📚 Ampere's Law and Its Applications | 安培定律及其应用
Ampere's law is one of the fundamental laws of electromagnetism. It provides a powerful method for calculating the magnetic field generated by a steady electric current, especially when the configuration possesses high symmetry. It is also known as Ampère's circuital law, and it constitutes one of the four Maxwell equations that govern classical electrodynamics. In this article we also examine the closely related Ampère force law, which describes the mechanical force on a current-carrying conductor in a magnetic field.
安培定律是电磁学的基本定律之一。它提供了一种计算稳恒电流所产生磁场的强大方法，尤其适用于具有高度对称性的电流结构。安培定律又称安培环路定理，是支配经典电动力学的四个麦克斯韦方程之一。本文还将讨论与之密切相关的安培力定律，其描述了载流导体在磁场中所受的机械力。
1. Statement of Ampère's Circuital Law | 安培环路定律的表述
The law states that the closed-loop line integral of the magnetic field B is proportional to the total current passing through the area enclosed by the loop:
该定律指出：磁场 B 沿闭合回路的线积分，与穿过回路所围面积的电流总量成正比：
∮ B · dl = μ₀ Ienc
where μ₀ is the vacuum permeability, dl is a small vector element along the loop, and Ienc is the net current enclosed by the loop. In SI units, μ₀ = 4π × 10⁻⁷ T·m/A.
其中 μ₀ 为真空磁导率，dl 为回路上的微元矢量，Ienc 为回路所包围的净电流。在国际单位制中，μ₀ = 4π × 10⁻⁷ T·m/A。
The direction of B follows the right-hand grip rule: if the thumb points along the current, the curled fingers show the direction of the magnetic field lines.
磁场 B 的方向由右手螺旋定则确定：拇指指向电流方向，四指弯曲的方向即为磁感线的环绕方向。
2. Sign Convention and Right-Hand Rule | 符号约定与右手定则
When applying Ampère's law, the sign of the enclosed current depends on the direction of integration around the loop. The standard convention is:
应用安培定律时，被包围电流的正负取决于积分路径的绕行方向。标准约定如下：
If the current threads the loop in the direction of the right-hand thumb when the fingers curl along the integration path, the current is taken as positive.
若当手指沿积分方向弯曲时，电流穿过回路的方向与右手拇指方向一致，则电流取正值。
If the current threads in the opposite direction, it is taken as negative.
若电流穿过回路的方向与拇指方向相反，则取负值。
Currents that lie entirely outside the chosen loop do not contribute to Ienc.
完全位于所选回路之外的电流对 Ienc 没有贡献。
This sign discipline is essential: a wrong sign produces a wrong field direction and loses marks in structured questions.
这种符号纪律至关重要：符号错误会导致磁场方向判断错误，在结构化题目中会丢分。
3. Application: Magnetic Field of a Long Straight Wire | 应用：长直导线的磁场
Consider an infinitely long straight wire carrying a steady current I. By symmetry, the magnetic field lines form concentric circles centred on the wire, and the magnitude of B is constant on any such circle.
考虑一根通有稳恒电流 I 的无限长直导线。由对称性可知，磁感线是以导线为圆心的同心圆，且 B 的大小在同一圆上处处相等。
Choose an Amperian loop that is a circle of radius r centred on the wire. Since B is parallel to dl at every point and has constant magnitude, the integral simplifies:
选取以导线为圆心、半径为 r 的圆形安培回路。由于 B 处处与 dl 平行且大小恒定，积分可简化为：
∮ B · dl = B × 2πr = μ₀ I
B = μ₀ I / (2πr)
This shows that the field decays as 1/r with distance from the wire. This result is used constantly in A-level questions involving wires, cables, and coaxial conductors.
这表明磁感应强度随距导线的距离按 1/r 衰减。这是 A-level 考试中涉及导线、电缆和同轴导体时反复使用的结果。
4. Application: Magnetic Field of a Solenoid | 应用：螺线管的磁场
A solenoid is a coil wound into a tightly packed helix. When current flows through it, the solenoid behaves like a bar magnet, producing a nearly uniform magnetic field inside its core.
螺线管是将导线紧密绕制而成的螺旋线圈。当电流通过时，螺线管的行为类似条形磁铁，在其内部产生近似均匀的磁场。
For an ideal solenoid with n turns per unit length carrying current I, choose a rectangular Amperian loop that runs along the central axis inside the solenoid and closes outside. The field outside is approximately zero, so only the inside segment contributes:
对理想螺线管，设单位长度匝数为 n，通有电流 I。选取一个矩形安培回路：一段沿螺线管内部轴线，另一段在外部闭合。外部磁场近似为零，因此只有内部线段有贡献：
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