📚 Energy Sources | 能源来源
Energy is the capacity to do work, and the way we obtain and convert energy from nature into usable forms powers modern civilisation. In the International A-level Physics syllabus, you must understand how different energy sources are classified, how they are harnessed, and how to analyse their efficiency and environmental impact quantitatively.
能量是做功的能力，我们从自然界获取并转化为可用形式的能量驱动着现代文明。在国际A-level物理课程中，你需要理解不同能源如何分类、如何被开发利用，以及如何定量分析它们的效率和环境影响。
1. Primary and Secondary Energy Sources | 一次能源与二次能源
A primary energy source is one that exists naturally, such as coal, wind, sunlight, or uranium. A secondary energy source is produced from a primary source by conversion; electricity is the most common secondary source. When you burn coal to generate electricity, chemical energy stored in the coal (primary) is converted into electrical energy (secondary).
一次能源是自然存在的能源，如煤、风、阳光或铀。二次能源是由一次能源经过转换而产生的；电力是最常见的二次能源。当你燃烧煤来发电时，煤中储存的化学能（一次）就被转化为电能（二次）。
Energy efficiency, η = Useful output energy ÷ Total input energy
The efficiency of any conversion process is the ratio of useful output energy to total input energy, usually expressed as a percentage. In power stations, the overall efficiency is typically between 30% and 45% because large amounts of energy are lost as heat to the surroundings.
任何转换过程的效率都是有用输出能量与总输入能量的比值，通常以百分比表示。在发电站中，总效率通常在30%到45%之间，因为大量的能量以热的形式散失到周围环境中。
2. Renewable vs Non-renewable Sources | 可再生能源与不可再生能源
Non-renewable sources (fossil fuels and nuclear fuels) exist in finite quantities and will eventually be exhausted. Their use also releases pollutants or radioactive waste. Renewable sources (solar, wind, hydroelectric, tidal, wave, geothermal, and biomass) are replenished naturally within a human timescale and produce little or no net carbon dioxide emission.
不可再生能源（化石燃料和核燃料）储量有限，最终将被耗尽。其使用还会释放污染物或放射性废物。可再生能源（太阳能、风能、水能、潮汐能、波浪能、地热能和生物质能）在人类时间尺度内自然补充，且几乎不产生净二氧化碳排放。
| Feature | Non-renewable | Renewable |
| Availability | Finite, depleted over time | Continuously replenished |
| Carbon emission | High (fossil fuels) | Near zero (except biomass) |
| Reliability | Very reliable, on demand | Intermittent or location-dependent |
| Energy density | High | Generally low |
Fossil fuels and nuclear fuel have very high energy density, meaning a small mass releases a large amount of energy. Renewable sources have much lower energy density, so large collecting areas are needed to produce comparable power output.
化石燃料和核燃料具有非常高的能量密度，即少量质量就能释放大量能量。可再生能源的能量密度要低得多，因此需要大面积收集装置才能产生相当的功率输出。
3. Fossil Fuels: Coal, Oil and Gas | 化石燃料：煤、石油和天然气
Fossil fuels are formed over millions of years from the remains of plants and organisms. In a conventional power station, the fuel is burned to heat water into steam, which drives a turbine connected to a generator. The chemical store of energy is first converted to internal energy of the steam, then to kinetic energy of the turbine, and finally to electrical energy.
化石燃料是由动植物遗骸经过数百万年形成的。在传统发电站中，燃料燃烧加热水产生蒸汽，蒸汽驱动连接发电机的涡轮机。化学能首先转化为蒸汽的内能，再转化为涡轮机的动能，最终转化为电能。
Chemical energy → Internal energy → Kinetic energy → Electrical energy
Burning fossil fuels produces carbon dioxide, a greenhouse gas, along with sulfur dioxide and nitrogen oxides that cause acid rain. Although cheap and reliable, their depletion and environmental harm make them unsustainable in the long term.
燃烧化石燃料会产生二氧化碳（一种温室气体），以及导致酸雨的二氧化硫和氮氧化物。虽然化石燃料便宜且可靠，但其耗竭和环境危害使它们长期不可持续。
4. Nuclear Fission | 核裂变
Nuclear power uses the fission of heavy nuclei such as uranium-235 or plutonium-239. A neutron absorbed by the nucleus causes it to split into two lighter nuclei, releasing a large quantity of energy and 2–3 additional neutrons, which sustain a chain reaction.
