📚 IB & OCR Science: Energy Key Concepts | IB OCR 科学：能量考点精讲
Energy is a unifying concept across IB and OCR science disciplines. Whether you are tackling physics, chemistry, or environmental systems, a solid grasp of energy stores, transfers, and conservation is essential for exam success. This guide breaks down every core energy topic with bilingual explanations, key equations, and practical study tips.
能量是贯穿 IB 与 OCR 科学课程的核心理念。无论你面对的是物理、化学还是环境系统，扎实掌握能量的储存、转移与守恒是考试制胜的关键。本文用双语拆解每一个核心能量考点，提供关键公式与实用备考建议。
1. What Is Energy? | 什么是能量？
Energy is defined as the capacity to do work. It is a scalar quantity, meaning it has magnitude but no direction. The SI unit of energy is the joule (J). One joule is equal to the work done when a force of one newton moves an object one metre in the direction of the force.
能量被定义为做功的能力。它是一个标量，只有大小没有方向。能量的国际单位是焦耳（J）。1 焦耳表示 1 牛顿的力使物体沿力的方向移动 1 米所做的功。
In everyday language, energy is often described as something that makes things happen. In science, we categorise energy into different stores and pathways to analyse changes in systems.
在日常语言中，能量常被描述为让事情发生的东西。在科学中，我们将能量归入不同的储存方式和转移途径，以便分析系统的变化。
2. Energy Stores and Systems | 能量的储存与系统
Energy can be stored in various forms: kinetic, gravitational potential, elastic potential, thermal (internal), chemical, nuclear, magnetic, and electrostatic. When describing energy changes, it is helpful to define the system (the object or group of objects we are studying) and the surroundings.
能量可以以多种形式储存：动能、重力势能、弹性势能、热（内）能、化学能、核能、磁能和静电势能。在描述能量变化时，明确系统（我们研究的对象或一组对象）与环境非常有用。
An open system allows both matter and energy to exchange with its surroundings. A closed system only exchanges energy, while an isolated system exchanges neither. In many exam questions, you will assume a closed system where total energy is conserved.
开放系统允许物质和能量与环境交换。封闭系统只交换能量，而孤立系统两者都不交换。在许多试题中，你会默认一个封闭系统，总能量保持不变。
3. Kinetic Energy | 动能
Kinetic energy (KE) is the energy an object possesses due to its motion. It depends on the object’s mass and speed. The equation is:
动能（KE）是物体由于运动而具有的能量。它取决于物体的质量和速度。公式为：
KE = ½ m v²
Where m is mass in kilograms (kg), v is speed in metres per second (m/s), and KE is in joules (J). Note that the speed is squared, meaning doubling the speed quadruples the kinetic energy. This is a common source of misconception: kinetic energy does not increase linearly with speed.
其中 m 为质量，单位千克（kg）；v 为速率，单位米/秒（m/s）；KE 单位为焦耳（J）。注意速率是平方的，速率加倍会使动能变为原来的四倍。这是常见的误区：动能并不随速率线性增长。
When solving problems, ensure all units are in SI. Convert grams to kilograms and centimetres per second to metres per second if necessary.
解题时，务必统一国际单位。如有必要，将克转为千克，将厘米/秒转为米/秒。
4. Gravitational Potential Energy | 重力势能
Gravitational potential energy (GPE) is the energy stored in an object due to its position in a gravitational field. The equation is:
重力势能（GPE）是物体因在引力场中的位置而储存的能量。公式为：
GPE = m g h
Here, m is mass (kg), g is the gravitational field strength (approximately 9.8 N/kg on Earth, often rounded to 10 N/kg in OCR exam papers), and h is the height above a reference point (m). GPE is directly proportional to both mass and height.
其中 m 为质量（kg），g 为引力场强度（地球表面约 9.8 N/kg，OCR 试卷中常取 10 N/kg），h 为相对于参考点的高度（m）。GPE 与质量和高度均成正比。
This equation assumes the height change is small compared to Earth’s radius, so g remains constant. For IB Physics HL, you may also encounter GPE in a radial field: GPE = -GMm/r.
