📚 Magnetic Flux Density | 磁通量密度
In CIE A Level Physics, magnetic flux density is one of the central ideas in electromagnetism. It describes the strength of a magnetic field and determines the force experienced by current-carrying conductors and moving charges. This article explains the definition, units, key equations, measurement methods and common exam traps.
在 CIE A Level 物理中，磁通量密度是电磁学的核心概念之一。它描述磁场的强弱，并决定载流导体和运动电荷所受的力。本文将讲解其定义、单位、关键公式、测量方法和常见考试陷阱。
1. Definition of Magnetic Flux Density | 磁通量密度的定义
Magnetic flux density, symbol B, is a vector quantity that measures the strength and direction of a magnetic field. It is defined by the force on a straight current-carrying conductor placed in the magnetic field.
磁通量密度，符号为 B，是一个矢量，用来衡量磁场的强弱和方向。它通过磁场中直载流导体所受的力来定义。
If a straight conductor of length L carries a current I and is placed perpendicular to a uniform magnetic field, the magnetic flux density B is given by the equation below, where F is the magnetic force on the conductor.
若长度为 L 的直导线载有电流 I，并垂直置于匀强磁场中，则磁通量密度 B 由下式给出，其中 F 是导体所受的磁力。
B = F / (IL)
or field contributes to the force, so the more general relationship becomes F = BIL sin θ.
该定义假设导体与磁场垂直。当导体与磁感线成 θ 角时，只有电流或磁场的垂直分量对力有贡献，因此更一般的关系式为 F = BIL sin θ。
2. Units and Dimensions | 单位与量纲
The SI unit of magnetic flux density is the tesla, symbol T. From the definition, 1 T is equivalent to 1 newton per ampere per metre.
磁通量密度的国际单位是特斯拉，符号为 T。根据定义，1 T 等于每安培每米 1 牛顿。
1 T = 1 N A⁻¹ m⁻¹
Because magnetic flux Φ is measured in webers (Wb) and area A in square metres, magnetic flux density can also be expressed as webers per square metre. Hence the tesla has an equivalent unit.
由于磁通量 Φ 的单位是韦伯 (Wb)，面积的单位是平方米，因此磁通量密度也可以表示为每平方米韦伯。所以特斯拉有一个等价单位。
1 T = 1 Wb m⁻²
The tesla is a relatively large unit. The Earth's magnetic flux density is of the order of 10⁻⁵ T to 10⁻⁴ T, while a strong laboratory electromagnet may produce 1 T to 3 T.
特斯拉是一个比较大的单位。地球的磁通量密度约为 10⁻⁵ T 到 10⁻⁴ T 的数量级，而强实验室电磁铁可以产生 1 T 到 3 T 的磁场。
3. Force on a Current-Carrying Conductor | 载流导体所受的力
The magnetic force on a straight conductor carrying current I in a uniform magnetic field B is described by Fleming's left-hand rule. The magnitude of the force is given below, where θ is the angle between the conductor and the magnetic field direction.
在匀强磁场 B 中，载有电流 I 的直导体所受的磁力可用弗莱明左手定则描述。力的大小由下式给出，其中 θ 是导体与磁场方向之间的夹角。
F = BIL sin θ
1, so F = BIL. When the conductor is parallel to the field, θ = 0° and the force is zero.
当导体与磁场垂直时，θ = 90°，sin θ = 1，所以 F = BIL。当导体与磁场平行时，θ = 0°，力为零。
The direction of the force is always perpendicular to both the current direction and the magnetic field direction. Fleming's left-hand rule links the directions of field, current and force.
力的方向始终垂直于电流方向和磁场方向。弗莱明左手定则将磁场、电流和力的方向联系起来。
4. Magnetic Flux and Flux Linkage | 磁通量与磁链
Magnetic flux Φ through a plane surface is defined as the product of the magnetic flux density perpendicular to the surface and the area of the surface.
通过某一平面的磁通量 Φ 定义为垂直于该表面的磁通量密度与表面面积的乘积。
Φ = BA cos θ
surface, θ = 0° and Φ = BA. If the field lies parallel to the surface, θ = 90° and Φ = 0.
这里 θ 是磁感线与表面法线之间的夹角。若磁场垂直于表面，θ = 0°，则 Φ = BA。若磁场平行于表面，θ = 90°，则 Φ = 0。
N turns, the magnetic flux linkage is NΦ = NBA cos θ. This quantity is central to Faraday's law of electromagnetic induction.
对于 N 匝线圈，磁链为 NΦ = NBA cos θ。这个量是法拉第电磁感应定律的核心。
NΦ = NBA cos θ
5. Magnetic Flux Density Inside a Solenoid | 螺线管内部的磁通量密度
A long solenoid carrying current I with n turns per unit length produces a nearly uniform magnetic field inside. The flux density at the centre is given by the equation below, where μ₀ is the permeability of free space.
载有电流 I、每单位长度 n 匝的长螺线管内部会产生近似匀强的磁场。中心处的磁通量密度由下式给出，其中 μ₀ 是真空磁导率。
B = μ₀ n I
Outside a long solenoid, the field is very weak, so the flux density is approximately zero. Inside, the field lines are parallel and evenly spaced, indicating a uniform flux density.
在长螺线管外部，磁场非常弱，因此磁通量密度近似为零。在内部，磁感线平行且均匀分布，表明磁通量密度是均匀的。
6. Measuring Magnetic Flux Density Using a Hall Probe | 用霍尔探头测量磁通量密度
A Hall probe is a practical device for measuring magnetic flux density. It makes use of the Hall effect: when a current flows through a thin semiconductor slice in a perpendicular magnetic field, a Hall voltage is generated across the slice.
