A-Level Physics | 磁场的产生与图示方法

📚 A-Level Physics | 磁场的产生与图示方法

Magnetic fields are a fundamental part of the electromagnetic theory covered in A-Level physics. This article explains how magnetic fields are produced by permanent magnets and by electric currents, and how they can be represented using field lines, flux density and magnetic flux.

磁场是 A-Level 物理电磁学理论中的基础内容。本文将讲解磁场如何由永磁体和电流产生，以及如何用磁感线、磁通密度和磁通量来图示磁场。

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1. What Is a Magnetic Field? | 什么是磁场？

A magnetic field is a region of space in which a magnetic force is experienced by a moving electric charge or by a magnetic material such as iron. It is a vector field, meaning that at every point it has both a direction and a magnitude.

磁场是空间中这样一个区域：在该区域内，运动的电荷或铁等磁性材料会受到磁力。磁场是矢量场，即空间每一点都有方向和大小。

Magnetic fields are produced fundamentally by moving charge. This includes currents in wires and the internal electron motion that gives permanent magnets their properties.

磁场从根本上由运动电荷产生，包括导线中的电流，以及使永磁体具有磁性的内部电子运动。

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2. Production by Permanent Magnets | 永磁体产生的磁场

Permanent magnets contain many small regions called magnetic domains. Within each domain, the atomic magnetic moments are aligned in the same direction. When a large number of domains point the same way, the material produces a net external magnetic field.

永磁体内部包含许多称为磁畴的小区域。在每个磁畴内，原子磁矩都沿同一方向排列。当大量磁畴方向一致时，材料就会产生净的外部磁场。

Every magnet has a north pole (N) and a south pole (S). Outside the magnet, field lines leave the north pole and enter the south pole; inside the magnet, they continue from the south pole back to the north pole, forming closed loops.

每个磁体都有北极（N）和南极（S）。在磁体外部，磁感线从北极出发进入南极；在磁体内部，磁感线从南极回到北极，形成闭合回路。

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3. Magnetic Field Around a Straight Current-Carrying Conductor | 直线载流导线周围的磁场

When a direct current flows through a straight wire, a magnetic field is produced around the wire. The magnetic field lines are concentric circles centred on the wire, lying in planes perpendicular to the wire.

当直流电通过直导线时，导线周围会产生磁场。磁感线是以导线为圆心的同心圆，位于与导线垂直的平面内。

The direction of the field is given by the right-hand grip rule: if the right thumb points in the direction of conventional current, the curled fingers show the direction of the magnetic field.

磁场方向由右手螺旋定则确定：若右手拇指指向电流方向，弯曲的四指所指方向即为磁场方向。

For a long straight wire in a vacuum, the magnetic flux density B at a perpendicular distance r from the wire is

在真空中，长直导线外距导线垂直距离 r 处的磁通密度 B 为

B = μ₀I / (2πr)

where I is the current and μ₀ is the permeability of free space. Thus the field strength is proportional to the current and inversely proportional to the distance from the wire.

式中 I 为电流，μ₀ 为真空磁导率。因此磁场强度与电流成正比，与到导线的距离成反比。

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4. Magnetic Field of a Flat Circular Coil | 平面圆形线圈的磁场

A single circular loop of wire carrying a current produces a magnetic field that is concentrated around the loop. Field lines pass through the centre of the coil, curve around outside, and return to the other side.

一个载流圆形线圈产生的磁场集中在线圈周围。磁感线穿过线圈中心，在外侧弯曲，再回到另一侧。

When viewed from one face, if the current flows anticlockwise, that face acts as a north pole; if the current flows clockwise, that face acts as a south pole.

从一侧面观察时，若电流沿逆时针方向流动，则该面等效为北极；若电流沿顺时针方向流动，则该面等效为南极。

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5. Magnetic Field of a Solenoid | 螺线管的磁场

A solenoid is a long coil formed by many turns of wire. Its magnetic field is very similar to that of a bar magnet. Inside the solenoid, the field lines are nearly parallel and equally spaced, so the field is approximately uniform.

螺线管是由多匝导线绕成的长线圈。它的磁场与条形磁铁的磁场非常相似。螺线管内部磁感线近似平行且间距相等，因此内部磁场近似匀强。

For an ideal solenoid, the magnetic flux density inside the solenoid is

理想螺线管内部的磁通密度为

B = μ₀nI

where n is the number of turns per unit length. The right-hand grip rule also applies: curl the fingers in the direction of the current, and the thumb points toward the north-pole end of the solenoid.

式中 n 为单位长度上的匝数。右手螺旋定则同样适用：四指沿电流方向弯曲，拇指指向螺线管的北极端。

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6. Representing Fields with Magnetic Field Lines | 用磁感线表示磁场

Magnetic field lines, also called flux lines, are a graphical way to describe a magnetic field. The direction of the field at any point is tangent to the line passing through that point.

磁感线，又称

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