核能利用铀-235或钚-239等重核的裂变反应。中子被原子核吸收后使其分裂为两个较轻的核，释放出大量能量和2到3个额外中子，从而维持链式反应。
²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3¹n + energy
The energy released per fission is about 200 MeV, which is enormously greater than the energy per chemical reaction. A typical fission of uranium-235 releases roughly 3.2 × 10⁻¹¹ J, so just one kilogram of uranium can produce the same energy as about 20,000 kilograms of coal.
每次裂变释放的能量约为200 MeV，远大于每次化学反应释放的能量。铀-235的一次典型裂变释放约3.2 × 10⁻¹¹ J，因此仅1千克铀就能产生相当于约20,000千克煤的能量。
Nuclear stations do not emit carbon dioxide during operation, but they produce radioactive waste that must be stored safely for thousands of years. The risk of accidents and high decommissioning costs are significant disadvantages.
核电站在运行期间不排放二氧化碳，但会产生必须安全储存数千年的放射性废物。事故风险和高昂的退役成本是重要的缺点。
5. Solar Energy | 太阳能
Solar energy can be harnessed in two ways: photovoltaic cells convert light directly into electrical energy, while solar thermal panels absorb sunlight to heat water or another fluid. Photovoltaic cells are semiconductor devices that generate a voltage when photons dislodge electrons in the material.
太阳能可以通过两种方式利用：光伏电池将光直接转化为电能，而太阳能热板吸收阳光加热水或其他流体。光伏电池是半导体器件，当光子使材料中的电子脱离时产生电压。
The power available from sunlight at the Earth’s surface is approximately 1.0 kW m⁻² under clear conditions, known as the solar constant at ground level. However, typical commercial photovoltaic cells have an efficiency of only 15%–20%, so a panel receiving 1 kW m⁻² of irradiance might generate around 150–200 W m⁻² of electrical power.
在晴朗条件下，地球表面的太阳功率约为1.0 kW m⁻²，这被称为地面太阳常数。然而，商用光伏电池的典型效率仅为15%–20%，因此接收1 kW m⁻²辐照度的电池板大约只能产生150–200 W m⁻²的电功率。
Maximum electrical power = Solar irradiance × Area × Efficiency
Solar power is clean and inexhaustible, but it is intermittent — no electricity is produced at night, and output falls substantially when cloud covers the sky. Large storage systems or backup sources are therefore required.
太阳能清洁且取之不尽，但它是间歇性的——夜间不发电，天空有云时输出大幅下降。因此需要大型储能系统或备用电源。
6. Wind Energy | 风能
Wind turbines convert the kinetic energy of moving air into electrical energy. The air passing through the swept area of the rotor carries kinetic energy, and the turbine extracts a fraction of this according to the Betz limit, which states that no turbine can capture more than about 59.3% of the incoming wind power.
风力涡轮机将运动空气的动能转化为电能。通过转子扫掠面积的气流携带动能，涡轮机按照贝茨极限提取其中一部分能量，该极限指出任何涡轮机最多只能捕获来流风功率的约59.3%。
P = ½ ρ A v³
Here, P is the power in the wind, ρ is the air density (about 1.2 kg m⁻³), A is the swept area in m², and v is the wind speed in m s⁻¹. Because power depends on the cube of wind speed, doubling the wind speed increases the available power by a factor of eight. This is why wind farms are sited in consistently windy locations.
其中P是风的功率，ρ是空气密度（约1.2 kg m⁻³），A是以m²为单位的扫掠面积，v是以m s⁻¹为单位的风速。由于功率与风速的三次方成正比，风速加倍会使可用功率增加八倍。这就是为什么风电场选址在持续多风的地方。
Wind energy is renewable and emission-free, but it is also intermittent and has a low capacity factor—typically 25%–40% of the installed capacity is actually delivered on average. Turbines are also considered visually intrusive and create noise.
风能是可再生且零排放的，但它同样是间歇性的，容量因子低——通常平均只能提供装机容量的25%–40%。涡轮机也被认为影响景观并产生噪音。
7. Hydroelectric Power | 水力发电
Hydroelectric power uses the gravitational potential energy of water stored in a reservoir behind a dam. Water flows down through a penstock to a turbine, converting potential energy first to kinetic energy and then to electrical energy. The available power is given by:
水力发电利用大坝后方水库中水的重力势能。水流经压力管道流向涡轮机，将势能先转化为动能再转化为电能。可用功率由下式给出：
P = ρ g h Q
where ρ is the density of water (1000 kg m⁻³), g is gravitational field strength (9.81 N kg⁻¹), h is the vertical height difference in metres, and Q is the volume flow rate in m³ s⁻¹. For example, a dam with a head of 50 m and a flow rate of 40 m³ s⁻¹ provides a theoretical power of 1000 × 9.81 × 50 × 40 ≈ 1.96 × 10⁷ W, or about 20 MW.
