Energy Changes Form
FoundationMoving things carry kinetic energy, high positions store potential energy, and food and fuel hold chemical energy; energy converts between these forms.
- Energy never vanishes; it only changes form.
- Moving things carry kinetic energy — faster means more.
- Height and changed shape store potential energy.
文體小知識：說明文把抽象的能量講得具體有趣。這一篇教你辨認動能、勢能和化學能，看能量怎樣變身。
§Text Passage
1Coast downhill on a bicycle and it speeds up without pedalling; lift a school bag with effort, loosen your hand, and it drops heavily. Behind all this is energy. Energy makes things active: it moves cars, lights lamps and lets us think, run and jump. Energy is invisible, yet it exists in everything in different forms.
2Animals running, wind stirring leaves, streams rushing — anything that moves carries kinetic energy. The faster and heavier, the more kinetic energy. The moment a volleyball player smashes, the ball flies so fast that opponents can hardly block it; wind turbines too use the kinetic energy of moving air to spin blades and make electricity.
3An object lifted high stores potential energy: the higher the bag, the more it stores; water rushing down from a high reservoir releases potential energy to spin generators. A stretched rubber band and a compressed spring also store elastic potential energy, springing out the moment we let go. Position and shape are both storerooms of energy.
4Food and fuel store chemical energy. The rice, noodles and meat we eat slowly release chemical energy in the body for moving, thinking and keeping warm; a battery turns chemical energy into electrical energy to drive fans and torches; cooking gas and petrol are treasure chests of chemical energy.
5Energy converts between forms: pedalling uphill turns the body's chemical energy into the bicycle's kinetic energy, which is stored as potential energy at the top; coasting down, the potential energy turns back into kinetic energy, faster and faster. Roller skating, swinging and trampolining are all energy's transformation shows.
6Energy conversions make life colourful, but energy is not easily earned — generating power uses up resources. So we should save energy: switch off lights, take the stairs, use public transport, and let every bit of energy go where it is worth, keeping Earth's resources usable for a long time.
1Summary 重點摘要
Moving things carry kinetic energy, high positions store potential energy, and food and fuel hold chemical energy; energy converts between these forms.
2Vocabulary cards Vocabulary
The power to make things move and work; it exists in different forms.
The energy of motion; the faster something moves, the more it has.
Energy stored because of high position or changed shape.
Energy stored in food, fuel and batteries.
Energy changing from one form to another, such as potential to kinetic.
The energy carried by electric current, turning into light, heat and motion.
The light and heat energy from the sun, a great source of energy on Earth.
Turning the energy of water, wind or fuel into electrical energy.
3Step by step Steps
- Step 1: Know what energy isEnergy is the power that makes things move and work. It cannot be seen or touched, yet it is everywhere: running, shining, sounding and heating all need energy. Scientists found an important rule: energy is never created out of nothing, nor does it vanish; it only converts from one form to another. Learning energy's forms is the first step to understanding how the world works.
- Step 2: Recognise kinetic energyAnything that moves carries kinetic energy. Running classmates, rolling balls, flowing rivers and wind stirring leaves all have it. Kinetic energy depends on speed and mass: the faster something moves, the more kinetic energy; the heavier it is, the more again. That is why a heavy lorry, though slow, has far more kinetic energy than a bicycle and needs a much longer distance to stop — take extra care around big vehicles when crossing roads.
- Step 3: Recognise potential energyAn object lifted high stores gravitational potential energy: the higher and heavier, the more it stores. Reservoirs hold water high up, and releasing it turns potential energy into kinetic energy to drive generators. Objects with changed shape store elastic potential energy: a drawn bow, a compressed spring, a twisted rubber band — let go and the stored energy is released. Remember: height stores power, changed shape stores spring; both are storerooms of energy.
- Step 4: Recognise chemical energyChemical energy hides inside substances and is released through chemical change. Food stores it, and after digestion our bodies use it — that is why breakfast gives us energy for lessons. Batteries store it and turn it into electrical energy in a circuit; gas, oil and wood release it as light and heat when burning. Chemical energy is the most common way energy is stored in daily life, and our three meals a day keep refilling it.
- Step 5: Watch examples of energy conversionEnergy keeps converting between forms. Pedalling uphill: the body's chemical energy becomes the bicycle's kinetic energy, then potential energy at the top; coasting down, potential becomes kinetic. Press a light switch: electrical energy becomes light and heat. Turn on a fan: electrical energy becomes kinetic energy. On a swing, potential energy peaks at the top and kinetic energy at the bottom, the two swapping until friction slowly carries the energy away. Look around — conversions are everywhere.
- Step 6: Take action to save energyEnergy conversions may seem free, but generating electricity burns fuel or uses water and wind, resources are limited, and pollution can follow. Everyone can save energy: switch off lights and air-conditioners when leaving a room; walk or take buses for short trips instead of private cars; choose energy-saving appliances and LED bulbs; set the air-conditioner at 25 to 26 degrees in summer. Cherish every unit of electricity, so energy keeps flowing and Earth's resources last for the next generation.
5Worked example Example
Example: Lok eats a bun, cycles to the top of a slope, rests a while, then coasts down, going faster and faster. Describe the energy conversions in this process.
- The bun stores chemical energy, which digestion makes available to the body.
- While pedalling, the body converts chemical energy into muscle power, driving the bicycle and giving it kinetic energy.
- Climbing the slope, the bicycle gains height, and kinetic energy gradually turns into stored potential energy, greatest at the top.
- Coasting down, height falls and potential energy turns back into kinetic energy, so the bicycle goes faster and faster.
- When braking, friction turns the kinetic energy into heat, and the bicycle stops.
- Answer: The journey's energy source is the bun's chemical energy: pedalling converts it into kinetic energy; climbing to the top stores the energy as potential energy; coasting down turns potential energy back into kinetic energy, making the bicycle faster and faster; braking finally turns kinetic energy into heat by friction. Energy only changes form — chemical → kinetic → potential → kinetic → heat — and never disappears.
6Common mistakes Mistakes
7Quick check Quick check
Q1What energy does a running classmate carry?
Q2What energy does a lifted object store?
Q3What energy does food store?
Q4When cycling downhill, what does potential energy turn into?
Q5What two forms of energy does a lamp make from electrical energy?
8Recap Recap
- Energy never vanishes; it only changes form.
- Moving things carry kinetic energy — faster means more.
- Height and changed shape store potential energy.
- Food, fuel and batteries store chemical energy.
- Save energy: switch off lights, walk more, cherish every unit.