📚 Conservation of Energy | 能量守恒
Energy is one of the most important ideas in KS3 science. This CIE KS3 revision guide explains the conservation of energy, energy stores, transfers, wasted energy and efficiency. It also includes worksheet-style examples to help you practise and prepare for tests.
能量是 KS3 科学中最重要的概念之一。这份 CIE KS3 复习指南讲解能量守恒、能量储存、能量转移、浪费的能量和效率，并提供练习题式的例子帮助你巩固和备考。
1. What is Energy? | 什么是能量？
Energy is the ability to do work. In physics, energy is measured in joules (J). One joule is the energy transferred when a force of 1 newton moves an object 1 metre.
能量是做功的能力。在物理学中，能量以焦耳（J）为单位。1 焦耳是 1 牛顿的力使物体移动 1 米时转移的能量。
Energy cannot be seen directly, but scientists can observe its effects. A glowing bulb, a moving car, a hot drink and a stretched spring all show that energy is present in different ways.
能量本身无法直接看到，但科学家可以观察到它的作用。发光的灯泡、行驶的汽车、热饮和被拉伸的弹簧都以不同方式表明能量的存在。
2. Energy Stores | 能量储存
Scientists describe energy as being stored in different ways. At KS3, the main energy stores include kinetic, gravitational potential, chemical, elastic potential, thermal, nuclear, and electrostatic or magnetic stores.
科学家将能量描述为以不同方式储存。在 KS3 阶段，主要的能量储存包括动能、重力势能、化学能、弹性势能、热能、核能以及静电或磁能储存。
| Energy store | 中文名称 | Example |
|---|---|---|
| Kinetic | 动能 | A moving bicycle |
| Gravitational potential | 重力势能 | A book on a high shelf |
| Chemical | 化学能 | Food, fuel, batteries |
| Elastic potential | 弹性势能 | A stretched rubber band |
| Thermal | 热能 | A hot cup of tea |
| Nuclear | 核能 | The core of an atom |
A moving car has kinetic energy because it is moving. Food, fuel and batteries store chemical energy. A ball held above the ground has gravitational potential energy because of its height.
行驶的汽车具有动能，因为它在运动。食物、燃料和电池储存化学能。被举高的球由于离地高度而具有重力势能。
3. 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. It can only be transferred from one store to another, or changed from one form to another. The total energy before a change is always equal to the total energy after the change.
能量守恒定律指出，能量既不能被创造，也不能被消灭。它只能从一个储存转移到另一个储存，或从一种形式转化为另一种形式。变化前的总能量总是等于变化后的总能量。
total energy before = total energy after
This law applies to every energy change in the universe. It applies to burning fuel, falling objects, electric circuits and living cells.
这一定律适用于宇宙中的每一次能量变化。它适用于燃料燃烧、物体下落、电路和活细胞。
If some energy seems to disappear, it has actually been transferred to the surroundings, usually as heat. Energy is never lost.
如果有些能量看起来消失了，实际上它已经转移到周围环境中，通常是以热的形式。能量永远不会丢失。
4. Energy Transfers | 能量转移
In any process, energy moves from one store to another. For example, when you switch on a kettle, electrical energy is transferred to the thermal energy store of the water and the heating element.
在任何过程中，能量都会从一个储存转移到另一个储存。例如，当你打开电水壶时，电能转移到加热元件和水的热能储存中。
A torch transfers energy from the chemical store of the battery to the electrical pathway, then to the thermal store of the bulb and finally to light. Some energy is transferred to the surroundings as heat.
手电筒将能量从电池的化学能储存转移到电路中的电能，再转移到灯泡的热能储存，最后转化为光。部分能量以热的形式转移到周围环境中。
A falling ball transfers gravitational potential energy to kinetic energy as it speeds up. When it hits the ground, energy is transferred to thermal and sound stores.
下落的小球随着速度增加，重力势能转化为动能。当它撞击地面时，能量又转移到热能和声能储存中。
5. Useful and Wasted Energy | 有用能量与浪费能量
In most energy transfers, only some of the input energy becomes useful output energy. The rest is wasted, usually as heat or sound. Wasted energy is not destroyed; it spreads into the surroundings.
在大多数能量转移中，只有部分输入能量转化为有用的输出能量，其余部分被浪费，通常以热或声的形式散失。浪费的能量并没有被消灭，而是扩散到周围环境中。
A light bulb is designed to produce light, but it also gets hot. For a filament bulb, most of the input electrical energy is wasted as heat, and only a small fraction becomes useful light.
灯泡的设计目的是发光，但它也会发热。对于白炽灯泡来说，输入的电能大部分以热的形式浪费，只有一小部分转化为有用的光。
In a car engine, chemical energy from petrol is transferred into kinetic energy to move the car. However, a large amount is also transferred as heat to the engine, exhaust gases and surroundings.
在汽车发动机中，汽油中的化学能转化为使汽车运动的动能。然而，大量能量也以热的形式转移到发动机、废气和周围环境中。
6. Dissipation and Spread | 能量散逸与扩散
When energy is wasted, it becomes spread out or dissipated. Heat energy from a hot cup of tea gradually warms the air around it. The energy is still there, but it is now less useful because it is too spread out.
能量被浪费时，会变得分散或散逸。热茶的热能会逐渐加热周围的空气。能量仍然存在，但由于过于分散，变得不太有用。
Dissipated energy is very difficult to gather back and use again. For example, the kinetic energy of a car becomes heat in the brakes and in the road. Once it has spread out, it cannot easily be turned back into movement.
散逸的能量很难再收集起来重新利用。例如，汽车的动能会转化为刹车和路面中的
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