Edexcel A-Level Physics (Combined Science 032): Electric Circuits – Current, Potential Difference and Resistance | Edexcel A-Level 物理（综合科学 032）：电路——电流、电势差与电阻

📚 Edexcel A-Level Physics (Combined Science 032): Electric Circuits – Current, Potential Difference and Resistance | Edexcel A-Level 物理（综合科学 032）：电路——电流、电势差与电阻

Electric circuits form a core topic in Edexcel A-Level Physics and also appear in combined science routes. This article covers current, potential difference, resistance, resistivity, I-V characteristics, internal resistance and potential dividers, with emphasis on definitions, graphs and practical calculations.

电路是 Edexcel A-Level 物理的核心主题，也出现在综合科学课程中。本文涵盖电流、电势差、电阻、电阻率、I-V 特性、内阻和分压器，重点讲解定义、图像和实际计算。

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1. Charge and Current | 电荷与电流

Electric current is the rate of flow of electric charge. Current I is measured in amperes A, charge Q in coulombs C and time t in seconds s. The defining equation is I = ΔQ / Δt, so one ampere is equal to one coulomb of charge passing a point per second. In a metal wire, current is carried by free electrons, while in an electrolyte it is carried by both positive and negative ions moving in opposite directions.

电流是电荷流动的速率。电流 I 以安培 A 为单位，电荷 Q 以库仑 C 为单位，时间 t 以秒 s 为单位。定义式为 I = ΔQ / Δt，因此 1 安培等于每秒通过某点的 1 库仑电荷。在金属导线中，电流由自由电子携带；在电解质中，电流由向相反方向运动的正、负离子共同携带。

I = ΔQ / Δt

Charge is conserved in any circuit: the total current entering a junction equals the total current leaving it. This is Kirchhoff’s first law and is particularly useful when analysing parallel branches. In a series circuit, the current is the same at every point because there is only one conducting path.

任何电路中电荷都是守恒的：流入节点的总电流等于流出节点的总电流。这就是基尔霍夫第一定律，在分析并联支路时特别有用。在串联电路中，由于只有一条导电路径，各点的电流都相同。

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2. Potential Difference and EMF | 电势差与电动势

Potential difference between two points is the energy transferred per unit charge moved between those points. It is measured in volts V, where one volt is one joule per coulomb. The equation is V = W / Q. Potential difference across a component shows how much electrical energy is being converted to other forms per coulomb passing through it.

两点之间的电势差是单位电荷在两点间移动时转移的能量。电势差以伏特 V 为单位，1 伏特等于每库仑 1 焦耳。公式为 V = W / Q。元件两端的电势差表示每库仑电荷通过该元件时有多少电能转化为其他形式的能量。

V = W / Q

Electromotive force (EMF), often written as ε, is the total chemical or mechanical energy converted into electrical energy per unit charge by a source such as a cell or battery. Although EMF and potential difference are both measured in volts, they are not identical: EMF refers to the energy supplied by the source, while terminal potential difference is the energy available to the external circuit after some energy is lost inside the source due to internal resistance.

电动势（EMF），常用 ε 表示，指电池等电源将化学能或机械能转化为每单位电荷电能的总量。虽然电动势和电势差都以伏特为单位，但二者并不相同：电动势指电源提供的能量，而端电势差是扣除因内阻在电源内部损失的能量后，外部电路可获得的能量。

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3. Resistance and Ohm’s Law | 电阻与欧姆定律

Resistance is a measure of how much a component opposes the flow of electric current. It is defined as R = V / I and is measured in ohms Ω. One ohm is the resistance when a potential difference of one volt drives a current of one ampere. Resistance depends on material, length, cross-sectional area and temperature.

