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OxfordAQA · International GCSE · Physics 주제 2 5:01 영어 내레이션 · 영어 + 중국어 자막 burned-in
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Nothing destroys energy.
没有什么能消灭能量。
It is only moved between stores, and part of it is dissipated into less useful forms.
能量只是在不同的储存方式之间转移，其中一部分耗散成不太有用的形式。
This topic builds the accounting: work and power, three energy-store equations, conservation with Sankey diagrams, and the choices behind a country's energy resources.
本主题建立能量的“账本”：功和功率、三个能量储存公式、用桑基图表示守恒，以及国家能源资源的选择。
Work is done when a force moves its point of application through a distance: W equals F times d.
当力使其作用点移动一段距离时，就做了功：W 等于 F 乘以 d。
A 50 newton force pushing a box through 4 metres does 200 joules of work.
50 牛的力把箱子推动 4 米，做功 200 焦耳。
Work against friction transfers the energy by heating, as in brakes and meteorites.
克服摩擦做功时，能量以加热的方式转移，比如刹车和流星。
Three stores, three equations.
三种储存方式，三个公式。
A stretched spring stores elastic energy, one half k e squared.
拉伸的弹簧储存弹性势能，二分之一 k e 平方。
A raised mass stores gravitational potential energy, m g h.
被举高的物体储存重力势能，m g h。
A moving mass carries kinetic energy, one half m v squared.
运动的物体具有动能，二分之一 m v 平方。
Power is the rate of transfer, energy divided by time.
功率是能量转移的快慢，能量除以时间。
Kinetic energy depends on the square of speed.
动能与速度的平方成正比。
At 10 metres per second a 1500 kilogram car carries 75 000 joules; at 20 it is 300 000, four times as much.
1500 千克的汽车以每秒 10 米行驶时具有 75 000 焦耳；以每秒 20 米行驶时是 300 000 焦耳，是原来的四倍。
The brakes must remove all of it, so the braking distance grows much faster than the speed.
刹车必须把这些能量全部转移掉，所以制动距离比速度增长得快得多。
A catapult's elastic has k equal to 100 newtons per metre and is stretched 0.20 metres.
弹弓的橡皮筋 k 为每米 100 牛，被拉长 0.20 米。
It stores one half times 100 times 0.20 squared, 2.0 joules.
它储存二分之一乘 100 乘 0.20 的平方，即 2.0 焦耳。
If all of it goes to a 0.050 kilogram stone, one half times 0.050 times v squared equals 2.0, so v squared is 80 and v is 8.9 metres per second.
如果全部给一颗 0.050 千克的石子，二分之一乘 0.050 乘 v 平方等于 2.0，所以 v 平方为 80，v 为每秒 8.9 米。
A ball is dropped from 5.0 metres.
一个球从 5.0 米高处落下。
As it falls, GPE becomes KE: m g h equals one half m v squared.
下落时重力势能变成动能：m g h 等于二分之一 m v 平方。
The mass cancels, so v is the square root of 2 g h, the square root of 2 times 10 times 5.0: 10 metres per second, for a heavy ball or a light one.
质量可以约去，所以 v 等于根号 2 g h，即根号 2 乘 10 乘 5.0：每秒 10 米，重球轻球都一样。
A crane lifts a 500 kilogram load through 12 metres in 20 seconds.
起重机在 20 秒内把 500 千克的货物提升 12 米。
The energy transferred is m g h: 500 times 10 times 12, 60 000 joules.
转移的能量是 m g h：500 乘 10 乘 12，即 60 000 焦耳。
Power is energy divided by time: 60 000 divided by 20, 3000 watts, which is 3.0 kilowatts.
功率等于能量除以时间：60 000 除以 20，即 3000 瓦，也就是 3.0 千瓦。
A pendulum swaps two stores.
单摆在两种储存方式之间交换能量。
At the top of each swing it stops for an instant: all its energy is gravitational potential.
在每次摆动的最高点，它瞬间静止：能量全是重力势能。
At the bottom it moves fastest: all kinetic.
在最低点它最快：全是动能。
Then it climbs and swaps back again.
然后它上升，再交换回来。
Air resistance dissipates a little each swing.
空气阻力每次摆动都耗散一点能量。
Efficiency is the useful energy out divided by the total energy in, times 100 per cent.
效率等于有用输出能量除以总输入能量，再乘以 100%。
A 60 watt lamp takes 60 joules in each second and gives 6 joules of light.
60 瓦的灯每秒输入 60 焦耳，发出 6 焦耳的光。
Its efficiency is 6 over 60, 10 per cent.
效率是 6 除以 60，即 10%。
The other 54 joules heat the lamp and the room.
其余 54 焦耳使灯和房间变热。
A motor takes 800 watts and gives out 600 watts of useful kinetic power, an efficiency of 75 per cent.
一台电动机输入 800 瓦，输出 600 瓦有用的动能功率，效率为 75%。
The other 200 watts is dissipated: in one minute, 12 000 joules heat the motor and its surroundings.
其余 200 瓦被耗散：一分钟内有 12 000 焦耳使电动机和周围环境变热。
On the Sankey diagram the useful arrow is three times as wide as the wasted one.
在桑基图上，有用箭头的宽度是浪费箭头的三倍。
Fuels are chosen for ease of storage, energy content and safety.
选择燃料时要考虑是否便于储存、能量含量和安全性。
A country choosing how to generate electricity trades renewability against cost and reliability, and counts the environmental cost of extraction, use and disposal.
一个国家选择发电方式时，要在可再生性与成本、可靠性之间权衡，还要计算开采、使用和处置的环境代价。
Four renewable resources.
四种可再生资源。
Wind needs no fuel and makes no emissions, but its output is unreliable and the turbines change the landscape.
风能不需要燃料、没有排放，但输出不稳定，风机还改变景观。
Solar works only in daylight.
太阳能只能在白天发电。
Hydroelectric power is reliable and starts quickly, but floods a valley.
水力发电稳定、启动快，但要淹没一个山谷。
Tidal power is predictable, but suits few coasts.
潮汐能可以预测，但适合的海岸很少。
Four common mistakes. Forgetting the square in one half m v squared or one half k e squared. Using the mass, not the weight, as the force when lifting. An efficiency above 100 per cent, which is impossible. And using a spring's length in centimetres instead of its extension in metres.
四个常见错误：忘了二分之一 m v 平方或二分之一 k e 平方中的平方；举起物体时把质量而不是重力当作力；效率超过 100%，这不可能；以及用弹簧的长度（厘米）代替伸长量（米）。
Check yourself.
自我检测。
A 50 newton force moving 4 metres does 200 joules of work.
50 牛的力移动 4 米，做功 200 焦耳。
A spring with k of 200 stretched 0.15 metres stores 2.25 joules.
k 为 200 的弹簧拉长 0.15 米，储存 2.25 焦耳。
A motor transferring 800 joules in 20 seconds has a power of 40 watts.
20 秒内转移 800 焦耳的电动机，功率为 40 瓦。
A lamp giving 12 joules of light per 100 in is 12 per cent efficient.
每输入 100 焦耳发出 12 焦耳光的灯，效率为 12%。