Generating and distributing electricity
OxfordAQA · International GCSE · Physics Topic 6 4:57 English narration · English + 中文 subtitles burned in
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Move a magnet near a coil and a potential difference appears.
在线圈附近移动磁铁，就会产生电势差。
This topic follows electricity from the generator, through the transformers of the national grid, to the appliances and safety systems of the home, and back to magnets in the motor effect.
本主题沿着电的路径：从发电机出发，经过国家电网的变压器，到达家中的电器和安全装置，最后回到磁体的电动机效应。
Push a magnet into a coil and the meter shows a potential difference: it is induced.
把磁铁推入线圈，电表显示出电势差：这是感应电势差。
Hold the magnet still and it drops to zero.
磁铁静止时，示数降为零。
Pull the magnet out, or turn it round, and the induced p.d. reverses.
把磁铁拉出，或把磁铁调转，感应电势差就反向。
Four things make the induced p.d. bigger: faster movement, a stronger field, more turns on the coil, and a larger coil area.
有四种方法能增大感应电势差：运动更快、磁场更强、线圈匝数更多、线圈面积更大。
Power stations use all of them, spinning big coils fast between strong magnets.
发电站全都用上：让大线圈在强磁体之间快速旋转。
An alternator spins a coil in a field and gives alternating current: the p.d. swings positive and negative.
交流发电机让线圈在磁场中旋转，产生交流电：电势差在正负之间摆动。
A dynamo uses a split connection so the output never changes sign: the humps all point one way.
直流发电机用换向器连接，输出永不变号：所有波峰都朝同一方向。
The power station generates at about 25 kilovolts.
发电站发出约 25 千伏的电。
A transformer steps it up to 400 kilovolts for the cables that carry it across the country.
变压器把它升到 400 千伏，由电缆输送到全国各地。
Near the town another transformer steps it down to 230 volts for homes.
在城镇附近，另一台变压器把电压降到230 伏供家庭使用。
Sending 100 megawatts at 25 kilovolts needs a current of 4000 amps.
以 25 千伏输送 100 兆瓦，需要 4000 安的电流。
At 400 kilovolts the same power needs only 250 amps.
以 400 千伏输送同样的功率，只需 250 安。
A smaller current heats the cables far less, so much less energy is wasted.
电流小，电缆发热就少得多，浪费的能量也少得多。
A transformer is two coils on a soft-iron core.
变压器是绕在软铁芯上的两个线圈。
Alternating current in the primary coil makes a changing field in the core, which induces a p.d. in the secondary coil.
初级线圈中的交流电在铁芯中产生变化的磁场，在次级线圈中感应出电势差。
The turns ratio sets the voltages: more turns on the secondary steps the voltage up.
匝数比决定电压：次级匝数多，电压就升高。
A laptop transformer changes 230 volts to 11.5 volts, with 2000 primary turns.
笔记本电脑的变压器把 230 伏变成 11.5 伏，初级线圈 2000 匝。
The secondary needs 2000 times 11.5 over 230: 100 turns.
次级线圈需要 2000 乘 11.5 除以 230，即 100 匝。
It gives 2.0 amps to the laptop; at 100 percent efficiency the primary current is 11.5 times 2.0 over 230: 0.10 amps.
它给电脑提供 2.0 安；效率为百分之百时，初级电流为 11.5 乘 2.0 除以 230，即 0.10 安。
On an oscilloscope the mains trace repeats.
在示波器上，市电的波形不断重复。
One complete cycle covers 4 squares at 5 milliseconds per square.
一个完整周期占 4 格，每格 5 毫秒。
Period: 20 milliseconds, 0.020 seconds.
周期为 20 毫秒，即 0.020 秒。
The frequency is 1 divided by 0.020: 50 hertz.
频率等于 1 除以0.020，即 50 赫兹。
If the live wire touches a metal casing, the earth wire carries the fault current away.
如果火线碰到金属外壳，地线会把故障电流导走。
That surge blows the fuse, which melts and cuts off the live wire.
这股大电流会熔断保险丝，切断火线。
A circuit breaker trips and does the same job, and can be reset.
断路器跳闸，起同样的作用，而且可以复位。
A wire sits between a N pole on the left and a S pole on the right, so the field points to the right.
一根导线位于左边的 N 极和右边的 S 极之间，所以磁场向右。
The current flows into the page.
电流流入纸面。
The wire is pushed downwards.
导线受力向下。
Reverse the current or the poles and the force is up.
把电流或磁极反过来，力就向上。
Fleming's left-hand rule predicts the direction.
弗莱明左手定则可以判断力的方向。
Point your first finger along the field, your second finger along the current, and your thumb gives the motion.
食指指向磁场，中指指向电流，拇指就指向运动方向。
A coil carrying current in a field turns: that is the electric motor.
通电线圈在磁场中会转动：这就是电动机。
A 230 volt, 1.1 kilowatt kettle.
一个 230 伏、1.1 千瓦的电水壶。
The current is the power divided by the voltage: 1100 over 230, about 4.8 amps.
电流等于功率除以电压：1100 除以 230，约 4.8 安。
Choose the next fuse rating above that: 5 amps, or 13.
选比它大的下一档保险丝：5 安或 13 安。
A 3 amp fuse would blow every time.
3 安的保险丝每次都会熔断。
A meter reads 12 345 kilowatt-hours at the start of a month and 12 465 at the end: 120 kilowatt-hours used.
电表月初读数 12 345 千瓦时，月末 12 465：用了 120 千瓦时。
At 0.9 per unit the bill is 108.
每度 0.9，电费为 108。
A 1500 watt heater on for 30 minutes uses 1.5 kilowatts times 0.5 hours: 0.75 kilowatt-hours.
一台 1500 瓦的取暖器开 30 分钟，用电 1.5 千瓦乘 0.5 小时，即 0.75 千瓦时。
Four common mistakes. A transformer does not work on d.c.: it needs a changing field.
四个常见错误：变压器不能用直流电：它需要变化的磁场。
High voltage is not faster: it means low current and less heating.
高压不是更快：而是电流小、发热少。
Kilowatt-hours need kilowatts and hours.
千瓦时要用千瓦和小时。
And the motor force is zero when the wire is parallel to the field, biggest at right angles.
导线与磁场平行时，电动机的力为零，垂直时最大。
Check yourself.
自我检测。
200 primary and 3000 secondary turns on 12 volts gives 180 volts.
初级 200 匝、次级 3000 匝，接 12 伏，输出 180 伏。
A 230 volt, 2.3 kilowatt heater draws 10 amps: fit a 13 amp fuse.
230 伏、2.3 千瓦的取暖器电流 10 安：用 13 安保险丝。
A 1.5 kilowatt kettle for 20 minutes uses 0.5 kilowatt-hours.
1.5 千瓦的电水壶用 20 分钟，耗电 0.5 千瓦时。
High voltage means low current and less heating.
高压意味着电流小、发热少。