📚 Electricity for AQA A-Level Physics | AQA A-Level物理电学核心精讲
Electricity is one of the most important topics in AQA A-Level Physics, linking the behaviour of charged particles with the operation of circuits and components. This article covers the key definitions, equations, graphs and practical skills you need for the exam. You should be able to define current, potential difference, resistance and power, solve series and parallel circuits, handle internal resistance, and interpret I-V characteristics accurately.
电学是 AQA A-Level 物理中最重要的主题之一，它将带电粒子的行为与电路和元件的工作联系起来。本文涵盖考试所需的核心定义、方程、图像和实验技能。你应当能够定义电流、电势差、电阻和功率，求解串联与并联电路，处理内阻问题，并能准确解释 I-V 特性曲线。
1. Electric Current and Charge | 电流与电荷
it of current is the ampere (A), which is equal to one coulomb per second (1 A = 1 C s⁻¹).
电流是电荷流动的速率。平均电流 I 由 I = ΔQ / Δt 给出，其中 ΔQ 是在时间 Δt 内通过某一点的电荷量。电流的国际单位是安培（A），等于每秒一库仑（1 A = 1 C s⁻¹）。
I = ΔQ / Δt Q = I t 1 A = 1 C s⁻¹
ich positive charge would flow, so it is opposite to the direction of electron drift. Charge is quantised: the elementary charge is e = 1.60 × 10⁻¹⁹ C, and any charge Q is an integer multiple of e, so Q = n e.
在金属中，电流由自由电子在正离子晶格中漂移形成。约定电流方向定义为正电荷流动的方向，因此与电子漂移方向相反。电荷是量子化的：基本电荷 e = 1.60 × 10⁻¹⁹ C，任何电荷 Q 都是 e 的整数倍，即 Q = n e。
× 5.0 = 10 C. This corresponds to n = Q / e = 10 / (1.60 × 10⁻¹⁹) = 6.25 × 10¹⁹ electrons.
示例：若 2.0 A 的电流持续 5.0 s，则转移的电荷为 Q = I t = 2.0 × 5.0 = 10 C，对应 n = Q / e = 10 / (1.60 × 10⁻¹⁹) = 6.25 × 10¹⁹ 个电子。
Current is not always carried by electrons alone. In an electrolyte, both positive and negative ions move, producing a current in the same overall direction. In a beam of charged particles, the beam current has the same direction as the motion of positive particles and the opposite direction to negative ones.
电流并不总是仅由电子输送。在电解质中，正离子和负离子同时移动，整体电流方向相同。在带电粒子束中，束流方向与带正电粒子的运动方向相同，与带负电粒子的运动方向相反。
2. Potential Difference and Electromotive Force | 电势差与电动势
e W is the work done or energy transferred and Q is the charge. The unit is the volt (V), and 1 V = 1 J C⁻¹.
两点之间的电势差（p.d.）是电荷在两点间移动时每单位电荷所转移的能量，定义为 V = W / Q，其中 W 为做功或转移的能量，Q 为电荷量。单位是伏特（V），且 1 V = 1 J C⁻¹。
The electromotive force (e.m.f.) of a source is the energy supplied per unit charge by the source to drive charge around a complete circuit. Although the name suggests a force, e.m.f. is a voltage and is measured in volts. It is usually given the symbol ε.
电源的电动势（e.m.f.）是电源为驱动电荷绕完整电路一周、每单位电荷所提供的能量。尽管名称中含有"力"字，电动势实际上是电压而非力，单位为伏特，通常用符号 ε 表示。
The key distinction is the direction of energy conversion. In the source, e.m.f. describes energy converted from chemical, mechanical or light energy into electrical energy per unit charge. In components, p.d. describes energy converted from electrical energy into other forms, such as thermal energy in a resistor.
关键区别在于能量转化的方向。在电源内部，电动势描述每单位电荷由化学能、机械能或光能转化为电能的量。在元件中，电势差描述每单位电荷由电能转化为其他形式（例如电阻中的热能）的量。
V = W / Q ε = W / Q（电源内部） 1 V = 1 J C⁻¹
In a closed loop, the sum of the e.m.f.s equals the sum of the p.d.s, which is simply a statement of conservation of energy applied to a circuit. Energy per unit charge supplied must equal energy per unit charge transferred.
在闭合回路中，电动势之和等于电势差之和，这实质上是把能量守恒应用于电路。每单位电荷提供的能量必然等于每单位电荷转移出去的能量。
3. Resistance and Resistivity | 电阻与电阻率
s a component and I is the current through it. The unit is the ohm (Ω), and 1 Ω = 1 V A⁻¹.
电阻定义为 R = V / I，其中 V 是元件两端的电势差，I 是通过它的电流。单位是欧姆（Ω），1 Ω = 1 V A⁻¹。
Ohm's law states that, for a metallic conductor at constant temperature, the current through it is directly proportional to the potential difference across it, so I ∝ V and R is constant. Note that this is a special property of certain conductors, not a universal law.
欧姆定律指出，对于温度恒定的金属导体，通过它的电流与其两端电势差成正比，即 I ∝ V，R 为常数。注意这只是某些导体的特殊性质，而不是普遍适用的定律。
A is the cross-sectional area and L is the length of the sample. Its unit is the ohm metre (Ω m).
电阻率 ρ 是材料本身的属性，定义为 ρ = RA / L，其中 A 为横截面积，L 为样品长度。其单位是欧姆米（Ω m）。
R = ρL / A ρ = RA / L ρ 的单位：Ω m
In a metal, raising the temperature increases the amplitude of lattice vibrations, so electrons collide more frequently with the ions and resistance increases. In an NTC thermistor, the resistance decreases as temperature rises because more charge carriers are released.
在金属中，温度升高会增大晶格振动幅度，电子与离子碰撞更加频繁，因此电阻增大。而在 NTC 热敏电阻中，温度升高时释放出更多载流子，因此电阻减小。
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