Understanding Energy Transfer and Efficiency | 能量传递与效率

📚 Understanding Energy Transfer and Efficiency | 能量传递与效率

In IGCSE Science, energy is not created or destroyed; it is transferred from one store to another. Efficiency tells us how much useful energy we get compared to the total energy put in. This article explains the key ideas, calculations, and real-world examples you need for your Edexcel exam.

在 IGCSE 科学中，能量不会被创造或消灭，只会从一种储能形式转移到另一种储能形式。效率告诉我们相对于输入的总能量，我们获得了多少有用能量。本文解释了你在 Edexcel 考试中需要掌握的关键概念、计算方法和真实案例。

1. Energy Stores and Transfers | 能量储存与转化

Energy can be stored in eight main ways: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, magnetic, and electrostatic. When something happens, energy is transferred between these stores.

能量主要有八种储存方式：动能、重力势能、弹性势能、热能、化学能、核能、磁能和静电势能。当某个事件发生时，能量在这些储存形式之间发生转化。

Kinetic energy: stored in moving objects. | 动能：运动物体储存的能量。
Gravitational potential energy: stored in objects at a height. | 重力势能：高处物体储存的能量。
Elastic potential energy: stored in stretched or compressed materials. | 弹性势能：被拉伸或压缩的物体储存的能量。
Thermal energy: stored in hot objects due to particle movement. | 热能：因粒子运动而在热物体中储存的能量。
Chemical energy: stored in fuels, food, and batteries. | 化学能：燃料、食物和电池中储存的能量。

When you switch on a lamp, electrical energy is transferred to light and thermal energy. In a car, chemical energy from fuel is transferred to kinetic energy, sound, and thermal energy. Always state the original store and the final useful store in exam answers.

当你打开电灯时，电能被转化为光能和热能。在汽车中，燃料的化学能被转化为动能、声能和热能。在考试答题时，务必说明初始的储能形式和最终的有用储能形式。

2. The Law of 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 or transformed. 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，只能被转化或转移。在封闭系统中，总能量保持不变。

Total energy before = Total energy after (if no energy leaves the system)

In a simple pendulum, the gravitational potential energy at the highest point is converted into kinetic energy at the lowest point. If we ignore air resistance, the total energy stays the same. In real situations, some energy is transferred to the surroundings by heating and sound, so the useful energy decreases.

在简单摆中，最高点的重力势能转化为最低点的动能。如果忽略空气阻力，总能量保持不变。在实际情况中，部分能量通过热传递和声音转移到了周围环境，因此有用能量减少。

3. What Is Efficiency? | 什么是效率？

Efficiency is a measure of how well a device converts input energy into useful output energy. It is always less than 100% in real devices because some energy is wasted, usually as thermal energy or sound.

效率是衡量设备将输入能量转化为有用输出能量的能力。实际设备中效率总是低于 100%，因为部分能量被浪费，通常以热能或声音的形式散发。

For example, an LED lamp may convert 40% of electrical energy to light, while the remaining 60% becomes thermal energy. A filament bulb is much less efficient, converting only about 5% of electrical energy to light.

例如，LED 灯可能将 40% 的电能转化为光能，其余 60% 变为热能。白炽灯效率低得多，仅将约 5% 的电能转化为光能。

4. Calculating Efficiency | 计算效率

Efficiency can be calculated using energy or power. The formula is:

效率可以用能量或功率来计算。公式为：

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

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

Power is the rate of energy transfer, measured in watts (W). 1 watt = 1 joule per second. When using the power formula, ensure both powers are in the same units.

功率是能量传递的速率，单位为瓦特（W）。1 瓦特 = 1 焦耳每秒。使用功率公式时，确保两个功率单位一致。

Worked example: A motor has an input power of 200 W and produces 150 W of useful mechanical power. Calculate its efficiency.

例题：一个电动机输入功率为 200 W，输出有用机械功率为 150 W。计算其效率。

Efficiency = (150 W ÷ 200 W) × 100% = 75%

Always write the formula, substitute the numbers, and give the unit in your final answer.

答题时一定要写公式、代入数值，并在最终答案中给出单位。

5. Sankey Diagrams | 桑基图

A Sankey diagram is a visual way to show energy transfers. The width of each arrow represents the amount of energy. The total width of the input arrow equals the sum of the useful and wasted output arrows.

桑基图是一种直观展示能量转化的图表。每条箭头的宽度代表能量多少。输入箭头的总宽度等于有用输出与浪费输出箭头的宽度之和。

To draw a Sankey diagram in an exam:

在考试中画桑基图时：

Draw a thick horizontal arrow for input energy. | 画一条粗的水平箭头表示输入能量。
Draw a horizontal arrow to the right for useful energy. | 向右画一条水平箭头表示有用能量。
Draw a downward or sloped arrow for wasted energy. | 向下或倾斜画一条箭头表示浪费的能量。
Label each arrow with its energy value in joules. | 在每条箭头上标注能量数值（焦耳）。

If an input of 100 J results in 45 J of useful light and 55 J of wasted heat, the useful arrow width is 45% of the input width, and the wasted arrow width is 55%.

