Energy, work and power · 能量、做功与功率
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| kinetic energy/kɪˈnetɪk ˈenədʒi/ | 动能 | dòng néng |
| gravitational potential energy/ˌɡrævɪˈteɪʃənl pəˈtenʃl ˈenədʒi/ | 重力势能 | zhòng lì shì néng |
| conservation of energy/ˌkɒnsəˈveɪʃn ɒv ˈenədʒi/ | 能量守恒 | néng liàng shǒu héng |
| Sankey diagram/ˈsæŋki ˈdaɪəɡræm/ | 桑基图 | sāng jī tú |
| work done/wɜːk dʌn/ | 做功 | zuò gōng |
| joule/dʒuːl/ | 焦耳 | jiāo ěr |
| power/ˈpaʊə/ | 功率 | gōng lǜ |
| efficiency/ɪˈfɪʃənsi/ | 效率 | xiào lǜ |
| watt/wɒt/ | 瓦特 | wǎ tè |
Energy never disappears
- At the top of a roller-coaster hill the car is slow but high; at the bottom it is fast but low.
- The energy did not vanish — it just moved from one store to another.
- This is the big idea of this lesson: energy is stored, transferred, and always conserved.
能量从不消失
- 在一座过山车山丘的顶部,车慢但高;在底部它快但低。
- 能量没有消失——它只是从一个储存移动到另一个。
- 这是这节课的大想法:能量被储存、转移,并总是守恒。
Energy stores and transfers
- Energy can be stored as: kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic, and internal (thermal).
- It is transferred between stores by: forces (mechanical work), electric currents, heating, and waves (such as light and sound).
- "Using energy" really means moving it from one store to another.
Wind turbines transfer energy from moving air to electricity
能量储存与转移
- 能量能被储存为:动能、引力势能、化学能、弹性(应变)能、核能、静电能和内(热)能。
- 它在储存之间被转移,通过:力(机械做功)、电流、加热和波(如光和声)。
- "使用能量"实际上意味着把它从一个储存移到另一个。
风力涡轮机把能量从运动的空气转移到电
Energy flow & efficiency · 能量流动与效率
Input energy splits into useful output and wasted energy; efficiency is the useful fraction, and the total is always conserved. · 输入能量分成有用输出和浪费的能量;效率是有用的那一份,而总量始终守恒。
Energy, work & power · 能量、做功与功率
PE + KE = constant · PE + KE = 恒定
As an object falls, potential · 势 energy becomes kinetic — total fixed. · 当一个物体下落时,势能变成动能——总量固定。
Select all · 所有 of these that are energy stores. · 选择这些中所有是能量储存的。
Kinetic, chemical and gravitational potential are energy stores. "Loud" and "fast" describe things, but they are not energy stores. · 动能、化学能和引力势能是能量储存。"响亮"和"快速"描述事物,但它们不是能量储存。
Kinetic and potential energy
- Kinetic energy 动能 is the energy of a moving object:
- Lifting a mass by a height $\Delta h$ changes its gravitational potential energy 重力势能:
- A falling object turns gravitational PE into kinetic energy.
动能与势能
- 动能(kinetic energy)是一个运动物体的能量:
- 把一个质量举高一个高度 $\Delta h$ 改变它的引力势能(gravitational potential energy):
- 一个下落的物体把引力势能转变成动能。
A $2.0\ \text{kg}$ ball moves at $3.0\ \text{m/s}$. What is its kinetic energy, in J? · 一个 $2.0\ \text{kg}$ 的球以 $3.0\ \text{m/s}$ 运动。它的动能是多少,以 J 计?
$E_k = \tfrac{1}{2}mv^2 = \tfrac{1}{2} \times 2.0 \times 3.0^2 = 9.0\ \text{J}$. · $E_k = \tfrac{1}{2}mv^2 = \tfrac{1}{2} \times 2.0 \times 3.0^2 = 9.0\ \text{J}$。
Conservation of energy 能量守恒
- The principle of conservation of energy: energy is never made or destroyed — it only moves between stores.
- Some energy is always wasted — usually spread out as heat to the surroundings.
- A Sankey diagram 桑基图 shows the input energy splitting into useful energy and wasted energy.
