定律
The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another. The total energy before a change is always equal to the total energy after the change. This means the total energy in a closed system remains constant.
能量守恒定律指出：能量不能被创造或消灭，只能从一种储存形式转移到另一种储存形式。变化前的总能量总是等于变化后的总能量。这意味着在封闭系统中，总能量保持不变。
Consider a pendulum swinging. At the highest point, it has maximum gravitational potential energy and zero kinetic energy. At the lowest point, it has maximum kinetic energy and minimum gravitational potential energy. If there were no air resistance, the pendulum would swing forever. In reality, some mechanical energy is transferred to thermal energy due to air resistance and friction at the pivot.
以单摆为例。在最高点，它具有最大的重力势能和零动能。在最低点，它具有最大的动能和最小的重力势能。如果没有空气阻力，单摆会永远摆动。而实际上，由于空气阻力和支点处的摩擦，部分机械能会转化为热能。
Total energy before = Total energy after | 变化前总能量 = 变化后总能量
In exam questions, always look for energy transfers that are “wasted” – these are usually thermal energy transfers to the surroundings. The useful energy is the part that does what you want it to do.
在考试题目中，务必找出“浪费”的能量转移——这些通常是以热能形式转移到周围环境中的部分。有用能量是完成你所期望工作的那部分能量。
3. Useful and Wasted Energy | 有用能量与浪费能量
When a device transfers energy, not all of it is used for the intended purpose. The energy that is used for the intended purpose is called useful energy. Energy that is not usefully transferred is called wasted energy, and it often ends up as thermal energy in the surroundings.
当设备进行能量转移时，并非所有能量都用于预期目的。用于预期目的的能量称为有用能量。未被有效转移的能量称为浪费能量，它通常以热量的形式散失到周围环境中。
Useful energy is measured in joules (J). Wasted energy is also measured in joules. The total energy input is the sum of useful energy output and wasted energy output.
有用能量以焦耳（J）为单位测量，浪费能量同样以焦耳（J）为单位。总输入能量等于有用能量输出加上浪费能量输出。
- Useful energy | 有用能量：the energy transferred to where it is wanted, in the form that is wanted
- Wasted energy | 浪费能量：the energy transferred to where it is not wanted, or in a form that is not useful
- Input energy | 输入能量：the total energy supplied to the device or system
For example, in an electric lamp, the useful energy is light. The wasted energy is thermal energy. In a car engine, the useful energy is kinetic energy, while wasted energy includes heat from the exhaust, sound, and heat from friction between moving parts.
例如，在电灯中，有用能量是光，浪费能量是热能。在汽车发动机中，有用能量是动能，而浪费能量包括排气管排出的热量、声音以及运动部件之间摩擦产生的热量。
Total input energy = Useful output energy + Wasted output energy | 总输入能量 = 有用输出能量 + 浪费输出能量
4. Efficiency Calculations | 效率计算
Efficiency tells us what fraction of the input energy is converted into useful energy. It can be written as a decimal, a fraction, or a percentage. The higher the efficiency, the less energy is wasted.
效率告诉我们输入能量中有多少比例被转化为有用能量。它可以写成小数、分数或百分比。效率越高，浪费的能量越少。
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, where power is the rate of energy transfer measured in watts (W).
效率也可以用功率来计算，功率是衡量能量转移速率的物理量，单位是瓦特（W）。
Efficiency = (Useful output power ÷ Total input power) × 100%
Example: An electric motor is supplied with 600 J of electrical energy. It produces 480 J of kinetic energy. Calculate its efficiency.
例题：一台电动机输入 600 J 电能，输出 480 J 动能。计算其效率。
Efficiency = (480 ÷ 600) × 100% = 0.8 × 100% = 80%
The remaining 20% is wasted as thermal and sound energy. Notice that efficiency has no units – it is simply a ratio or percentage.
