📚 IGCSE WJEC Physics: Capacitance – Key Concepts Explained | IGCSE WJEC 物理：电容 考点精讲
Capacitance is a fundamental topic in the WJEC IGCSE Physics syllabus, bridging electrostatics and circuit theory. Understanding capacitors – devices that store electric charge and energy – is essential for analysing how circuits behave in cameras, flash units, timing devices, and smoothing circuits. This article breaks down every key point you need for the exam, presented in clear paired English‑Chinese paragraphs with worked examples and common pitfalls.
电容是 WJEC IGCSE 物理考纲中的基础主题，连接了静电学和电路理论。理解电容器——储存电荷和电能的器件——对于分析照相机闪光灯、定时设备及滤波电路的工作原理至关重要。本文将逐一剖析考试所需的所有关键点，以清晰的中英对照段落呈现，并配有计算示例和常见误区。
1. What is Capacitance? | 什么是电容？
Capacitance describes the ability of a component to store electric charge per unit potential difference across it. Imagine two metal plates separated by an insulator; when connected to a battery, opposite charges build up on each plate, creating a voltage. The larger the capacitance, the more charge can be stored at a given voltage. Symbol C, measured in farads (F).
电容描述元件在单位电势差下储存电荷的能力。可以想象两块被绝缘体隔开的金属板；当连接电池时，两板分别积聚等量异种电荷，形成电压。电容越大，在给定电压下能储存的电荷越多。符号为 C，单位是 法拉（F）。
For example, a 1 000 µF capacitor stores much more charge at 5 V than a 10 µF capacitor. In practice, most capacitors have values in microfarads (µF), nanofarads (nF) or picofarads (pF).
例如，在 5 V 电压下，一个 1 000 µF 电容器储存的电荷远多于 10 µF 电容器。实际中大多数电容器的数值为微法（µF）、纳法（nF）或皮法（pF）。
2. Capacitors and Their Symbols | 电容器及其符号
A capacitor consists of two conducting plates separated by an insulating layer called the dielectric. When connected in a circuit, it stores energy in the electric field between the plates. The circuit symbol is two parallel lines of equal length, with a gap between them. Some capacitors are polarised (e.g. electrolytic capacitors) and must be connected the correct way round; their symbol adds a curved plate or a ‘+’ sign.
电容器由两片被绝缘层（称为电介质）隔开的导体板构成。接入电路时，能量以两板间电场的形式储存。电路符号为两条等长的平行线段，中间留有间隙。某些电容器有极性（如电解电容），必须按正确方向连接；其符号会添加弧形板或“+”标记。
Non‑polarised capacitor: two plain parallel lines.
无极性电容器：两条简单的平行线段。
Polarised capacitor: one straight plate and one curved plate, indicating the negative terminal.
有极性电容器：一块直板、一块弧形板，弧形板表示负极。
3. The Capacitance Formula C = Q / V | 电容公式 C = Q / V
The defining equation links charge (Q, in coulombs), capacitance (C, in farads) and potential difference (V, in volts):
电容的定义式将电荷（Q，单位库仑）、电容（C，单位法拉）和电势差（V，单位伏特）联系起来：
C = Q / V
This means one farad is one coulomb per volt. A capacitor of 1 F stores 1 C of charge when the voltage across it is 1 V. Rearranging, Q = C V or V = Q / C.
这意味着 1 法拉等于 1 库仑每伏特。一个 1 F 的电容器在两端电压为 1 V 时储存 1 C 的电荷。移项可得 Q = C V 或 V = Q / C。
Example: A 470 µF capacitor is connected to a 9 V battery. Calculate the charge stored.
示例：一个 470 µF 电容器连接到 9 V 电池。计算储存的电荷。
Q = C V = 470 × 10⁻⁶ F × 9 V = 4.23 × 10⁻³ C (4.23 mC)
Always convert sub‑multiples to farads before calculating. Watch out for unit prefixes: 1 µF = 10⁻⁶ F, 1 nF = 10⁻⁹ F, 1 pF = 10⁻¹² F.
计算前务必把倍数单位换算为法拉。注意单位前缀：1 µF = 10⁻⁶ F，1 nF = 10⁻⁹ F，1 pF = 10⁻¹² F。
4. Factors Affecting Capacitance | 影响电容的因素
For a parallel‑plate capacitor, capacitance depends on three factors:
对于平行板电容器，电容取决于三个因素：
Area of overlap of the plates, A – larger area gives greater capacitance
板间重叠面积 A – 面积越大电容越大
Separation distance, d – smaller gap increases capacitance
板间距离 d – 间距越小电容越大
Permittivity of the dielectric material, ε – better insulating materials yield higher capacitance
电介质材料的介电常数 ε – 绝缘性能越好的材料电容越高
These are combined in the formula:
这些因素结合在公式中：
C = ε A / d
where ε = ε₀ εᵣ, with ε₀ the permittivity of free space (8.85 × 10⁻¹² F y (dielectric constant) of the material. A vacuum has εᵣ = 1; other materials have εᵣ > 1.
