Magnetic fields due to currents · 电流产生的磁场
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| iron core/ˈaɪən kɔː/ | 铁芯 | tiě xīn |
| right-hand grip rule/raɪt hænd ɡrɪp ruːl/ | 右手定则 | yòu shǒu dìng zé |
| bar magnet/bɑː ˈmæɡnɪt/ | 条形磁铁 | tiáo xíng cí tiě |
| solenoid/ˈsəʊlənɔɪd/ | 螺线管 | luó xiàn guǎn |
A magnet with an off switch
- A scrapyard crane picks up a tonne of steel, swings it across the yard, and drops it by flicking a switch.
- No permanent magnet can do that. What the crane carries is a coil of wire around an iron core, and its magnetism exists only while the current flows.
- Every field in this lesson comes from a current, which means every one of them can be switched, reversed or made stronger at will.
- This lesson is the field patterns that currents produce, and the forces between two currents.
带开关的磁铁
- 废料场的吊车吊起一吨钢材,横穿场地,然后一按开关就把它放下。
- 没有哪块永磁体能做到这一点。吊车带的是绕在铁芯上的一圈导线,它的磁性只在电流流动时存在。
- 这一课里的每一个磁场都来自电流,这意味着它们每一个都能被随意开关、反向或增强。
- 这一课讲电流产生的场的图样,以及两条电流之间的力。
The field around a straight wire
- The field forms concentric circles around the wire, in the plane perpendicular to it.
- Their spacing widens with distance, because the field weakens as you move away.
- The direction comes from the right-hand grip rule 右手定则: grip the wire with your right hand, thumb along the current, and your fingers curl the way the field points.
- For a current into the page, that gives a clockwise field.
Circles, widening, each with an arrow
直导线周围的场
- 磁场在垂直于导线的平面内形成同心圆。
- 它们的间距随距离变宽,因为场随着远离而减弱。
- 方向由右手定则(right-hand grip rule)给出:用右手握住导线,拇指沿电流,四指弯曲的方向就是场的方向。
- 对流入纸面的电流,这给出顺时针的场。
一圈圈的圆,间距变宽,每条都带箭头
Current field rule lab · 电流磁场规则实验
Connect current direction to the circular magnetic field around a wire. · 将电流方向与导线周围的环形磁场联系起来。
The magnetic field around a long straight wire forms: · 长直导线周围的磁场形成：
Concentric circles, with the direction given by the right-hand grip rule. · 同心圆，方向由右手螺旋定则给出。
The right-hand grip rule gives the direction of the field around a current-carrying wire. · 右手螺旋定则给出了载流导线周围磁场的方向。
Thumb along the current, curled fingers show the way the circular field points. · 拇指指向电流方向，弯曲手指指示环形磁场的方向。
The flat circular coil
- Bend the wire into a loop and the circular fields around each side reinforce through the centre.
- So the field at the centre is at right angles to the plane of the coil, and strongest there.
- Seen from one face the coil behaves as a north pole, since the field leaves it there; from the other face, a south pole. A single loop is already a small bar magnet 条形磁铁.
平面圆线圈
- 把导线弯成一个环,环两侧周围的圆形磁场就在中心处相互加强。
- 所以中心处的场垂直于线圈平面,并在那里最强。
- 从一面看,线圈表现为北极,因为场从那里出来;从另一面看是南极。单单一个环就已经是一块小条形磁铁(bar magnet)。
What is the direction of the magnetic field at the centre of a flat circular coil? · 平面圆形线圈中心的磁场方向是什么？
The circular fields around the two sides reinforce through the centre, so the coil acts as a small bar magnet, north from one face and south from the other. · 两侧周围的环形磁场在中心处相互加强，因此线圈表现得像一个小条形磁铁，一面为北极，另一面为南极。
The solenoid
- A solenoid 螺线管 is many such loops in a row. Their fields add along the axis, so inside the field is nearly uniform and parallel to the axis, exactly like a stretched bar magnet.
- Outside it spreads out and falls off fast, and the ends behave as N and S poles.
- Three things strengthen it: more turns per unit length, a larger current, and an iron core 铁芯.
Uniform down the axis, and poles at the ends
螺线管
- 螺线管(solenoid)是许多这样的环排成一列。它们的场沿轴线相加,所以内部的场几乎均匀且平行于轴线,恰如一根拉长的条形磁铁。
- 外部它散开并迅速减弱,两端表现为 N 极和 S 极。
- 有三样东西能增强它:单位长度上更多的匝数、更大的电流,以及一个铁芯(iron core)。
沿轴线均匀,两端是磁极
Inside a long solenoid the magnetic field is nearly . · 长螺线管内部的磁场几乎是。
Like a stretched bar magnet — the field inside is uniform and parallel to the axis. · 像拉伸的条形磁铁——内部磁场均匀且平行于轴线。
Adding an iron core greatly increases a solenoid's magnetic field. · 加入铁芯可极大地增强螺线管的磁场。
The iron's atomic magnets line up and add to the field — used in electromagnets and transformers. · 铁芯的原子磁矩排列并叠加磁场——用于电磁铁和变压器中。
Which changes increase the magnetic field inside a solenoid? Select all · 所有 that apply. · 哪些变化会增加螺线管内部的磁场？选择所有适用项。
Turns per unit length, current and a ferrous core. A thicker wire lowers the resistance but does not itself change the field for a given current. · 单位长度匝数、电流和铁磁性芯材。较粗的导线会降低电阻，但对于给定电流而言不会直接改变磁场。
Worked example: why the core helps
- Explain why inserting an iron core greatly increases the magnetic field of a solenoid.