此公式假设高度的变化远小于地球半径，因此 g 可视为常数。对于 IB 物理高阶课程，你还可能接触到径向场中的重力势能公式：GPE = -GMm/r。
5. Elastic Potential Energy | 弹性势能
Elastic potential energy (EPE) is the energy stored in an object when it is stretched or compressed, provided the object obeys Hooke’s Law. The equation is:
弹性势能（EPE）是物体在拉伸或压缩时储存的能量，前提是物体遵循胡克定律。公式为：
EPE = ½ k x²
k is the spring constant (N/m), and x is the extension or compression from the equilibrium position (m). Like kinetic energy, the extension is squared, so doubling the deformation quadruples the stored energy.
k 为劲度系数（N/m），x 为相对于平衡位置的伸长量或压缩量（m）。与动能类似，形变量是平方的，因此形变加倍会使储存的能量变为四倍。
Remember that Hooke’s Law within its elastic limit states that F = k x. The work done in stretching the spring is equal to the area under the force-extension graph, which gives rise to the ½ factor in the energy equation.
记住，在弹性限度内，胡克定律 F = k x。拉伸弹簧所做的功等于力-伸长量图下的面积，这就是能量公式中 ½ 因子的来源。
6. Work Done and Energy Transfer | 做功与能量转移
Work is the transfer of energy by a force. When a force moves an object, work is done. The work done is given by:
功是通过力进行的能量转移。当一个力使物体移动时，我们就说做功了。做功的计算公式为：
W = F d cos θ
F is the force (N), d is the displacement (m), and θ is the angle between the force and displacement directions. In simple cases where force and displacement are parallel, W = F d. This is the central link between mechanical work and energy changes.
F 为力（N），d 为位移（m），θ 为力与位移方向之间的夹角。在力与位移平行的情况下，简化为 W = F d。这是机械功与能量变化之间的核心纽带。
Work transfers energy between stores: for example, lifting a weight increases its GPE, while friction between surfaces dissipates energy as thermal energy. Understanding this pathway is critical for solving energy problems.
做功会使能量在不同储存形式间转移：例如，举起重物会增大其重力势能，而表面之间的摩擦会以热能形式耗散能量。理解这条转移路径是解决能量问题的关键。
7. Power | 功率
Power is the rate of doing work or transferring energy. It is a scalar quantity measured in watts (W). One watt equals one joule per second. The relationship is:
功率是做功或转移能量的速率。它是一个标量，单位为瓦特（W）。1 瓦特等于 1 焦耳/秒。关系式为：
P = W / t and P = E / t
P is power (W), W is work done (J), E is energy transferred (J), and t is time (s). Many appliances are labelled with their power rating, which helps calculate energy consumption using the kilowatt-hour (kW·h), a common OCR topic.
P 为功率（W），W 为功（J），E 为转移的能量（J），t 为时间（s）。许多电器都标有额定功率，这有助于通过千瓦时（kW·h）计算能耗，这也是 OCR 的常见考点。
A useful alternative equation for mechanical power is P = F v, where v is velocity, derived from the work equation. This is especially relevant in vehicle motion problems.
机械功率的另一个实用公式是 P = F v，其中 v 为速度，由功的公式推导而来。这在车辆运动问题中尤为常见。
8. Efficiency | 效率
The efficiency of a device or process tells us how much of the input energy is converted into useful output energy. No real machine is 100% efficient; some energy is always dissipated, often as heat. Efficiency can be calculated as:
设备或过程的效率表示输入能量中有多大比例转化为有用的输出能量。现实中没有机器能达到 100% 效率，总会有一部分能量耗散，通常以热的形式。效率的计算公式为：
Efficiency = (Useful output energy / Total input energy) × 100%
Efficiency can also be expressed in terms of power: Efficiency = (Useful power output / Total power input) × 100%. It is a ratio and has no units. A high-efficiency appliance wastes less energy.
效率也可以用功率表示：效率 = (有用输出功率 / 总输入功率) × 100%。它是一个比值，没有单位。高效率的电器浪费的能量更少。
When analysing thermal systems, waste heat is the most common loss. Improving efficiency often involves reducing friction, using insulation, or redesigning systems to minimise energy dispersal.
分析热系统时，废热是最常见的损失。提高效率通常需要减少摩擦、使用隔热材料或重新设计系统以尽可能减少能量散逸。
9. Conservation of Energy | 能量守恒
The principle of conservation of energy states that energy cannot be created or destroyed. It can only be transferred, stored, or dissipated. This is a foundational law in all sciences.
能量守恒定律指出，能量不能被创造或消灭，只能被转移、储存或耗散。这是所有科学中的基本定律。
In a closed system, total energy remains constant. For example, a swinging pendulum continuously converts GPE into KE and back, but (ignoring air resistance) the sum of GPE and KE stays the same. Any energy that appears to be ‘lost’ has been dissipated to the surroundings, increasing their thermal store.