其中 ε = ε₀ εᵣ，ε₀ 是真空介电常数（8.85 × 10⁻¹² F m⁻¹），εᵣ 为材料的相对介电常数（介电常数）。真空 εᵣ = 1；其他材料 εᵣ > 1。
5. Dielectrics and Their Role | 电介质及其作用
A dielectric is an insulating material placed between the plates. It serves two purposes: it keeps the plates apart to prevent short circuits, and it increases the capacitance by reducing the effective electric field. Polar molecules in the dielectric align with the field, partially cancelling it, allowing more charge to be stored for the same voltage.
电介质是置于两板之间的绝缘材料。它有两个作用：使两板保持分离以防短路，并通过削弱有效电场来增大电容。电介质中的极性分子沿电场排列，部分抵消电场，从而在相同电压下储存更多电荷。
The dielectric constant εᵣ quantifies this effect. For example, mica has εᵣ ≈ 6, so a mica‑filled capacitor has six times the capacitance of an identical air‑filled one.
常见电介质包括空气、纸、陶瓷、云母和电解液。介电常数 εᵣ 量化了这种效应。例如，云母的 εᵣ ≈ 6，因此填充云母的电容器的电容是相同空气电容器的 6 倍。
6. Charging a Capacitor | 电容器充电
When a capacitor is connected in series with a resistor and a DC source, the voltage across it rises exponentially. At the start, current is high because the potential difference between supply and capacitor is large; as the capacitor voltage approaches the supply voltage, current drops to zero. The charging curves for voltage V and current I are:
当电容器与电阻和直流电源串联时，其两端电压呈指数上升。初始时电流很大，因为电源与电容器间电势差大；随着电容器电压趋近电源电压，电流降至零。电压 V 和电流 I 的充电曲线为：
Voltage: V = V₀ (1 – e⁻ᵗ/ᴿᶜ)
电压：V = V₀ (1 – e⁻ᵗ/ᴿᶜ)
Current: I = I₀ e⁻ᵗ/ᴿᶜ
电流：I = I₀ e⁻ᵗ/ᴿᶜ
The product RC (resistance × capacitance) is the time constant, symbol τ (tau), in seconds. After one time constant, V reaches about 63% of the supply voltage; after 5 RC, the capacitor is considered fully charged (over 99%).
乘积 RC（电阻×电容）为时间常数，符号 τ（tau），单位秒。经过一个时间常数，电压达到电源电压的约 63%；经过 5 RC 后，电容器视为完全充电（超过 99%）。
7. Discharging a Capacitor | 电容器放电
Removing the source and connecting the charged capacitor across a resistor leads to exponential decay of both voltage and current. The discharge equations mirror the charge equations without the '1 – ':
移去电源并将已充电的电容器接到电阻两端，会导致电压和电流均呈指数衰减。放电方程与充电方程对称，只是去掉了“1 – ”：
Voltage: V = V₀ e⁻ᵗ/ᴿᶜ
电压：V = V₀ e⁻ᵗ/ᴿᶜ
Current: I = – I₀ e⁻ᵗ/ᴿᶜ (direction reversed)
电流：I = – I₀ e⁻ᵗ/ᴿᶜ（方向相反）
After one time constant, the voltage falls to about 37% of its initial value. After 5 RC, it is nearly zero. This behaviour is used in timing circuits (e.g. automatic lights, oscillators).
经过一个时间常数，电压降至初始值的约 37%。经过 5 RC，电压近于零。此特性可用于定时电路（如自动灯、振荡器）。
8. Energy Stored in a Capacitor | 电容器储存的能量
A charged capacitor stores energy in its electric field. The energy can be calculated using the potential difference and charge or capacitance:
充电的电容器在电场中储存能量。能量可用电势差和电荷或电容计算：
E = ½ Q V = ½ C V² = ½ Q² / C
Energy E is measured in joules (J). For instance, a 1 000 µF capacitor charged to 10 V stores: E = ½ × 1 000 × 10⁻⁶ × (10)² = 0.05 J. This is sufficient to power a small flash lamp momentarily.
