Magnetism and Current-Carrying Wires · 磁性与载流导线
An electric motor: current + magnet = spin
- Inside every electric motor, a current-carrying wire sits in a magnetic field — and gets pushed.
- Because a current is just moving charges, the field pushes the whole wire sideways.
- Reverse the current every half-turn and that push becomes continuous rotation.
- This "motor effect", and its reverse, power almost all our machines.
电动机:电流 + 磁铁 = 旋转
- 在每一台电动机内部,载流导线位于磁场中——并被推动。
- 因为电流就是运动的电荷,场把整根导线往旁边推。
- 每半圈反转一次电流,那个推力就变成连续的旋转。
- 这个"电动机效应"及其逆过程,驱动着我们几乎所有的机器。
The motor effect
- A wire carrying current $I$ and length $L$ in a field $B$ feels a force $F = BIL$.
- Like the force on a single charge, it is perpendicular to both the current and the field.
- Reverse the current or the field and the force flips direction.
- Bigger current, stronger field, or longer wire all give a bigger force.
电动机效应
- 一根载电流 $I$、长度 $L$ 的导线在场 $B$ 中感到力 $F = BIL$。
- 和作用在单个电荷上的力一样,它垂直于电流和场两者。
- 反转电流或场,力就翻转方向。
- 更大的电流、更强的场,或更长的导线,都给出更大的力。
Force on a current · 电流所受的力
Change the field, current and length and watch the motor-effect force respond. · 改变磁场、电流和长度，观察电动机效应力的响应。
A $0.5\ \text{m}$ wire carries $4\ \text{A}$ across a $2\ \text{T}$ field. What force does it feel, in $\text{N}$? · 一根$0.5\ \text{m}$导线载有电流$4\ \text{A}$穿过磁场$2\ \text{T}$。它受到的力是多少，单位为$\text{N}$？
$F = BIL = 2 \times 4 \times 0.5 = 4\ \text{N}$.
The force on a current-carrying wire in a magnetic field is: · 载流导线在磁场中受到的力为：
$F = BIL$ — the motor-effect force on a current. · $F = BIL$ —— 作用于电流的电动机效应力。
Reversing the current through the wire reverses the direction of the force. · 反转通过导线的电流会反转力的方向。
Reversing the current (or the field) flips the force direction. · 反转电流（或磁场）会翻转力的方向。
Select all · 所有 changes that increase the motor-effect force on a wire. · 选择所有能增大导线电动机效应力的变化。
$F = BIL$ grows with field, current and length. Parallel current gives zero force. · $F = BIL$随磁场、电流和长度增大。平行电流产生的力为零。
Fleming's left-hand rule
- Point your left hand: first finger = field, second finger = current, thumb = force.
- Hold them at right angles and they give the three perpendicular directions.
- It is the quick way to find which way the wire is pushed.
- (Use the left hand for the force on a current; the right hand was for a positive charge's velocity.)
弗莱明左手定则
- 伸出左手:食指 = 场,中指 = 电流,拇指 = 力。
- 让它们成直角,就给出三个相互垂直的方向。
- 这是判断导线被推向哪个方向的快捷办法。
- (对电流所受的力用左手;右手是用于正电荷的速度的。)
Which hand's rule gives the force on a current-carrying wire? · 哪只手的规则给出载流导线所受的力？
Fleming's left-hand rule gives the force on a current: field, current, force. · 弗莱明左手定则给出电流所受的力：磁场、电流、力。
A current makes its own field too
- The reverse is also true: a current-carrying wire creates a magnetic field around itself.
- Coil the wire and the fields add to make a strong electromagnet.
- Switch the current off and the field vanishes — a controllable magnet.
- Electromagnets lift scrap cars, ring doorbells and drive loudspeakers.
电流也产生自己的场
- 反过来也成立:载流导线在自己周围产生一个磁场。
- 把导线绕成线圈,各处的场相加,构成一个强的电磁铁。
- 关掉电流,场就消失——一个可控的磁铁。
- 电磁铁能吊起废车、按响门铃、驱动扬声器。
A coil of current-carrying wire acts as an . · 载流线圈相当于一个。
A coil's magnetic fields add to make a strong, switchable electromagnet. · 线圈的磁场叠加形成强大的、可开关的电磁铁。
Use the left hand (Fleming's left-hand rule) for the force on a current-carrying wire — it is easy to grab the wrong hand. And the force needs the current to run across the field: a wire carrying current parallel to the field feels no force.
对载流导线所受的力用左手(弗莱明左手定则)——很容易抓错手。而且力需要电流横越场:载电流平行于场的导线不受力。
A wire of length $0.5\ \text{m}$ carries $4\ \text{A}$ across a $2\ \text{T}$ field.
- $F = BIL = 2 \times 4 \times 0.5 = 4\ \text{N}$.
Double the current to $8\ \text{A}$ and the force doubles to $8\ \text{N}$.
一根长 $0.5\ \text{m}$ 的导线载 $4\ \text{A}$ 横越 $2\ \text{T}$ 的场。
- $F = BIL = 2 \times 4 \times 0.5 = 4\ \text{N}$。
把电流加倍到 $8\ \text{A}$,力加倍到 $8\ \text{N}$。
A current-carrying wire in a magnetic field feels the motor-effect force $F = BIL$, perpendicular to the current and field (Fleming's left-hand rule). Reversing the current or field flips it. A current also creates a field — the basis of the electromagnet.
载流导线在磁场中感到电动机效应力 $F = BIL$,垂直于电流和场(弗莱明左手定则)。反转电流或场就翻转它。电流也产生场——电磁铁的基础。