IB Physics: Field Lines and Field Strength | IB物理：描述场的力线与场强

📚 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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