📚 Energy Transfers and Efficiency | 能量转换与效率
Energy is a core concept in IGCSE Science. Understanding how energy is transferred, conserved, and wasted is essential for solving problems in physics, chemistry, and biology. This revision guide covers the key ideas you need for your Edexcel exam.
能量是 IGCSE 科学的核心概念。理解能量如何转换、守恒和耗散，对于解决物理、化学和生物中的问题至关重要。本复习指南涵盖了 Edexcel 考试所需的关键知识点。
1. Forms of Energy | 能量的形式
Energy exists in many different forms. The main ones you need to know are: kinetic (movement), gravitational potential, elastic potential, chemical, thermal (heat), electrical, light, and sound.
能量以多种形式存在。你需要掌握的主要形式有：动能（运动）、重力势能、弹性势能、化学能、热能（热量）、电能、光能和声能。
- Kinetic energy – energy of an object due to its motion.
- 动能 – 物体因运动而具有的能量。
- Gravitational potential energy – energy stored in an object due to its height above the ground.
- 重力势能 – 物体因高于地面而储存的能量。
- Chemical energy – energy stored in bonds between atoms, released in reactions.
- 化学能 – 原子间化学键中储存的能量，在反应中释放。
- Thermal energy – energy associated with the temperature of a substance.
- 热能 – 与物质温度相关的能量。
2. Conservation of Energy | 能量守恒
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one form to another. The total energy before a change equals the total energy after the change.
能量守恒定律指出：能量不能被创造或毁灭，只能从一种形式转换为另一种形式。变化前的总能量等于变化后的总能量。
Total energy before = Total energy after
变化前总能量 = 变化后总能量
For example, when a ball is dropped, its gravitational potential energy is converted into kinetic energy as it falls. Just before it hits the ground, almost all the potential energy has become kinetic energy.
例如，当一个小球下落时，它的重力势能转化为动能。在即将落地时，几乎所有势能都变成了动能。
3. Energy Transfer Diagrams | 能量转换图
We can represent energy transfers using chain diagrams or flow diagrams. An arrow shows one energy form changing into another.
我们可以用链式图或流程图来表示能量转换。箭头表示一种能量形式变成另一种形式。
Chemical → Kinetic → Electrical
化学能 → 动能 → 电能
This is how a generator works: fuel burns (chemical) to heat water (thermal), steam spins a turbine (kinetic), which turns a generator (electrical).
发电机就是这样工作的：燃料燃烧（化学能）加热水（热能），蒸汽带动涡轮机（动能），涡轮机带动发电机（电能）。
4. Useful and Wasted Energy | 有用能与废能
In every energy transfer, some energy is transferred usefully and some is wasted. Wasted energy is usually transferred to the surroundings as thermal energy, making the surroundings warmer.
在每一次能量转换中，部分能量被有效利用，部分被浪费。浪费的能量通常以热能形式转移到周围环境中，使环境变暖。
| Example | Useful energy | Wasted energy |
| Electric lamp | Light | Thermal |
| Electric motor | Kinetic | Thermal + sound |
| Television | Light + sound | Thermal |
In your exam, you may be asked to identify the useful and wasted forms of energy for a device.
考试中，你可能会被要求指出某个设备的有效能和废能形式。
5. Efficiency | 效率
Efficiency tells us how much of the total input energy is transferred to useful output energy. It is calculated as a fraction or a percentage.
效率表示总输入能量中有多少被转化为有用的输出能量。它可以表示成分数或百分比。
Efficiency = (Useful output energy ÷ Total input energy) × 100%
效率 = (有用输出能量 ÷ 总输入能量) × 100%
Example: A light bulb uses 100 J of electrical energy and produces 15 J of light. Its efficiency is (15 ÷ 100) × 100% = 15%.
例如：一个灯泡消耗 100 J 电能，产生 15 J 光能。其效率为 (15 ÷ 100) × 100% = 15%。
Efficiency is always less than 100% in real devices, except in theory.