On a roller-coaster, stored energy at the top becomes kinetic energy lower down
能量守恒
- 能量守恒原理(principle of conservation of energy):能量从不被创造或毁灭——它只在储存之间移动。
- 一些能量总是被浪费——通常作为热散布到周围环境。
- 一张桑基图(Sankey diagram)显示输入能量分成有用能量和浪费能量。
在一辆过山车上,顶部的储存能量变成下方的动能
When a machine wastes energy, where does the wasted energy usually go? · 当一台机器浪费能量时,浪费的能量通常去哪里?
Energy is conserved — it is never destroyed. Wasted energy is usually spread out as heat, which is hard to use again. · 能量被守恒——它从不被毁灭。浪费的能量通常作为热散布,它难以再次使用。
Work
- Work done 做功 equals the energy transferred. When a force moves an object along its direction:
- The unit of work and energy is the joule 焦耳 (J).
- Lifting a heavier load, or moving it further, does more work.
A swinging pendulum trades gravitational potential energy for kinetic energy and back
做功
- 做功(work done)等于转移的能量。当一个力沿它的方向移动一个物体时:
- 做功和能量的单位是焦耳(joule,J)。
- 举一个更重的负载,或移动它更远,做更多功。
一个摆动的摆把引力势能换成动能再换回来
A force of $50\ \text{N}$ pushes a box $4.0\ \text{m}$ along the floor. How much work is done, in J? · 一个 $50\ \text{N}$ 的力把一个盒子沿地板推 $4.0\ \text{m}$。做了多少功,以 J 计?
$W = Fd = 50 \times 4.0 = 200\ \text{J}$. · $W = Fd = 50 \times 4.0 = 200\ \text{J}$。
Power 功率 and efficiency 效率
- Power is the work done (energy transferred) each second:
- The unit is the watt 瓦特 (W); $1\ \text{W} = 1\ \text{J/s}$.
- Efficiency is how much of the input energy becomes useful:
- It is always less than $100\%$ because some energy is always wasted.
A Sankey diagram: the band widths are proportional to the energy, so the input splits into useful output and wasted heat
功率与效率
- 功率(power)是每秒做的功(转移的能量):
- 单位是瓦特(watt,W);$1\ \text{W} = 1\ \text{J/s}$。
- 效率(efficiency)是多少输入能量变成有用的:
- 它总是小于 $100\%$,因为一些能量总是被浪费。
桑基图:带的宽度与能量成正比,所以输入分成有用输出和浪费的热
A motor does $200\ \text{J}$ of work in $10\ \text{s}$. What is its power, in W? · 一个马达在 $10\ \text{s}$ 内做 $200\ \text{J}$ 的功。它的功率是多少,以 W 计?
$P = \dfrac{W}{t} = \dfrac{200}{10} = 20\ \text{W}$. · $P = \dfrac{W}{t} = \dfrac{200}{10} = 20\ \text{W}$。
A lamp takes in $50\ \text{J}$ of electrical energy and gives out $30\ \text{J}$ of useful light. What is its efficiency, in %? · 一盏灯吸入 $50\ \text{J}$ 的电能并给出 $30\ \text{J}$ 的有用光。它的效率是多少,以 % 计?
efficiency · 效率 $= \dfrac{\text{useful}}{\text{total}} \times 100\% = \dfrac{30}{50} \times 100\% = 60\%$. · 效率 $= \dfrac{\text{useful}}{\text{total}} \times 100\% = \dfrac{30}{50} \times 100\% = 60\%$。
You've got it
- $E_k = \tfrac{1}{2}mv^2$ · $\Delta E_p = mg\,\Delta h$
- energy is conserved — wasted energy usually ends up as heat
- work $W = Fd$ (joule); power $P = \dfrac{W}{t}$ (watt)
- efficiency $= \dfrac{\text{useful}}{\text{total}} \times 100\%$, always below $100\%$
你掌握了
- $E_k = \tfrac{1}{2}mv^2$ · $\Delta E_p = mg\,\Delta h$
- 能量被守恒——浪费的能量通常最终成为热
- 做功 $W = Fd$(焦耳);功率 $P = \dfrac{W}{t}$(瓦特)
- 效率 $= \dfrac{\text{useful}}{\text{total}} \times 100\%$,总是低于 $100\%$