Magnetic Fields and Electromagnetism | 磁场与电磁学

📚 Magnetic Fields and Electromagnetism | 磁场与电磁学

Magnetic fields and electromagnetism form one of the central pillars of CIE A-Level Physics. From the force on a current-carrying wire to electromagnetic induction and transformers, these ideas link electric currents, force and energy transfer in a coherent way.

磁场与电磁学是 CIE A-Level 物理的核心支柱之一。从通电导线在磁场中的受力，到电磁感应和变压器，这些概念将电流、力与能量传递有机地联系起来。

1. Magnetic Field Concepts | 磁场基本概念

A magnetic field is a region of space in which a moving charge, a current-carrying conductor or a magnetic material experiences a force. The direction of a magnetic field at a point is the direction of the force on a north pole placed there.

磁场是空间中运动电荷、通电导线或磁性材料会受到力的区域。磁场的方向定义为置于该点的北极所受力的方向。

Magnetic flux density B is defined as the force per unit current per unit length acting on a straight conductor placed perpendicular to the field. Its SI unit is the tesla (T), where 1 T = 1 N A⁻¹ m⁻¹.

磁通量密度 B 定义为垂直于磁场的直导线单位电流、单位长度所受的力。其国际单位是特斯拉（T），1 T = 1 N A⁻¹ m⁻¹。

Field lines are used to represent magnetic fields. They point from north to south outside a magnet, and their spacing indicates field strength: closer lines mean a stronger field.

磁感线用于表示磁场。磁体外部的磁感线从北极指向南极，其疏密表示磁场强弱：磁感线越密，磁场越强。

2. Magnetic Force on a Current-Carrying Conductor | 电流在磁场中的力

A current-carrying conductor placed in a magnetic field experiences a force as long as the current is not parallel to the field. The magnitude is given by F = BIL sin θ, where θ is the angle between the field B and the current I.

通电导线放在磁场中时，只要电流方向与磁场不平行，就会受到力的作用。其大小由 F = BIL sin θ 给出，其中 θ 是磁场 B 与电流 I 之间的夹角。

F = BIL sin θ

The direction of this force is given by Fleming’s left-hand rule: the first finger points in the field direction, the second finger in the conventional current direction, and the thumb gives the force direction.

该力的方向由弗莱明左手定则判断：食指指向磁场方向，中指指向电流方向，拇指指向力的方向。

When the conductor is perpendicular to the field, sin θ = 1 and the force is maximum: F = BIL. When parallel, sin θ = 0 and no force acts.

当导线与磁场垂直时，sin θ = 1，力最大：F = BIL。当导线与磁场平行时，sin θ = 0，不受力。

3. Force on a Moving Charge | 运动电荷在磁场中的力

A single charge q moving with velocity v in a magnetic field B experiences a force F = Bqv sin θ, where θ is the angle between v and B. For a negative charge, the force direction is opposite to that predicted for a positive charge.

单个电荷 q 以速度 v 在磁场 B 中运动时，会受到

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