实际设备的效率总是低于 100%，除理论情况外。
6. Sankey Diagrams | 桑基图
A Sankey diagram is a flow diagram where the width of the arrow represents the amount of energy. The left side shows input energy; the right splits into useful and wasted energy.
桑基图是一种流程图，箭头的宽度表示能量的大小。左侧显示输入能量，右侧分为有用能和废能。
Input energy ──→ Useful energy
↘ Wasted energy
When drawing a Sankey diagram, make sure the total width of the output branches equals the total width of the input arrow.
画桑基图时，确保输出分支的总宽度等于输入箭头的总宽度。
7. Kinetic and Potential Energy Calculations | 动能与势能计算
You need to recall the equations for kinetic energy and gravitational potential energy. These are frequently tested in IGCSE Science.
你需要记住动能和重力势能的公式。这些在 IGCSE 科学中经常考查。
Kinetic energy (KE) = ½ × mass × velocity²
动能 = ½ × 质量 × 速度²
Gravitational potential energy (GPE) = mass × gravitational field strength × height
重力势能 = 质量 × 重力场强度 × 高度
On Earth, the gravitational field strength is approximately 10 N/kg. Always check the question for the exact value.
在地球上，重力场强度约为 10 N/kg。做题时请务必检查题目给出的具体数值。
8. Work Done and Power | 做功与功率
Work is done when a force moves an object over a distance in the direction of the force. Energy transferred equals work done.
当力使物体在力的方向上移动一段距离时，就做了功。转移的能量等于所做的功。
Work done = Force × Distance
功 = 力 × 距离
Power is the rate of transfer of energy, or the rate of doing work. It is measured in watts (W), where 1 W = 1 J/s.
功率是能量转换的速率，也就是做功的速率。其单位是瓦特 (W)，1 W = 1 J/s。
Power = Work done ÷ Time
功率 = 功 ÷ 时间
9. Energy Resources | 能源资源
Energy resources can be renewable or non-renewable. Renewable resources are replenished naturally over short timescales; non-renewable resources will eventually run out.
能源资源可分为可再生能源和不可再生能源。可再生能源在较短时间内能自然补充；不可再生能源终将耗尽。
| Renewable 可再生 | Non-renewable 不可再生 |
| Solar, wind, hydroelectric, geothermal, biomass, tidal | Coal, oil, natural gas, nuclear |
For each resource, you should understand the main advantage and disadvantage. For example, solar is clean but not available at night; wind is free but unreliable.
对于每种资源，你需要了解其主要优点和缺点。例如，太阳能清洁但夜间不可用；风能免费但不稳定。
10. Practical: Investigating Energy Transfer | 实验：探究能量转换
A common experiment is to measure the efficiency of an electric heater. You can measure the electrical energy input using a joulemeter or power meter, and the useful thermal energy gained by the water using the equation: energy = mass × specific heat capacity × temperature change.
一个常见实验是测量电加热器的效率。你可以使用焦耳计或功率计测量输入的电能，并用水吸收的热量来计算有用热能，公式为：能量 = 质量 × 比热容 × 温度变化。
Always evaluate experimental errors: heat is lost to the surroundings, and the thermometer may not be fully immersed.
始终要评估实验误差：热量会散失到周围环境中，温度计可能没有完全浸入。
11. Exam Tips | 考试技巧
When answering energy questions, always state the energy form correctly and identify the ‘useful’ and ‘wasted’ outputs. Show all working in calculations and include units.
解答能量问题时，务必正确表述能量形式，并指明”有用”和”浪费”的输出。计算时写出全部步骤并带上单位。
- Learn the equations by heart, not just the symbols.
- 注意区分公式，不要混淆。
- Convert units before substituting: kJ → J, g → kg.
- 代入前先统一单位：kJ → J，g → kg。
- Check that your final answer makes sense physically.
- 检查你的最终答案是否符合物理常识。
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