📚 Oxford AQA International AS Physics: Electricity Concepts Explained | 牛津AQA国际AS物理：电学概念详解
Electricity is a foundational topic in AS Physics, linking the microscopic behaviour of charges to circuit analysis. This article explains all key concepts required for the Oxford AQA International AS Level specification, from basic definitions of current and voltage to more complex ideas such as internal resistance and Kirchhoff’s laws. Understanding these principles will not only prepare you for the topic test but also build a solid base for later electromagnetism and electronics topics.
电学是AS物理的基础主题，它将电荷的微观行为与电路分析联系起来。本文详细解释了牛津AQA国际AS考试大纲要求的所有核心概念，从电流和电压的基本定义到内阻和基尔霍夫定律等更复杂的内容。理解这些原理不仅能帮助你应对章节测试，还能为后续的电磁学和电子学内容打下扎实基础。
1. Electric Current and Charge | 电流与电荷
Electric current is the rate of flow of electric charge. In metal wires, the charge carriers are free electrons, which move when a potential difference is applied. The conventional direction of current is from positive to negative, opposite to the electron flow.
电流是电荷流动的速率。在金属导线中，载流子是自由电子，当施加电势差时它们会定向移动。电流的常规方向是从正极到负极，与电子流动方向相反。
I = ΔQ / Δt
The SI unit of current is the ampere (A), where 1 A = 1 C s⁻¹. Charge Q is measured in coulombs (C). For a steady current, if 2 C of charge passes a point in 4 s, the current is 0.5 A.
I = ΔQ / Δt
电流的国际单位是安培(A)，1 A = 1 C s⁻¹。电荷Q的单位是库仑(C)。对于恒定电流，若4秒内通过某点的电荷为2 C，则电流为0.5 A。
Total charge transferred can be found from the area under a current-time graph. For alternating current (AC), the direction of flow reverses periodically, but many AS circuits use direct current (DC) from batteries or rectified supplies.
转移的总电荷可以通过电流-时间图线下的面积求得。对于交流电(AC)，流动方向周期性反转，但许多AS电路使用来自电池或整流电源的直流电(DC)。
2. Potential Difference and Electromotive Force | 电势差与电动势
Potential difference (p.d. or V) between two points is the energy transferred per unit charge as charge moves between those points. Electromotive force (e.m.f., symbol ε) is the total energy supplied per unit charge by a source such as a battery or generator.
两点之间的电势差(p.d. 或V)是单位电荷在这两点间移动时转移的能量。电动势(e.m.f., 符号ε)是电池或发电机等电源对每单位电荷提供的总能量。
V = W / Q
One volt is one joule per coulomb. If a cell with e.m.f. 6 V does 12 J of work in moving 2 C of charge around the whole circuit, the terminal p.d. across the cell may be slightly less due to internal resistance.
V = W / Q
一伏特等于一焦耳每库仑。如果电动势为6 V的电池使2 C的电荷绕整个电路移动时做功12 J，则电池的端电压可能会因内阻而略低。
3. Resistance and Ohm’s Law | 电阻与欧姆定律
Resistance is a measure of the opposition to current flow. For an ohmic conductor at constant temperature, the current through it is directly proportional to the potential difference across it, as stated in Ohm’s law.
电阻是对电流阻碍作用的量度。对于温度恒定的欧姆导体，通过它的电流与其两端的电势差成正比，这就是欧姆定律。
R = V / I
The SI unit of resistance is the ohm (Ω). A component has a resistance of 1 Ω if a p.d. of 1 V produces a current of 1 A. Many components, such as metallic wires at constant temperature, are ohmic, but filament lamps and diodes are non-ohmic.
R = V / I
电阻的国际单位是欧姆(Ω)。如果1 V的电势差产生1 A的电流，则该组件的电阻为1 Ω。恒温下的金属导线等许多组件是欧姆性的，但灯丝和二极管等是非欧姆性的。
4. Resistivity and Conductivity | 电阻率与电导率
The resistance of a wire depends on its length L, cross-sectional area A, and the material’s resistivity ρ. Resistivity is an intrinsic property that indicates how strongly a material opposes current.
导线的电阻取决于其长度L、横截面积A和材料的电阻率ρ。电阻率是反映材料对电流阻碍作用强弱的固有属性。
R = ρ × L / A
Resistivity is measured in ohm-metres (Ω·m). Conductivity σ is the reciprocal of resistivity: σ = 1/ρ. Good conductors like copper have very low resistivity.
R = ρ × L / A
电阻率的单位是欧姆·米(Ω·m)。电导率σ是电阻率的倒数: σ = 1/ρ。铜等良导体的电阻率非常低。
5. Electrical Power and Energy | 电功率与电能
The power P dissipated or transferred in a circuit component relates the current through it and the p.d. across it. Energy is the product of power and time.
电路组件中耗散或转移的功率P与通过它的电流和其两端的电压有关。能量是功率与时间的乘积。
P = I × V
Using Ohm’s law, alternate forms are P = I² R and P = V² / R. A resistor carrying 2 A with a p.d. of 5 V dissipates 10 W. The energy transferred in 60 s is 600 J.
P = I × V
利用欧姆定律，可得到替代形式 P = I² R 和 P = V² / R。一个通有2 A电流且两端电压为5 V的电阻器耗散10 W。60秒内转移的能量为600 J。
6. Series and Parallel Circuits | 串联与并联电路
When resistors are connected in series, the same current flows through each, and the total resistance is the sum of individual resistances. In parallel, the p.d. across each resistor is the same, and the reciprocal of total resistance equals the sum of the reciprocals.
