Electricity and magnetism
OxfordAQA · International GCSE · Physics Topic 5 4:50 English narration · English + 中文 subtitles burned in
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Electricity is charge on the move.
电就是运动的电荷。
Potential difference drives the charge, every component resists it, and every current makes a magnetic field.
电势差推动电荷，每个元件都阻碍它，而每一股电流都会产生磁场。
This topic builds the circuit toolkit, then the electromagnets that follow.
本主题先建立电路工具箱，再学习由此而来的电磁铁。
Current is the rate of flow of charge, Q equals I t.
电流是电荷流动的速率，Q 等于 I 乘 t。
A torch bulb carries 0.50 amps for 2.0 minutes from a 9.0 volt battery.
一个手电筒灯泡从 9.0 伏电池获得 0.50 安的电流，持续 2.0 分钟。
First change the time to seconds: 120.
先把时间换成秒：120 秒。
The charge is 0.50 times 120, 60 coulombs.
电荷量为 0.50 乘 120，即 60 库仑。
Each coulomb carries 9.0 joules, so the energy is 540 joules.
每库仑携带 9.0 焦耳，所以能量为 540 焦耳。
Learn the standard symbols: a cell, a lamp, a resistor, a diode, an ammeter and a voltmeter.
要记住标准符号：电池、灯泡、电阻、二极管、电流表和电压表。
The ammeter goes in series, in the loop; the voltmeter goes across the component it measures.
电流表串联在回路中；电压表并联在所测元件两端。
At constant temperature a resistor gives a straight line through the origin: current is proportional to potential difference.
在温度不变时，电阻的图像是一条过原点的直线：电流与电势差成正比。
A filament lamp curves: as the current rises the filament gets hot, its ions vibrate more, and its resistance rises.
灯丝灯的图像是弯曲的：电流增大时灯丝变热，离子振动加剧，电阻增大。
A diode lets current through in one direction only.
二极管只让电流朝一个方向通过。
In the reverse direction there is no current at all.
反向时完全没有电流。
In the forward direction it conducts once the p.d. passes a small threshold.
正向时，电势差超过一个小的阈值后就开始导电。
An LED is a diode that emits light, and wastes far less energy than a filament lamp.
发光二极管（LED）就是会发光的二极管，比灯丝灯浪费的能量少得多。
As a thermistor gets hotter, its resistance falls, steeply at first.
热敏电阻温度升高时，电阻下降，开始时下降得很快。
That makes it a temperature sensor for thermostats.
所以它可用作恒温器中的温度传感器。
A light-dependent resistor behaves the same way with light: in bright light its resistance falls, which switches automatic lighting.
光敏电阻对光的反应相同：光越强，电阻越小，可用来控制自动照明。
A 2.0 ohm and a 4.0 ohm resistor in series with a 12 volt battery.
一个 2.0 欧和一个 4.0 欧的电阻与 12 伏电池串联。
Total resistance is 6.0 ohms, so the current is 12 divided by 6.0: 2.0 amps everywhere in the loop.
总电阻为 6.0 欧，所以电流为 12 除以 6.0，回路中处处都是 2.0 安。
The 2.0 ohm resistor takes 4.0 volts; the 4.0 ohm resistor takes 8.0 volts.
2.0 欧的电阻分得 4.0 伏；4.0 欧的电阻分得 8.0 伏。
4 plus 8 is 12.
4 加 8 等于 12。
A 6.0 ohm and a 3.0 ohm resistor in parallel across 6.0 volts.
一个 6.0 欧和一个 3.0 欧的电阻并联在 6.0 伏电源上。
Each branch has the full 6.0 volts, so the 6.0 ohm branch carries 1.0 amp and the 3.0 ohm branch 2.0 amps.
每条支路都有完整的 6.0 伏，所以 6.0 欧支路电流 1.0 安，3.0 欧支路电流 2.0 安。
The battery supplies 3.0 amps, so the combined resistance is 2.0 ohms: less than either resistor.
电池提供 3.0 安，所以总电阻为 2.0 欧：比任何一个电阻都小。
Two 4 ohm resistors across 12 volts.
两个 4 欧的电阻接在 12 伏电源上。
In series the resistance is 8 ohms and the supply gives 1.5 amps.
串联时总电阻 8 欧，电源提供 1.5 安。
In parallel the resistance is 2 ohms and the supply gives 6 amps: four times the current from the same parts.
并联时总电阻 2 欧，电源提供 6 安：同样的元件，电流是四倍。
Like poles repel and unlike poles attract, without touching.
同名磁极相斥，异名磁极相吸，不需要接触。
The field lines show the direction a north pole would be pushed: they run from N to S outside the magnet, and are closest together, where the field is strongest, at the poles.
磁感线表示北极受力的方向：在磁铁外部从 N 指向 S，在磁极处最密集，那里磁场最强。
Put opposite poles facing each other and the field between them is uniform: the field lines are parallel and equally spaced, so the field has the same strength and direction everywhere in the gap.
让异名磁极相对，它们之间的磁场是匀强磁场：磁感线平行且间距相等，所以间隙中各处磁场的大小和方向都相同。
A current-carrying wire has circular field lines around it.
通电导线周围有环形磁感线。
A solenoid, a coil of wire, adds the fields of its turns together, so outside it the field is like a bar magnet.
螺线管就是一圈圈导线，它把每一匝的磁场叠加起来，所以外部磁场就像条形磁铁。
Put a soft-iron core inside and it becomes an electromagnet.
在里面放入软铁芯，它就成了电磁铁。
Three ways to make an electromagnet stronger: more turns on the coil, a larger current, and a soft-iron core.
让电磁铁更强有三种方法：增加线圈匝数、增大电流、加入软铁芯。
Because it can be switched on and off, it lifts scrap in a crane, and works relays and door locks.
因为它可以通断，所以能用在起重机上吊废铁，也能用于继电器和门锁。
A permanent magnet has its own field and can attract or repel.
永磁体有自己的磁场，既能吸引也能排斥。
An induced magnet is a magnetic material that becomes a magnet only while it is in a field.
感应磁体是一种磁性材料，只有处在磁场中时才成为磁体。
It always attracts, and loses most of its magnetism when it is removed.
它总是吸引，离开磁场后大部分磁性就消失了。
Four common mistakes. Leaving time in minutes in Q equals I t: use seconds.
四个常见错误：在 Q = It 中用分钟：要用秒。
Adding resistances in parallel: the combined resistance is less than either.
并联电阻直接相加：总电阻比任何一个都小。
Drawing a voltmeter in series: it goes across the component.
把电压表串联：它应并联在元件两端。
And saying a hot thermistor's resistance rises: it falls.
以及说热敏电阻变热时电阻增大：其实是减小。
Check yourself.
自我检测。
2 amps for 30 seconds is 60 coulombs.
2 安持续 30 秒是 60 库仑。
12 volts pushing 0.5 amps means 24 ohms.
12 伏推动 0.5 安，电阻为 24 欧。
A heated thermistor's resistance falls.
热敏电阻受热时电阻减小。
To strengthen an electromagnet, add more turns or a larger current.
要增强电磁铁，可增加匝数或增大电流。