霍尔探头是测量磁通量密度的实用仪器。它利用霍尔效应：当电流流过置于垂直磁场中的薄半导体片时，薄片两端会产生霍尔电压。
VH = BI / (n q t)
Since the Hall voltage is proportional to B for a fixed current, the probe can be calibrated with a known field. The probe is placed in the field and rotated until the maximum reading is obtained; this corresponds to the field direction being perpendicular to the probe face.
由于在电流固定的情况下霍尔电压与 B 成正比，因此探头可以用已知磁场进行校准。将探头放入磁场并旋转，直到获得最大读数；这对应于磁场方向垂直于探头表面。
The Hall probe is particularly useful for measuring flux density between the poles of magnets and inside solenoids, where a direct mechanical force measurement would be difficult.
霍尔探头特别适用于测量磁铁两极之间和螺线管内部的磁通量密度，因为这些位置难以直接通过机械力测量。
7. Motion of Charged Particles in a Magnetic Field | 带电粒子在磁场中的运动
is the angle between v and B. The force is always perpendicular to the velocity, so it changes direction but not speed.
以速度 v 在磁场 B 中运动的带电粒子会受到磁力 F = Bqv sin θ，其中 θ 是 v 与 B 之间的夹角。该力始终垂直于速度，因此它改变方向但不改变速度大小。
F = Bqv sin θ
If the particle moves perpendicular to a uniform magnetic field, it follows a circular path. The magnetic force provides the centripetal force, giving the radius of the circular motion.
若粒子垂直于匀强磁场运动，它会沿圆形路径运动。磁力提供向心力，从而得出圆周运动的半径。
Bqv = mv² / r → r = mv / (Bq)
This relationship is used in mass spectrometers and particle accelerators. It also shows that for a fixed B and charge q, the radius depends on the momentum mv of the particle.
这一关系用于质谱仪和粒子加速器。它还表明，在 B 和电荷 q 固定的情况下，半径取决于粒子的动量 mv。
8. Superposition of Magnetic Fields | 磁场的叠加
Magnetic flux density is a vector, so when several magnetic fields overlap, the resultant flux density is the vector sum of the individual flux densities.
磁通量密度是矢量，因此当多个磁场重叠时，合磁通量密度是各个磁通量密度的矢量和。
B_resultant = B₁ + B₂ + B₃ + ...
For example, the field at a point between two parallel conductors depends on the directions of the currents. If the currents are in the same direction, the fields between the wires oppose each other; if the currents are opposite, the fields reinforce between the wires.
例如，两根平行导体之间某一点的磁场取决于电流方向。若电流同向，导线之间的磁场相互削弱；若电流反向，导线之间的磁场相互增强。
Vector superposition is essential for calculating the field of coils, solenoids and combinations of magnets.
矢量叠加对于计算线圈、螺线管以及磁铁组合的磁场至关重要。
9. Common Misconceptions and Exam Tips | 常见误区与应试技巧
A common mistake is to confuse magnetic flux density B with magnetic flux Φ. B is the field strength per unit area, while Φ is the total magnetic field passing through a given area.
一个常见错误是混淆磁通量密度 B 与磁通量 Φ。B 是单位面积上的磁场强度，而 Φ 是穿过给定面积的总磁场量。
d the field; in Φ = BA cos θ, θ is the angle between the field and the normal to the area.
另一个错误是用错角度。在 F = BIL sin θ 中，θ 是导体与磁场之间的夹角；而在 Φ = BA cos θ 中，θ 是磁场与面积法线之间的夹角。
| 公式 | 角度 θ 的含义 |
| F = BIL sin θ | 导体与磁场的夹角 |
| Φ = BA cos θ | 磁场与面积法线的夹角 |
| F = Bqv sin θ | 速度与磁场的夹角 |
Always remember that the magnetic force is perpendicular to both the current and the field, as described by Fleming's left-hand rule. If the conductor is parallel to the field, the force is zero, not maximum.
始终记住，磁力垂直于电流和磁场，正如弗莱明左手定则所述。若导体与磁场平行，力为零，而不是最大。
10. Worked Example and Summary | 实例与总结
A short worked example can illustrate the key ideas. A 0.20 m wire carries a current of 3.0 A and is perpendicular to a magnetic field of flux density 0.50 T. The force on the wire is calculated below.
一个简短的例题可以说明关键概念。一根 0.20 m 的导线载有 3.0 A 的电流，并垂直于磁通量密度为 0.50 T 的磁场。导线所受的力计算如下。
F = BIL = 0.50 × 3.0 × 0.20 = 0.30 N
kes an angle of 30° with the field, the force becomes F = BIL sin 30° = 0.30 × ½ = 0.15 N.
如果导线旋转到与磁场成 30° 角，则力变为 F = BIL sin 30° = 0.30 × ½ = 0.15 N。
F = 0.50 × 3.0 × 0.20 × sin 30° = 0.15 N
ion Φ = BA cos θ and the solenoid equation B = μ₀ n I are all central to CIE examinations.
总之，磁通量密度是一个矢量，它将磁场与电流和运动电荷所受的力联系起来。定义式 B = F / (IL sin θ)、磁通量公式 Φ = BA cos θ 以及螺线管公式 B = μ₀ n I 都是 CIE 考试的核心内容。
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