电阻器串联时，流过每个电阻器的电流相同，总电阻等于各电阻之和。并联时，每个电阻器两端的电压相同，总电阻的倒数等于各电阻倒数之和。
| Property | Series | Parallel |
|---|---|---|
| Current I | Same everywhere | Splits at junctions, Itotal = I1 + I2 + … |
| p.d. V | Divides: Vtotal = V1 + V2 + … | Same across each branch |
| Resistance R | Rtotal = R1 + R2 + … | 1/Rtotal = 1/R1 + 1/R2 + … |
系列/并联电路的特点，用表格概括。
7. Internal Resistance and Terminal PD | 内阻与端电压
Real power sources, such as batteries, have internal resistance (r), which causes a loss of energy inside the cell. The terminal p.d. V across the source is less than the e.m.f. ε when a current I flows.
真实的电源（如电池）具有内阻(r)，这会导致电池内部有能量损耗。当有电流I流过时，电源的端电压V会低于电动势ε。
V = ε − I r
The “lost volts” are I r. The full circuit equation is ε = I (R + r). Measuring V and I for different load resistances allows determination of ε and r from the intercept and gradient of a V–I graph.
V = ε − I r
“损失的电压”为I r。全电路方程为ε = I (R + r)。通过测量不同负载电阻下的V和I，可以从V-I图线的截距和斜率确定ε和r。
8. Potential Dividers | 分压器
A potential divider uses two resistors in series to provide a variable output voltage from a fixed input. The output is taken across one of the resistors.
分压器利用两个串联电阻器从固定输入电压中获得可变的输出电压。输出电压取自其中一个电阻器两端。
Vout = (R2 / (R1 + R2)) × Vin
If R1 and R2 are replaced by a variable resistor or sensor, the output voltage can be adjusted dynamically. Potential dividers are essential in sensor circuits and volume controls.
Vout = (R2 / (R1 + R2)) × Vin
若将R1和R2替换为可变电阻器或传感器，输出电压可动态调节。分压器在传感器电路和音量控制中至关重要。
9. Sensing Circuits: Thermistors and LDRs | 传感电路：热敏电阻与光敏电阻
Thermistors (especially NTC types) decrease their resistance as temperature rises. Light-dependent resistors (LDRs) show a large drop in resistance when light intensity increases. These components are used in potential dividers to produce a p.d. that varies with physical conditions.
热敏电阻（尤其是NTC型）的阻值随温度升高而减小。光敏电阻(LDR)在光照强度增加时电阻大幅下降。这些元件用于分压器中，产生随物理条件变化的电压。
For example, placing an NTC thermistor in the R2 position of a potential divider with a fixed R1 results in Vout increasing as temperature rises. This can trigger a warning system.
例如，将NTC热敏电阻放在分压器的R2位置，与固定R1配合，温度升高时Vout增大，可触发报警系统。
10. I-V Characteristics | 电流-电压特性曲线
The current-voltage (I-V) graph of a component reveals its behaviour. An ohmic conductor gives a straight line through the origin with constant gradient (resistive). A filament lamp’s curve bends because resistance increases with temperature.
组件的电流-电压(I-V)图线展示其特性。欧姆导体呈现一条通过原点的直线，斜率恒定（电阻不变）。灯丝的曲线会弯曲，因为电阻随温度升高而增大。
A diode has a very high resistance in reverse bias and conducts easily above a threshold voltage (approx. 0.6 V for silicon) in forward bias, showing an exponential-like increase in current. These non-linear characteristics are important for understanding rectification and logic circuits.
二极管在反向偏置时电阻极高，在正向偏置且超过阈值电压（硅管约0.6 V）后容易导通，电流呈指数级增长。这些非线性特性对于理解整流和逻辑电路非常重要。
11. Kirchhoff’s Laws | 基尔霍夫定律
Kirchhoff’s first law (current law) states that the sum of currents entering a junction equals the sum of currents leaving it. This is a consequence of charge conservation. Kirchhoff’s second law (voltage law) states that in any closed loop, the sum of the e.m.f.s equals the sum of the p.d.s (voltage drops).
基尔霍夫第一定律（电流定律）指出，流入一个节点的电流之和等于流出该节点的电流之和。这是电荷守恒的结果。基尔霍夫第二定律（电压定律）指出，在任何闭合回路中，电动势之和等于电势差（电压降）之和。
∑ Iin = ∑ Iout
∑ ε = ∑ I R
These laws are powerful tools for analysing complex circuits with multiple loops and branches, often combined with Ohm’s law.
∑ Iin = ∑ Iout
∑ ε = ∑ I R
这些定律是分析多回路、多支路复杂电路的有力工具，通常与欧姆定律结合使用。
12. Practical Skills: Measuring Resistance and EMF | 实验技能：测量电阻和电动势
Common practical tasks involve determining the resistance of a wire using an ammeter and voltmeter (V/I method), and finding the internal resistance and e.m.f. of a cell. For the latter, a variable resistor is used to change circuit current while recording terminal p.d. The resulting V–I graph has intercept ε and gradient −r.
常见的实验任务包括利用安培表和伏特表（伏安法）测定导线电阻，以及测量电池的内阻和电动势。对于后者，使用可变电阻器改变电路电流，同时记录端电压。得到的V-I图线的截距为ε，斜率为−r。
Using a potential divider, you can also investigate the variation of output voltage with sensor resistance. Always take repeat readings and consider sources of uncertainty such as contact resistance and meter precision.
利用分压器，你还可以研究输出电压随传感器阻值的变化。务必进行重复读数，并考虑接触电阻和仪表精度等不确定度来源。
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导