其中ρ是水的密度（1000 kg m⁻³），g是重力场强度（9.81 N kg⁻¹），h是以米为单位的垂直落差高度，Q是以m³ s⁻¹为单位的体积流量。例如，一个水头50米、流量40 m³ s⁻¹的大坝理论功率为1000 × 9.81 × 50 × 40 ≈ 1.96 × 10⁷ W，约20 MW。
Hydroelectric power is highly reliable, can respond quickly to demand, and has very low running costs. However, large dams flood vast areas of land, displace local communities, and alter river ecosystems. In pumped storage schemes, water is pumped back uphill during off-peak hours to store energy for later use.
水力发电非常可靠，能快速响应需求变化，运行成本极低。然而，大型水坝会淹没大片土地，迫使当地社区迁移，并改变河流生态系统。在抽水蓄能方案中，低谷时段将水抽回高处储存能量以备后用。
8. Tidal and Wave Energy | 潮汐能与波浪能
Tidal power harnesses the rise and fall of sea levels caused by the gravitational pull of the Moon and Sun. A tidal barrage traps water at high tide and releases it through turbines at low tide. The theoretical mean power of a tidal basin is:
潮汐能利用月球和太阳引力引起的海平面升降。潮汐坝在高潮时蓄水，低潮时通过涡轮机放水。潮汐盆地的理论平均功率为：
P = ½ ρ g A R² per tide cycle
where A is the basin area and R is the tidal range. Tides are perfectly predictable, making tidal power reliable, but the construction of barrages is expensive and ecologically disruptive. Wave energy converters extract energy from the oscillatory motion of surface waves.
其中A是盆地面积，R是潮差。潮汐完全可预测，使潮汐能可靠，但拦潮坝的建设造价高昂且破坏生态。波浪能转换装置则从表面波的振荡运动中提取能量。
The UK has some of the best tidal resources in the world. However, tidal power output varies over the lunar cycle, producing two high tides and two low tides per day, so generation is periodic rather than continuous.
英国拥有世界上最好的潮汐资源之一。然而，潮汐能输出随月相周期变化，每天出现两次高潮和两次低潮，因此发电是周期性的而非连续性的。
9. Geothermal and Biomass | 地热能及生物质能
Geothermal energy comes from heat stored in the Earth’s crust, originating from radioactive decay and residual heat from planetary formation. In volcanic regions, hot water or steam can be brought to the surface to drive turbines directly. In other areas, ground-source heat pumps extract low-grade heat for space heating.
地热能来自地壳中储存的热量，其来源是放射性衰变和地球形成时的余热。在火山地区，热水或蒸汽可以直接引到地表驱动涡轮机。在其他地区，地源热泵提取低品位热量用于空间供暖。
Biomass involves burning organic material such as wood chips, agricultural waste, or purpose-grown energy crops. While biomass combustion releases carbon dioxide, the plants absorbed the same carbon dioxide during their growth, making biomass roughly carbon-neutral. However, transport and processing add emissions, and land used for energy crops competes with food production.
生物质能通过燃烧木片、农业废弃物或专为能源种植的作物来获取能量。虽然生物质燃烧释放二氧化碳，但植物在生长期间吸收了同等的二氧化碳，因此生物质大致是碳中性的。然而，运输和加工会增加排放，而且用于能源作物的土地与粮食生产竞争。
Geothermal energy is very reliable and has minimal visual impact, but it is only economically viable in tectonically active regions. Biomass is dispatchable (can be turned on and off as needed) but has lower efficiency than fossil fuels and produces particulate pollution when burned.
地热能非常可靠且对景观影响极小，但只有在构造活跃地区才具有经济可行性。生物质是可调度能源（可按需开启和关闭），但效率低于化石燃料，且燃烧时会产生颗粒物污染。
10. Efficiency, Sankey Diagrams and the National Grid | 效率、桑基图与国家电网
A Sankey diagram is a scale drawing in which the width of each arrow is proportional to the energy or power it represents. The input arrow splits into useful output energy and wasted energy streams. Sankey diagrams allow quick visual comparison of different energy conversion devices.
