📚 IB Physics: Field Lines and Field Strength | IB物理：描述场的力线与场强
Fields are regions of space in which an object experiences a force without physical contact. The gravitational field, electric field and magnetic field are essential topics in IB Physics. Two complementary ways are used to describe these fields: field lines provide a visual and qualitative picture, while field strength gives a quantitative vector measurement. This article explains how to draw and interpret field lines, link them to field strength, and apply the key equations with confidence.
场是空间中一个物体无需物理接触就能感受到力的区域。引力场、电场和磁场是IB物理的核心内容。描述这些场有两种互补的方式：场线提供直观的定性图像，而场强则给出定量的矢量测量。本文将系统讲解如何绘制和解读场线，将场线与场强联系起来，并自信地应用关键公式。
1. What Are Field Lines? | 什么是场线？
A field line, sometimes called a line of force, is an imaginary curve drawn so that its tangent at any point gives the direction of the field at that point. For a gravitational field the arrow points towards the mass; for an electric field the arrow points in the direction of the force on a positive test charge; for a magnetic field the arrow points from the north pole to the south pole outside a magnet.
场线，有时也称为力线，是一种想象的曲线，其绘制规则是：曲线上任意一点的切线方向就是该点场的方向。引力场中箭头指向物体；电场中箭头指向正试探电荷所受力的方向；磁场中，磁体外部箭头从北极指向南极。
Field lines can be plotted using small test charges, small test masses, or iron filings. They are not physical objects but useful models that help us visualise how a field acts in space.
场线可以用小试探电荷、小试探质量或铁粉来描绘。它们并不是真实存在的物体，而是帮助我们形象化理解场在空间中如何作用的模型。
2. Rules for Drawing Field Lines | 绘制场线的规则
The following rules are central to drawing and interpreting field lines correctly.
以下规则是正确绘制和解读场线的核心。
- Field lines begin on positive charges and end on negative charges, or at infinity for isolated charges.
- 电场线从正电荷出发，终止于负电荷，或延伸到无穷远。
- For gravity, field lines always terminate on the mass causing the field; they never begin on ordinary matter.
- 引力场线总是终止于产生场的物体；普通物质不可能成为引力场线的起点。
- Magnetic field lines form continuous closed loops, with no starting point or ending point.
- 磁感线形成连续的闭合回路，没有起点和终点。
- Field lines never cross.
- 场线永不相交。
- The density of lines represents the relative magnitude of the field: closer lines mean a stronger field.
- 场线的疏密程度表示场的相对强弱：线越密，场越强。
- In a uniform field, lines are parallel and equally spaced.
- 在匀强场中，场线平行且间距相等。
3. Gravitational Field Strength and Field Lines | 引力场强度与引力场线
The gravitational field strength g at a point is the force per unit mass acting on a small test mass placed at that point:
引力场强度g定义为作用在置于该点的小试探质量上的单位质量力：
g = F/m
The unit of g is N kg⁻¹, which is equivalent to m s⁻². For a point mass M, the field strength at distance r is given by the inverse-square law:
g的单位为N kg⁻¹，与m s⁻²等价。对于点质量M，距离r处的场强由平方反比定律给出：
g = GM/r²
The field lines point radially inward toward the mass. Because gravitational force is always attractive, field lines never point outward from a mass. Near the Earth's surface, the field is approximately uniform, so the lines are almost parallel and equally spaced.
引力场线径向指向质量。由于引力始终是吸引力，场线永远不会从质量向外。在地球表面附近，引力场近似匀强，因此场线几乎平行且等距。
4. Electric Field Strength and Field Lines | 电场强度与电场线
The electric field strength E is the force per unit positive charge:
电场强度E是作用在单位正电荷上的力：
E = F/q
Its units are N C⁻¹ or V m⁻¹. For a point charge Q, the field strength at distance r is:
其单位为N C⁻¹或V m⁻¹。对于点电荷Q，距离r处的场强为：
E = kQ/r² = Q/(4πε₀r²)
Field lines begin on positive charges and end on negative charges. For a positive point charge, lines radiate outward; for a negative charge, lines radiate inward. Between two parallel plates carrying equal but opposite charge, the electric field is uniform: field lines are equally spaced straight lines from the positive plate to the negative plate.
电场线从正电荷出发，终止于负电荷。正点电荷的场线向外辐射；负点电荷的场线向内汇聚。在两块带等量异种电荷的平行板之间，电场是匀强的：场线是等距平行直线，从正极板指向负极板。
5. Magnetic Field Lines and Magnetic Flux Density | 磁感线与磁感应强度
Magnetic fields are represented by magnetic field lines. The quantitative measure of a magnetic field is the magnetic flux density B, often called the magnetic field strength. Its unit is the tesla (T). The magnitude of B can be defined from the force on a current-carrying conductor:
磁场用磁感线表示。磁场的定量量度是磁感应强度B，通常也称为磁场强度，其单位为特斯拉(T)。B的大小可由载流导体所受的力来定义：
F = BIL sin θ
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