GCSE Physics - AQA Specification - Energy Topic
Energy 是 AQA GCSE Physics 试卷中最核心的模块之一，权重占整个课程的约 18%，贯穿电学、波和粒子模型等几乎所有其他主题。
很多同学觉得能量"简单"，但 AQA 偏偏在这里设置高分陷阱：混淆 energy store 和 energy transfer、忽略单位换算、不知道什么时候用 E = ½mv² 什么时候用 E = mgh。这篇精讲逐个击破所有考点，助你稳拿满分。
一、系统概念与8种储能方式 | Energy Systems and the 8 Energy Stores
AQA 要求你理解"系统"（System）：当能量变化时，我们只关心被选中的物体——其余都是"周围环境"。A closed system has no net change in total energy.
Energy is never "used up" — it is transferred between stores. 官方列出 8 种储存形式，考试必须用这些术语：
Kinetic（动能） — 运动物体，E = ½mv²。 / Gravitational Potential（重力势能） — 高度决定，E = mgh。 / Elastic Potential（弹性势能） — 拉伸/压缩，E = ½ke²。 / Thermal（热能） — 粒子动能和势能总和。 / Chemical（化学能） — 化学键中储存，如燃料和电池。 / Magnetic（磁能） — 磁铁在磁场中的能量。 / Electrostatic（静电势能） — 电荷在电场中的能量。 / Nuclear（核能） — 原子核中释放的能量。
Exam Tip: AQA 评分极为严格——写 "heat energy" 不给分，必须是 "thermal energy"。写 "movement energy" 不给分，必须是 "kinetic energy"。每年都有考生因术语不精确失分。
二、能量转移的四种途径 | Four Pathways: Mechanical, Heating, Electrical, Radiation
能量在 store 之间移动时，必须通过四种转移途径之一：
Mechanically / By Forces（做功力转移） — When a force moves an object through a distance, work is done. Work done (J) = Force (N) × Distance (m). Lifting a book transfers energy from your chemical store to the book's GPE store.
By Heating（热传递） — Energy transfers from hotter to cooler objects via conduction, convection, or radiation. AQA uses "heating" to describe the pathway, not "heat".
Electrically（电转移） — When current flows through a circuit, energy transfers from a battery's chemical store to components' thermal store.
By Radiation / Waves（辐射/波转移） — Energy transferred by electromagnetic waves or sound. The Sun transfers energy to Earth through the vacuum of space via EM radiation.
答题模板: "Energy is transferred from the [store] of [object A] to the [store] of [object B] via [pathway]." 起点 store、起点物体、终点 store、终点物体、转移途径——五个要素缺一不可。
三、KE与GPE公式：两道必出计算题 | Kinetic Energy and GPE: Guaranteed Calculation Questions
Kinetic Energy: Eₖ = ½ × m × v² — m = mass (kg), v = speed (m/s)。正确顺序：先平方速度，再乘质量，最后除 2。(½mv)² 和 ½mv² 结果完全不同。A car of mass 1200 kg at 15 m/s: Eₖ = ½ × 1200 × 225 = 135,000 J.
Gravitational Potential Energy: Eₚ = m × g × h — g = 9.8 N/kg，AQA 有时取 10，务必看清题目说明。50 kg 学生爬 12 m 楼梯: Eₚ = 50 × 9.8 × 12 = 5,880 J.
组合题技巧: "assume no energy losses" 直接令 KE = GPE: ½mv² = mgh，消掉 m，v² = 2gh，v = √(2gh)。这个 shortcut 几乎每份 past paper 都出现。
四、弹性势能与比热容：两个核心公式对比 | Elastic Potential vs Specific Heat Capacity
Elastic Potential Energy: Eₑ = ½ × k × e² — k = spring constant (N/m), e = extension (m)。注意 e 是伸长量，不是弹簧总长度。仅当未超过弹性限度时适用。Spring with k = 400 N/m stretched 0.05 m: Eₑ = ½ × 400 × 0.0025 = 0.5 J.
Specific Heat Capacity: ΔE = m × c × Δθ — c = specific heat capacity (J/kg°C), Δθ = temperature change (°C)。水的 c 值极高 (4200 J/kg°C)，海水升温慢降温也慢——历年 favorite explain 题。2 kg 水从 20°C 到 100°C: ΔE = 2 × 4200 × 80 = 672,000 J.