桑基图是一种按比例绘制的图表，其中每条箭头的宽度与它所代表的能量或功率成正比。输入箭头分为有用输出能量和耗散能量流。桑基图可以快速直观地比较不同能量转换装置的性能。
In electric power transmission, energy is lost as heat in the cables due to resistance. The power loss is given by P_loss = I²R. To minimise this loss, power stations step up the voltage using transformers, which reduces the current for a given transmitted power, and therefore reduces the I²R loss dramatically. Step-down transformers then reduce the voltage to safe levels for consumers.
在电力传输中，能量因电缆电阻而以热的形式损失。功率损失由P_loss = I²R给出。为了最小化这种损失，发电站使用变压器升压，从而在传输给定功率时降低电流，大幅减少I²R损耗。然后降压变压器将电压降到用户安全水平。
P_loss = I²R → reduced by step-up transformers (V ↑ so I ↓)
Overall efficiency of the electricity supply system can be calculated by dividing the useful electrical energy delivered to consumers by the total chemical or nuclear energy input at the power station. Typical overall values are 30%–40% for fossil fuel stations and about 33% for nuclear stations.
供电系统的总效率可以通过用户获得的有用电能除以发电站输入的化学能或核能总量来计算。化石燃料电站的典型总效率为30%–40%，核电站约为33%。
11. Energy Density and Specific Energy | 能量密度与比能量
Two important quantities are used to compare fuels. Specific energy is the energy released per unit mass of fuel, measured in J kg⁻¹. Energy density is the energy released per unit volume, measured in J m⁻³. These determine how much fuel must be carried and how much storage space is needed.
有两个重要量用于比较燃料。比能量是单位质量燃料释放的能量，单位为J kg⁻¹。能量密度是单位体积燃料释放的能量，单位为J m⁻³。它们决定了需要携带多少燃料以及需要多少储存空间。
| Fuel | Specific energy (MJ kg⁻¹) | Energy density (MJ m⁻³) |
| Coal | ~24 | ~40,000 |
| Natural gas | ~53 | ~38 (at atmospheric pressure) |
| Uranium-235 (fission) | ~80,000,000 | ~4 × 10¹⁴ |
| Hydrogen | ~142 | ~10 (gas), ~8,000 (liquid) |
When solving problems, always identify whether the question asks for specific energy or energy density. To find the mass of fuel needed for a given energy output, divide the required energy by the specific energy; to find the volume, divide by the energy density.
解题时，务必判断题目问的是比能量还是能量密度。要计算产生给定能量所需的燃料质量，用所需能量除以比能量；要计算体积，则除以能量密度。
12. Environmental Impact and Sustainability | 环境影响与可持续性
Each energy source has a unique environmental footprint. Fossil fuels contribute to climate change through CO₂ emissions and acid rain through SO₂ and NOₓ. Nuclear power produces radioactive waste requiring long-term geological storage. Large hydroelectric dams cause methane release from decomposing vegetation and disrupt fish migration. Wind turbines cause bird and bat mortality and visual pollution.
每种能源都有独特的环境足迹。化石燃料通过CO₂排放加剧气候变化，通过SO₂和NOₓ导致酸雨。核电产生需要长期地质储存的放射性废物。大型水坝导致植被腐烂释放甲烷并阻碍鱼类洄游。风力涡轮机导致鸟类和蝙蝠死亡以及视觉污染。
The concept of sustainability requires meeting present energy needs without compromising the ability of future generations to meet theirs. A sustainable energy mix typically combines renewable sources with energy storage systems, demand-side management, and improvements in energy efficiency across all sectors.
可持续性的概念要求满足当前能源需求的同时不损害后代满足其需求的能力。可持续的能源结构通常将可再生能源与储能系统、需求侧管理以及各部门能源效率的提升相结合。
In the International A-level examinations, you may be asked to evaluate the viability of different sources for a specific location, calculate efficiency from data, or discuss trade-offs between cost, reliability, and environmental impact. Always support your arguments with quantified calculations using the formulas and data given in the question.
在国际A-level考试中，你可能会被要求评估不同能源在特定地点的可行性，根据数据计算效率，或讨论成本、可靠性和环境影响之间的权衡。务必使用题目中给出的公式和数据，以定量计算支持你的论证。
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