能量 E 的单位是焦耳（J）。例如，一个 1 000 µF 电容器充电至 10 V 储存的能量为：E = ½ × 1 000 × 10⁻⁶ × (10)² = 0.05 J。这足以瞬间点亮小型闪光灯。
Common misconception: energy stored does not equal Q V, but half that because the average voltage during charging is V/2.
常见误区：储存的能量 不等于 Q V，而是其一半，因为充电过程中的平均电压为 V/2。
9. Time Constant and RC Circuits | 时间常数与 RC 电路
The time constant τ = R wledge of τ allows you to estimate the voltage at any time t without solving exponentials:
时间常数 τ = R C 是衡量电容器充放电快慢的关键量。增大 R 或 C 会增大 τ，减缓过程。知道 τ 后你无需解指数方程即可估算任意时刻 t 的电压：
t = τ, V ≈ 0.63 V₀ (charge) or 0.37 V₀ (discharge)
t = τ 时，V ≈ 0.63 V₀（充电）或 0.37 V₀（放电）
t = 5τ, V ≈ 0.99 V₀ (charge) or 0.01 V₀ (discharge)
t = 5τ 时，V ≈ 0.99 V₀（充电）或 0.01 V₀（放电）
WJEC exam questions often ask you to find τ from a graph, recognise half‑life of discharge (t₁/₂ = 0.69 RC), or calculate R or C from given τ.
WJEC 试题常要求你从图中找出 τ、识别放电半衰期（t₁/₂ = 0.69 RC），或根据给定的 τ 计算 R 或 C。
10. Capacitors in Series and Parallel | 电容器的串联与并联
Capacitor networks follow opposite rules to resistors. For series connection, the total capacitance is less than any individual value (because the effective plate separation increases):
电容器网络的规则与电阻器相反。串联时总电容小于任一单个电容（因为等效板间距增大）：
1 / Ctotal = 1 / C₁ + 1 / C₂ + …
For parallel connection, capacitances simply add because plate areas effectively increase:
并联时，电容直接相加，因为等效板面积增大：
Ctotal = C₁ + C₂ + …
Example: 3 µF and 6 µF in series give Ctotal = (1/3 + 1/6)⁻¹ = 2 µF. In parallel, Ctotal = 9 µF.
示例：3 µF 和 6 µF 串联，Ctotal = (1/3 + 1/6)⁻¹ = 2 µF；并联时 Ctotal = 9 µF。
11. Practical Applications of Capacitors | 电容器的实际应用
Capacitors appear in many everyday and industrial devices:
电容器出现在许多日常生活和工业设备中：
Flash camera: stores energy slowly from a battery and discharges rapidly to create a bright flash.
照相机闪光灯：从电池缓慢储能，快速放电产生强烈闪光。
Smoothing circuits: after rectification, a large capacitor smooths voltage ripples in DC power supplies.
滤波电路：整流后，大电容平滑直流电源中的电压波纹。
Timing circuits: combined with a resistor, the charge/discharge curve controls delays (e.g. interval wipers, burglar alarms).
定时电路：与电阻组合，充放电曲线控制延迟（如间歇雨刷、防盗报警器）。
Tuning circuits: with inductors, capacitors select specific frequencies in radios.
调谐电路：与电感器一起，电容器在收音机中选择特定频率。
Back‑up power: supercapacitors supply short‑term power to memory chips when the main supply fails.
备用电源：超级电容器在主电源故障时为存储芯片提供短期电力。
12. Key Exam Points and Summary | 考点总结
For WJEC IGCSE, focus on these essentials:
针对 WJEC IGCSE，请关注以下要点：
| Topic | Must‑know |
|---|---|
| Definition & Formula | C = Q / V; unit farad; micro, nano, pico prefixes. |
| Parallel‑plate factors | C ∝ A, C ∝ 1/d, C ∝ ε. Use ε = ε₀ εᵣ. |
| Energy stored | E = ½ Q V = ½ C V². Remember the ½ factor. |
| Charging / discharging graphs | Exponential curves; interpret V–t and I–t; identify τ and half‑life. |
| Time constant | τ = R C; after 5τ fully charged/discharged; t₁/₂ = 0.69 RC. |
| Series & parallel | Series: 1/C = Σ 1/C; parallel: C = Σ C. Opposite to resistors. |
| Applications | Flash, smoothing, timing, tuning, backup. |
Remember to practise unit conversions and graph interpretation. Many students lose marks by using the wrong prefix or misreading exponential axes. Solid command of these principles will earn you full marks on capacitance questions.
记得练习单位换算和图表解读。许多学生因单位前缀错误或误判指数坐标而失分。扎实掌握这些原理将确保你在电容题目中拿到满分。
Published by TutorHao | Physics Revision Series | aleveler.com
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