电阻是衡量元件阻碍电流流动程度的物理量。其定义为 R = V / I，单位是欧姆 Ω。1 欧姆表示在 1 伏特电势差下产生 1 安培电流时的电阻。电阻取决于材料、长度、横截面积和温度。

R = V / I

Ohm’s law states that, for a metallic conductor at constant temperature, the current is directly proportional to the potential difference across it. Under these conditions the resistance is constant and the I-V graph is a straight line through the origin. Many components, however, do not obey Ohm’s law because their resistance changes with temperature, light or applied voltage.

欧姆定律指出，对于恒温下的金属导体，电流与导体两端的电势差成正比。在这种条件下电阻保持恒定，I-V 图像是一条过原点的直线。然而，许多元件不服从欧姆定律，因为它们的电阻会随温度、光照或外加电压而变化。

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4. Resistivity and Conductivity | 电阻率与电导率

Resistivity ρ is a property of the material itself and describes how strongly the material opposes current flow. The resistance of a uniform wire is given by R = ρL / A, where L is the length and A is the cross-sectional area. Resistivity is measured in ohm metres Ω m. A longer wire has greater resistance, while a thicker wire has lower resistance.

电阻率 ρ 是材料本身的属性，描述材料阻碍电流流动的强弱。均匀导线的电阻由 R = ρL / A 给出，其中 L 为长度，A 为横截面积。电阻率的单位是欧姆米 Ω m。导线越长电阻越大，导线越粗电阻越小。

R = ρL / A

Conductivity σ is the reciprocal of resistivity: σ = 1 / ρ. It is useful when comparing how well different materials conduct electricity. Metals such as copper and silver have very low resistivity and therefore high conductivity, while insulators such as glass and rubber have extremely high resistivity. Semiconductors lie between these two extremes and form the basis of diodes and transistors.

电导率 σ 是电阻率的倒数：σ = 1 / ρ。它在比较不同材料的导电性能时很有用。铜和银等金属的电阻率很低，因此电导率很高；玻璃和橡胶等绝缘体的电阻率极高。半导体介于两者之间，是二极管和晶体管的基础。

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5. I-V Characteristics of Components | 元件的 I-V 特性

The I-V characteristic graph shows how current changes with potential difference for a component. An ohmic conductor gives a straight line through the origin, showing a constant resistance. A filament lamp gives a curve that becomes shallower as voltage increases because the filament heats up and its resistance rises.

I-V 特性图显示元件电流随电势差变化的情况。欧姆导体呈现一条过原点的直线，表明电阻恒定。白炽灯的曲线随着电压升高而变得平缓，因为灯丝受热后电阻增大。

A semiconductor diode conducts virtually no current in reverse bias, but in forward bias it conducts once the applied voltage exceeds the threshold voltage of about 0.6 V for silicon. The graph is therefore very different in the two directions. Thermistors and LDRs are also non-ohmic: their resistance decreases with temperature or light intensity respectively.

半导体二极管在反向偏置时几乎不导电，但在正向偏置下，只要外加电压超过硅管约 0.6 V 的阈值电压就开始导通。因此两个方向的图像差异很大。热敏电阻和光敏电阻也是非欧姆元件：它们的电阻分别随温度或光照强度升高而降低。

• Ohmic conductor: straight line through origin | 欧姆导体：过原点的直线
• Filament lamp: curved, resistance increases with temperature | 白炽灯：曲线，电阻随温度升高而增大
• Diode: conducts only in forward bias above threshold | 二极管：仅在正向偏置且超过阈值电压时导通

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6. Power and Energy Dissipation | 功率与能量耗散

Electrical power is the rate at which energy is transferred in a circuit. Power P is measured in watts W and can be calculated using P = VI. For a resistor, combining this with Ohm’s law gives two more useful forms: P = I²R and P = V² / R. These equations assume that all electrical energy is converted into heat or other useful forms in the component.

电功率是电路中能量转移的速率。功率 P 以瓦特 W 为单位，可用 P = VI 计算。对于电阻器，将其与欧姆定律结合可得到另外两个常用形式：P = I²R 和 P = V² / R。这些公式假设所有电能都转化为元件中的热能或其他有用形式的能量。

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