如果输入 100 J，产生 45 J 有用光能和 55 J 废热，那么有用箭头宽度为输入宽度的 45%，浪费箭头宽度为 55%。

6. Reducing Energy Waste | 减少能量浪费

In homes and machines, wasted energy usually transfers to the surroundings by heating. Reducing this waste improves efficiency and saves money and fuel.

在家里和机器中，浪费的能量通常以热传递的形式散失到周围环境。减少这种浪费可以提高效率，节省金钱和燃料。

Lubricating moving parts reduces friction, which reduces thermal energy waste. | 润滑运动部件可以减少摩擦，从而减少热能浪费。
Adding insulation to loft walls reduces heat loss by conduction and convection. | 在阁楼墙壁上加装隔热层可减少热传导和对流造成的热量损失。
Using double glazing traps air between panes, reducing heat loss through windows. | 双层玻璃在玻璃板之间封存空气，减少窗户的热损失。
Fitting draught excluders reduces heat loss by convection through gaps. | 安装防风条可减少通过缝隙对流造成的热量损失。

In machines, using better bearings or more aerodynamic shapes reduces wasted energy due to friction and air resistance.

在机器中，使用更好的轴承或更流线型的形状可减少因摩擦和空气阻力造成的能量浪费。

7. Work, Power, and Energy Links | 功、功率与能量的联系

Work is done when a force moves an object. The amount of work done equals the energy transferred. Both are measured in joules (J).

当力使物体移动时，就做了功。做功的量等于传递的能量。两者的单位都是焦耳（J）。

Work done (J) = Force (N) × Distance moved in direction of force (m)

Power is the rate of doing work or the rate of energy transfer:

功率是做功的速率或能量传递的速率：

Power (W) = Energy transferred (J) ÷ Time taken (s)

A machine that transfers 500 J of energy in 10 s has a power of 50 W. If the same work is done in half the time, the power doubles.

一台机器在 10 秒内传递 500 J 能量，其功率为 50 W。如果相同功在一半时间内完成，功率则翻倍。

8. Efficiency in Everyday Devices | 日常生活中设备的效率

Different devices have different efficiencies. You should know approximate values for common appliances.

不同设备的效率不同。你应该知道常见电器的大致效率值。

Device	Useful output	Typical efficiency
LED lamp	Light	40–50%
Filament lamp	Light	~5%
Electric motor	Kinetic energy	70–90%
Petrol engine	Kinetic energy	20–30%
Human body	Muscle movement	~25%

Higher efficiency means less wasted energy and lower running costs. However, efficiency is not the only factor; initial cost, reliability, and environmental impact also matter.

效率越高意味着浪费的能量越少，运行成本越低。但效率不是唯一因素；初始成本、可靠性和环境影响也很重要。

9. Common Exam Mistakes | 常见考试错误

Students often lose marks by making simple errors. Avoid these common mistakes:

学生常常因为简单的错误而丢分。请避免以下常见错误：

Using the wrong formula: check whether the question gives energy values or power values. | 用错公式：要检查题目给的是能量值还是功率值。
Forgetting to multiply by 100 for a percentage. | 忘记乘以 100 得到百分数。
Not stating the unit (J or W) in the final answer. | 最终答案未写单位（J 或 W）。
Confusing “useful energy” with “total input energy”. | 混淆”有用能量”和”总输入能量”。
Thinking efficiency can be more than 100% – it cannot in real devices. | 认为效率可能超过 100% —— 实际设备中这是不可能的。

Always read the question carefully to identify whether you need to calculate energy efficiency or power efficiency.

答题前务必仔细阅读题目，判断需要计算能量效率还是功率效率。

10. Practice Questions | 练习题目

Try these questions to check your understanding.

尝试回答以下问题以检验你的理解。

Question 1: A microwave oven transfers 800 J of electrical energy. It produces 600 J of useful thermal energy in the food. Calculate the efficiency.

题目 1：一台微波炉传递 800 J 电能，其中 600 J 转化为食物中的有用热能。计算其效率。

Efficiency = (600 ÷ 800) × 100% = 75%

Question 2: An engine has an input power of 5000 W and a useful output power of 1500 W. What is its efficiency?

题目 2：一台发动机输入功率为 5000 W，有用输出功率为 1500 W。其效率是多少？

Efficiency = (1500 ÷ 5000) × 100% = 30%

Question 3: A kettle transfers 250 000 J of electrical energy. If its efficiency is 80%, how much useful thermal energy is transferred to the water?

题目 3：一个水壶传递 250 000 J 电能。若其效率为 80%，有多少有用热能传递到水中？

Useful energy = 0.80 × 250 000 = 200 000 J

Check your answers and review the method if you made any errors.

核对你的答案，如有错误请回顾解题方法。

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Understanding Energy Transfer and Efficiency | 能量传递与效率