Edexcel A-Level Physics: Electric and Magnetic Fields (Combined 073) | A-Level 物理：电场与磁场（综合 073）

📚 Edexcel A-Level Physics: Electric and Magnetic Fields (Combined 073) | A-Level 物理：电场与磁场（综合 073）

This revision guide covers the Edexcel A-Level Physics content often labelled ‘Combined 073’, bringing together electric fields, magnetic fields, capacitance and electromagnetic induction. It focuses on definitions, key equations, graphical interpretation and structured problem-solving.

本复习指南涵盖 Edexcel A-Level 物理中常标为 ‘Combined 073’ 的专题，综合了电场、磁场、电容和电磁感应。内容重点包括定义、核心方程、图像解释和结构化解题方法。

1. Electric Fields and Coulomb’s Law | 电场与库仑定律

An electric field is a region surrounding a charged object where a small positive test charge would experience a force. Coulomb’s law gives the magnitude of the electrostatic force between two point charges Q₁ and Q₂ separated by distance r as F = kQ₁Q₂/r², where k = 1/(4πε₀) ≈ 8.99×10⁹ N m² C⁻².

电场是带电体周围能够使试探正电荷受力的区域。库仑定律给出两个点电荷 Q₁ 与 Q₂ 相距 r 时静电力的大小：F = kQ₁Q₂/r²，其中 k = 1/(4πε₀) ≈ 8.99×10⁹ N m² C⁻²。

F = kQ₁Q₂/r²

The force acts along the line joining the centres of the two charges. Like charges repel and opposite charges attract, so the sign of the product Q₁Q₂ determines the direction of the force in vector calculations.

静电力沿两电荷中心的连线方向作用。同种电荷相互排斥，异种电荷相互吸引，因此 Q₁Q₂ 的符号在矢量计算中决定力的方向。

• Like charges: repulsive force | 同种电荷：排斥力
• Opposite charges: attractive force | 异种电荷：吸引力
• Point charges: separation must be in metres | 点电荷：距离必须以米为单位

2. Electric Field Strength and Uniform Fields | 电场强度与匀强电场

Electric field strength E is defined as the force per unit positive charge: E = F/q. In a uniform electric field between two parallel plates, the field strength is E = V/d, where V is the potential difference and d is the plate separation. The unit of E is either V m⁻¹ or N C⁻¹.

电场强度 E 定义为单位正电荷所受的电场力：E = F/q。在两块平行板之间的匀强电场中，电场强度为 E = V/d，其中 V 是电势差，d 是板间距。E 的单位可以是 V m⁻¹ 或 N C⁻¹。

E = F/q    and    E = V/d

In a uniform field, electric field lines are parallel and evenly spaced, pointing from the positive plate to the negative plate. A charged particle moving perpendicular to the field experiences a constant force, producing a parabolic path similar to projectile motion under gravity.

在匀强电场中，电场线平行且等距，由正极板指向负极板。垂直于电场方向运动的带电粒子会受到恒定的电场力，其轨迹类似于重力场中的抛体运动，呈抛物线形。

3. Electric Potential and Work Done | 电势与做功

Electric potential V at a point in a field is the work done per unit positive charge in bringing it from infinity to that point. For a point charge Q, the potential at distance r is V = kQ/r. Electric potential is a scalar quantity and is measured in volts (V).

电场中某点的电势 V 是将单位正电荷从无穷远处移到该点所做的功。对于点电荷 Q，距离 r 处的电势为 V = kQ/r。电势是标量，单位为伏特 (V)。

V = kQ/r    and    ΔW = QΔV

The work done on or by a charge q moving through a potential difference ΔV is ΔW = qΔV. This gives rise to the electronvolt: 1 eV = 1.60×10⁻¹⁹ J. Equipotential surfaces are perpendicular to field lines, so no work is done when a charge moves along an equipotential.

电荷 q 通过电势差 ΔV 时所做的功为 ΔW = qΔV。由此可以定义电子伏特：1 eV = 1.60×10⁻¹⁹ J。等势面与电场线垂直，因此电荷沿等势面运动时不做功。

4. Capacitance and Energy Storage | 电容与储能

Capacitance C is defined as the charge stored per unit potential difference: C = Q/V. The unit is the farad (F). For a parallel-plate capacitor, C = ε₀εᵣA/d, where A is the plate area, d is the separation, ε₀ is the permittivity of free space and εᵣ is the relative permittivity of the dielectric.

电容 C 定义为单位电势差下储存的电荷量：C = Q/V。单位为法拉 (F)。对于平行板电容器，C = ε₀εᵣA/d，其中 A 是板面积，d 是板间距，ε₀ 是真空中介电常数，εᵣ 是电介质的相对介电常数。

C = Q/V    and    C = ε₀εᵣA/d

The energy stored in a capacitor is U = ½QV = ½CV² = ½Q²/C. During discharge, the charge, voltage and current all decay exponentially with

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