剩余的 20% 作为热能和声能浪费掉了。注意效率没有单位——它只是一个比值或百分比。
5. Reducing Energy Loss | 减少能量损失
Reducing wasted energy is important for saving money, saving fuel, and reducing environmental impact. There are several methods used in homes and machines to improve efficiency.
减少浪费能量对于节约资金、节省燃料和降低环境影响非常重要。家庭和机械设备中有多种方法可以提高效率。
| Method | 方法 | How it reduces waste | 如何减少浪费 |
| Lubrication | 润滑 | Reduces friction between moving parts, reducing thermal energy loss | 减少运动部件之间的摩擦，降低热能损失 |
| Thermal insulation | 保温隔热 | Prevents heat from escaping from buildings, ovens and pipes | 防止热量从建筑物、烤箱和管道中散失 |
| Streamlining | 流线型设计 | Reduces air resistance for vehicles, saving fuel | 减少车辆所受的空气阻力，节省燃料 |
| Lower-friction tyres | 低摩擦轮胎 | Reduce rolling resistance in cars | 减少汽车的滚动阻力 |
| Condensing boilers | 冷凝锅炉 | Capture heat from exhaust gas that would otherwise be lost | 回收本来会散失的废气热量 |
In homes, cavity wall insulation, loft insulation, double glazing, and draught excluders all reduce unwanted heat transfers. These all help increase efficiency and reduce fuel bills.
在家庭中，空心墙保温、阁楼保温、双层玻璃和防风条都能减少不必要的热量传递。这些方法都有助于提高效率、减少燃料费用。
Remember: lubrication always reduces friction, but friction is not always wasted – in brakes and tyres, friction is essential for useful energy transfer.
请记住：润滑总是减少摩擦，但摩擦并不总是浪费——在刹车和轮胎中，摩擦力是实现有用能量转移的必需条件。
6. Sankey Diagrams | 桑基图
A Sankey diagram is a scale drawing that shows energy transfers. The thickness of each arrow represents the amount of energy. The input arrow is drawn on the left. The useful output arrow points to the right. The wasted energy arrow points downwards.
桑基图是一种按比例绘制的能量转移示意图。箭头的粗细表示能量的多少。输入箭头画在左侧，有用输出箭头指向右侧，浪费能量箭头指向下方。
Example: A light bulb has 100 J of input energy, gives 10 J of light and 90 J of heat.
示例：一只灯泡输入能量 100 J，输出光能 10 J，热能 90 J。
- Input arrow: 100 J wide, drawn horizontally | 输入箭头：宽度表示 100 J，水平绘制
- Useful output (light): 10 J wide, drawn horizontally right | 有用输出（光能）：宽度表示 10 J，水平向右
- Wasted output (heat): 90 J wide, drawn downwards | 浪费输出（热能）：宽度表示 90 J，向下绘制
From the diagram you can immediately see the efficiency. In this case, efficiency = 10%, which is typical for a filament lamp. LED lamps are much more efficient, converting around 40–50% of input energy into light.
从图中可以立即看出效率。在这个例子中，效率 = 10%，这是白炽灯的典型值。LED 灯泡的效率要高得多，能将大约 40%–50% 的输入能量转化为光能。
When drawing a Sankey diagram in an exam, always use a ruler and make sure the widths are roughly to scale. Label each arrow with the type of energy and its value in joules.
在考试中绘制桑基图时，务必使用直尺，并确保箭头宽度大致按比例。在每条箭头上标注能量类型及其以焦耳为单位的值。
7. Energy and Power | 能量与功率
Power is the rate at which energy is transferred. It is measured in watts (W). One watt is equal to one joule per second. A common formula is: Power = Energy ÷ Time.
功率是能量转移的速率。它用瓦特（W）来计量。1 瓦特等于每秒 1 焦耳。常用公式为：功率 = 能量 ÷ 时间。
P = E ÷ t or E = P × t
For example, if a device uses 2000 J of energy in 50 seconds, its power is 2000 ÷ 50 = 40 W. If you know the power rating of an appliance, you can calculate how much energy it uses over a given time.