在封闭系统中，总能量保持不变。例如，摆动的钟摆不断将重力势能转化为动能，再转化回去，但（忽略空气阻力）GPE 与 KE 之和始终不变。任何看似“消失”的能量实际上都耗散到了环境中，增加了环境的热储存。
This principle allows you to solve problems by equating energy transfers, such as using KE lost = thermal energy gained by brakes, or GPE lost = KE gained by a falling object.
利用这一定律，你可以通过能量转化关系来解题，例如动能损失 = 刹车获得的热能，或重力势能损失 = 下落物体获得的动能。
10. Energy Resources and Sustainability | 能源与可持续性
Understanding energy resources is essential for both IB Environmental Systems and Societies and OCR Gateway/21st Century Science. Energy resources are categorised as renewable or non-renewable. The table below summarises key examples.
理解能源对于 IB 环境系统与社会以及 OCR Gateway/21 世纪科学课程都至关重要。能源可分为可再生与不可再生。以下表格总结了主要示例。
| Resource | Renewable? | Advantages | Disadvantages |
|---|---|---|---|
| Fossil fuels (化石燃料) | No | Reliable, high energy density | Produces CO₂, non-renewable |
| Nuclear (核能) | No | No CO₂ during operation, very high output | Radioactive waste, risk of accidents |
| Wind (风能) | Yes | No fuel costs, low carbon | Intermittent, visual/noise impact |
| Solar (太阳能) | Yes | Abundant, low maintenance | Weather-dependent, requires battery storage |
| Hydroelectric (水力发电) | Yes | Reliable, can meet peak demand | Habitat flooding, high initial cost |
| Tidal (潮汐能) | Yes | Predictable, long lifespan | Expensive, limited locations |
When evaluating an energy resource, consider factors such as reliability, cost, environmental impact, and societal implications. Both IB and OCR exams expect you to compare resources critically.
评估能源时，要考虑可靠性、成本、环境影响和社会因素。IB 和 OCR 考试都要求你批判性地比较各种能源。
11. Sankey Diagrams and Energy Analysis | 桑基图与能量分析
A Sankey diagram is a visual tool used to represent energy transfers. The width of each arrow is proportional to the amount of energy. A Sankey diagram for an inefficient light bulb, for example, would show a thin arrow for useful light and a much wider arrow for wasted heat.
桑基图是一种用于表示能量转移的视觉工具。每个箭头的宽度与能量大小成正比。例如，低效灯泡的桑基图会显示一条细箭头代表有用光能，以及一条宽得多的箭头代表浪费的热能。
To draw a Sankey diagram, calculate the useful output and waste components, and ensure the total width on the right (output side) equals the input width. Efficiency improvements can be illustrated by making the useful arrow wider.
绘制桑基图时，需计算有用输出和浪费能源，并确保右侧（输出端）的总宽度与输入宽度相等。提高效率可以通过加宽有用箭头来表示。
These diagrams are frequently examined in OCR Gateway Physics (B) and also appear in IB MYP Science tasks. Practice interpreting and sketching them proportionally.
这类图表在 OCR Gateway 物理（B）中常考，也出现在 IB 中学项目科学任务中。多练习按比例解读与绘制桑基图。
12. Exam Success Tips | 应考贴士
Use consistent SI units. Always convert mass into kg, distance into m, and time into s unless the question specifies otherwise. Marks are often lost on unit errors.
使用统一的国际单位。除非题目另有说明，始终将质量转换为 kg，距离转换为 m，时间转换为 s。因单位错误而失分的情况很常见。
Memorise the core equations. Write down KE, GPE, EPE, W, and P on your exam paper as soon as you are allowed to. Check whether g = 10 N/kg is specified in the OCR rubric.
记熟核心公式。 一旦可以动笔，立刻在试卷上写下 KE、GPE、EPE、W 和 P 的公式。检查 OCR 评分说明中是否规定 g = 10 N/kg。
Show full working. Even if your final answer is wrong, clear substitution and rearrangement will earn method marks. State the principle of conservation of energy if linking different stores.
展示完整步骤。即使最终答案错误，清晰的代入和公式变形也能得到过程分。如果涉及不同储存形式间的联系，要说明能量守恒原理。
Practise Sankey diagrams. Be ready to complete or sketch proportionality-based diagrams and calculate efficiency from them. Use a ruler for accuracy.
练习桑基图。做好完成或绘制比例图表的准备，并根据图表计算效率。使用直尺保证准确性。
Connect concepts. IB questions often integrate energy with other topics like momentum, waves, or thermodynamics. Look for energy links in multi-step problems.
联系概念。IB 考题常将能量与动量、波或热力学等其他主题融合。在多步问题中要寻找能量方面的联系。
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