- The core is a ferrous material, so the solenoid's field magnetises it: its atomic magnets line up with the field.
- The magnetised core then produces its own magnetic field, which adds to the field of the current.
- The two together are far stronger than the current's field alone. Both steps, magnetised and adds its own field, are needed for the marks. "It concentrates the field lines" is not the marked answer.
例题:铁芯为什么有用
- 解释插入铁芯为什么会大大增强螺线管的磁场。
- 铁芯是含铁材料,所以螺线管的场把它磁化:它的原子磁体沿着磁场排列起来。
- 被磁化的铁芯于是产生它自己的磁场,而这个场叠加到电流的场上。
- 两者合起来远强于单独电流的场。得分需要两步都有:被磁化,并且叠加自己的场。"它把场线聚集起来"不是评分认可的答案。
Put the explanation of why an iron core strengthens a solenoid in order. · 按顺序说明铁芯为何能增强螺线管磁场。
Both middle steps are needed for the marks: magnetised, and adding its own field. "It concentrates the field" is not the marked wording. · 中间两步都需要才能得分：磁化，以及产生自身磁场。“它集中了磁场”不是标准表述。
Two parallel currents
- Each wire sits in the field of the other, so each experiences a motor-effect force $F = BIL$.
- Apply the left-hand rule to each wire in turn, using the field the other one makes. The result is symmetric, as Newton's third law requires.
- Currents in the same direction attract. Currents in opposite directions repel.
- This force is the basis of the historical definition of the ampere.
两条平行电流
- 每根导线都处在另一根的场中,所以各自受到电动机效应的力 $F = BIL$。
- 依次对每根导线用左手定则,用的是另一根产生的场。结果是对称的,这正是牛顿第三定律所要求的。
- 同方向的电流相吸。反方向的电流相斥。
- 这个力是安培历史定义的基础。
Two parallel wires carrying currents in the same · 相同 direction: · 两根通有同向电流的平行导线：
Same-direction currents attract; opposite-direction currents repel. This is the basis of the ampere. · 同向电流相互吸引；反向电流相互排斥。这是安培定义的基础。
Match each task to the rule you use. · 将每个任务与其使用的规则匹配。
A parallel-wires question needs both: the right hand for the field one wire makes, then the left hand for the force it exerts on the other. · 平行导线问题需要两者结合：先用右手判断一根导线产生的磁场，再用左手判断其对另一根导线的作用力。
Worked example: predicting the direction
- Two long parallel wires carry currents in the same direction. Show that they attract.
- Use the right-hand grip rule on the left wire: in the region of the right wire, its field points into the page, say.
- Now use the left-hand rule on the right wire: field into the page, current up the page, so the force on it is towards the left wire.
- Repeat for the other wire and the force is towards the first. They attract. Note that like currents attract, which is the opposite of what like charges do.
例题:预测方向
- 两根长平行导线载有同方向的电流。证明它们相吸。
- 对左边那根用右手定则:在右边那根所在的区域,比方说它的场指入纸面。
- 现在对右边那根用左手定则:场指入纸面,电流沿纸面向上,所以它受到的力朝向左边那根导线。
- 对另一根重复一次,力朝向第一根。它们相吸。注意同向电流相吸,这与同号电荷的行为恰好相反。
Marks that slip away
- Right hand for the field a current makes; left hand for the force on a current in a field. Both appear in a parallel-wires question and they are not interchangeable.
- The field around a wire is circles, not lines along the wire, and their spacing widens.
- The iron core is magnetised and adds its own field. Say both parts.
- Currents in the same direction attract, unlike charges of the same sign.
容易丢掉的分
- 求电流产生的场用右手;求场中电流受的力用左手。平行导线题里两者都会出现,而且不能互换。
- 导线周围的场是圆,不是沿导线的直线,而且它们的间距变宽。
- 铁芯是被磁化并叠加自己的场。两部分都要说。
- 同方向的电流相吸,与同号电荷不同。
You've got it
- a straight wire's field is concentric circles with widening spacing, direction from the right-hand grip rule
- a flat coil gives a field at right angles through its centre and acts as a small bar magnet; a solenoid is nearly uniform inside and has poles at its ends
- more turns per unit length, a larger current and an iron core all strengthen a solenoid, the core because it is magnetised and adds its own field
- parallel currents attract when in the same direction and repel when opposite, found by using the right-hand rule for the field and the left-hand rule for the force
你掌握了
- 直导线的场是间距变宽的同心圆,方向由右手定则确定
- 平面线圈在中心给出垂直于线圈平面的场,像一块小条形磁铁;螺线管内部几乎均匀,两端有磁极
- 单位长度更多匝数、更大电流和铁芯都能增强螺线管,铁芯是因为它被磁化并叠加自己的场
- 平行电流同向时相吸、反向时相斥,用右手定则求场、左手定则求力即可判定