例如，如果一个设备在 50 秒内消耗 2000 J 能量，其功率为 2000 ÷ 50 = 40 W。如果你知道电器的额定功率，就可以计算它在一段时间内消耗的能量。
Exam questions often combine power and efficiency. You might be given the input power and the efficiency, and asked to find the useful output power.
考试题常常将功率与效率结合起来。题目可能给出输入功率和效率，让你求有用输出功率。
Worked example: A hairdryer has an input power of 1200 W and an efficiency of 60%. What is the useful output power?
例题：某吹风机输入功率为 1200 W，效率为 60%。有用输出功率是多少？
Useful output power = 1200 W × 0.6 = 720 W
The remaining 480 W is wasted, mainly as heat and sound.
剩余的 480 W 被浪费，主要以热和声的形式散失。
8. Common Exam Mistakes | 常见考试错误
Students often make the same types of mistakes when answering energy and efficiency questions. Being aware of these can save you valuable marks.
学生在回答能量与效率问题时，经常犯类似错误。了解这些错误可以帮助你避免失分。
- Using the wrong units: Energy is in joules (J), power is in watts (W), time is in seconds (s). Never mix them up.
- Forgetting to multiply by 100%: Efficiency as a percentage requires the ×100%. As a decimal, it should be between 0 and 1.
- Not reading the question: Check whether the question asks for efficiency as a decimal, fraction, or percentage.
- Ignoring gravitational field strength (g): On Earth g = 10 N/kg (Edexcel IGCSE). Some questions may give a different value.
- Saying energy is “used up”: Energy is never used up; it is transferred to other stores. Use the phrase “transferred” or “dissipated”.
Units reminder: 1 kJ = 1000 J, 1 MJ = 1,000,000 J. If a question gives energy in kJ, always convert to J before calculating power or efficiency.
单位提醒：1 kJ = 1000 J，1 MJ = 1,000,000 J。如果题目给出的能量单位是 kJ，在计算功率或效率之前务必先转换为 J。
When answering “explain” questions, use the words “useful energy” and “wasted energy”. For higher marks, mention the specific energy stores involved and the specific way energy is wasted (for example, “thermal energy transferred to the surroundings due to friction”).
在回答“解释”类问题时，请使用“有用能量”和“浪费能量”这两个词。要获得更高分数，需要指出涉及的具体能量储存形式以及能量被浪费的具体方式（例如“由于摩擦，以热能形式转移到周围环境中”）。
9. Exam Strategy and Practice | 应试策略与练习
When you see an energy question in the Edexcel IGCSE Science exam, follow these steps: underline the key quantities, check the units, identify the useful energy store, write down the correct formula, substitute the numbers, and finish with the correct unit.
在 Edexcel IGCSE 科学考试中遇到能量题时，请按以下步骤操作：圈出关键量，检查单位，判断有用能量储存形式，写下正确的公式，代入数值，最后标注正确的单位。
For calculation questions, always show your working. Edexcel awards method marks even if the final answer is wrong. For practical investigations, be ready to describe how to measure energy changes – for example, heating water with an immersion heater and using E = P × t, or dropping a falling object and measuring its speed.
对于计算题，务必写出过程。Edexcel 考试即使最终答案错误，也会给方法分。对于实验题，要准备好描述如何测量能量变化——例如，用浸入式加热器加热水并用 E = P × t 计算能量，或者让物体下落并测量其速度。
Key words to remember: conservation, transfer, store, useful, wasted, efficiency, input, output, dissipation, rate.
需要记住的关键词：守恒、转移、储存、有用、浪费、效率、输入、输出、耗散、速率。
Practice by calculating the efficiency of everyday devices: a kettle, a TV, a bicycle. Ask yourself: what is the useful energy output? What is being wasted? This habit will make exam questions feel familiar and manageable.
练习时，可以计算日常设备的效率：水壶、电视机、自行车。问问自己：有用能量输出是什么？什么被浪费了？养成这个习惯后，考试题会显得熟悉且容易应对。
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