区分要点: Eₑ = ½ke² 用于弹簧和弹性物体；ΔE = mcΔθ 用于任何加热/冷却场景。e 是伸长量（米），Δθ 是温度变化（摄氏度），两者不可混淆。
五、功率与效率：最容易拿满分的两道计算题 | Power and Efficiency: High-Scoring Calculations
Power: P = E/t — rate of energy transfer, unit: watt (W), 1 W = 1 J/s. A 60 W bulb transfers 60 J every second. 另需掌握 P = F × v (power when a force moves an object at constant velocity). A motor lifting a 200 N weight through 6 m in 4 s: Work = 1200 J, Power = 1200 ÷ 4 = 300 W.
Efficiency = Useful Output / Total Input — no units, between 0 and 1 (or 0% to 100%). Can never exceed 1. AQA 必问："Why is efficiency less than 100%?" — 标准答案：Some energy is dissipated to the thermal store of the surroundings via heating, due to friction or electrical resistance.
可用能量或功率计算：Efficiency = Useful output energy / Total input energy = Useful power output / Total power input. An electric motor receiving 500 J, doing 350 J useful work: Efficiency = 0.7 = 70%. 其余 150 J dissipated as thermal energy.
六、能量守恒与耗散：解释题万能三步模板 | Conservation, Dissipation and a Universal Template
Energy can be transferred usefully, stored or dissipated, but can never be created or destroyed. 在封闭系统中，总能量保持不变——整个物理学的基石法则。
实际场景中几乎没有 100% 高效的过程——总有能量被 dissipate（耗散）到周围环境的热储中，变得越来越难再利用。AQA prefers "dissipated" over "wasted energy".
万能三步模板 — "Explain the energy transfers when...": Step 1: 初始 store — "Energy is transferred from the [chemical/kinetic] store of [object]..." Step 2: 有用转移 — "...to the [kinetic/GPE/thermal] store via [mechanical work/heating/electrically]." Step 3: 耗散说明 — "Some energy is dissipated to the thermal store of the surroundings via heating, due to [friction/air resistance], so the process is not 100% efficient."
Example — falling ball: "Energy transfers from its GPE store to kinetic store via mechanical work. Some energy dissipates to thermal stores due to air resistance. On impact, kinetic energy converts to thermal and sound, so the ball does not bounce to its original height."
七、必做实验：比热容测量与常见失分点 | Required Practical: Specific Heat Capacity
AQA 规定学生必须完成比热容测量实验（Required Practical 1）。Paper 1 至少考一道 4-6 分的相关题目。
实验步骤：Measure metal block mass (kg); wrap in insulation to reduce heat loss; insert heater and thermometer; record initial temperature; switch on power and start stopwatch simultaneously; record current (ammeter), p.d. (voltmeter), and temperature every minute for 10 min; calculate E = I × V × t; plot temperature vs energy — gradient = 1/(mc), so c = 1/(gradient × m).
三大失分点：(1) 用 g 而非 kg 记录质量——最常见 GCSE 单位错误；(2) 不加 insulation 却声称结果准确——大量热量散失使测出 c 值偏高；(3) 忘记搅拌液体以确保温度均匀（若实验对象是液体而非金属块）。
八、六大考试陷阱与避坑指南 | Six Exam Traps to Avoid
Trap 1: 单位陷阱 g vs kg — 所有能量公式用 kg。题目常给克（g），必须先除以 1000 再代入。忘转换直接零分。
Trap 2: 速度平方陷阱 — (½mv)² ≠ ½mv²。正确：先平方 v，再乘 m，最后除以 2。
Trap 3: Extension vs Length — Eₑ = ½ke² 中 e 是伸长量。弹簧原长 10 cm，拉至 15 cm，e = 0.05 m，不是 0.15 m。
Trap 4: g 值陷阱 — 计算题常用 9.8，但题目可能写 "take g = 10 N/kg"。用错 g 值整个答案都对不上。
Trap 5: 守恒题需写全 — "state conservation of energy": 必须写 energy can neither be created nor destroyed。只写 "energy is conserved" 只得半分。
Trap 6: Efficiency 不可能 > 100% — 算出效率超过 1 说明你颠倒了分子分母。Useful output 必须 ≤ total input。
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