Electric Charge and Current / 电荷与电流

Electric Charge and Current / 电荷与电流

Electric current is the rate of flow of charge: I = Q / t, where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds (s). Conventional current flows from positive to negative, opposite to electron flow. Ammeters are connected in series with very low resistance.

电流是电荷流动的速率：I = Q / t。习惯电流从正流向负，与电子流动方向相反。电流表串联连接且内阻极低。

In metals, current is carried by free electrons. One coulomb is the charge passing a point when one ampere flows for one second.

在金属中，电流由自由电子携带。一库仑是一安培电流在一秒内通过某点的电荷量。

Potential Difference and EMF / 电势差与电动势

Potential difference: V = W / Q, energy per unit charge. EMF is the total energy supplied per unit charge by a source. Terminal p.d. is less than EMF when current flows due to internal resistance. Voltmeters are connected in parallel with very high resistance.

电势差：V = W / Q，单位电荷的能量。EMF是电源单位电荷供应的总能量。有电流时端电压因内阻而小于EMF。电压表并联连接且内阻极高。

Resistance and Ohm's Law / 电阻与欧姆定律

Resistance: R = V / I, measured in ohms. Ohm's Law: current is proportional to voltage at constant temperature for ohmic conductors (straight-line I-V graph). Non-ohmic components include filament lamps and diodes.

电阻：R = V / I，单位欧姆。欧姆定律：恒温下欧姆导体的电流与电压成正比（I-V图为直线）。非欧姆元件包括白炽灯和二极管。

Resistance depends on: length (longer = higher R), cross-sectional area (thicker = lower R), material (resistivity ρ), and temperature. Formula: R = ρL / A.

电阻取决于：长度（越长R越大）、横截面积（越粗R越小）、材料（电阻率ρ）和温度。公式：R = ρL / A。

Series and Parallel Circuits / 串联与并联电路

Series: Same current everywhere (I = I₁ = I₂). Voltage divides: V = V₁ + V₂. Resistance adds: R = R₁ + R₂. 串联：各处电流相同，电压分配，电阻相加。

Parallel: Same voltage across each branch. Current divides: I = I₁ + I₂. Combined resistance: 1/R = 1/R₁ + 1/R₂. Total R is always less than the smallest individual R. 并联：各支路电压相同，电流分流，总电阻始终小于最小单个电阻。

For n identical resistors in parallel, R_total = R / n. This is a useful exam shortcut.

对于n个相同电阻并联，R_total = R / n。这是考试中有用的快捷方法。

Electrical Power and Energy / 电功率与电能

Power equations: P = IV = I²R = V²/R. Choose based on known quantities. Energy: E = P × t = I × V × t. Domestic energy is measured in kilowatt-hours (kWh): 1 kWh = 3.6 × 10⁶ J.

功率公式：P = IV = I²R = V²/R，根据已知量选择。电能：E = P × t = I × V × t。家庭用电以千瓦时（kWh）计量：1 kWh = 3.6 × 10⁶ J。

Mains Electricity and Safety / 市电与安全用电

Mains is AC at 50 Hz. Three wires: live (brown, carries voltage), neutral (blue, completes circuit), and earth (green/yellow, safety path to ground). Fuses melt when current exceeds rating; circuit breakers trip electromagnetically. Double-insulated appliances (square-in-square symbol) do not need earthing.

市电为 50 Hz 交流电。三根线：火线（棕色，传输电压）、零线（蓝色，构成回路）和地线（黄绿，安全接地）。保险丝过流熔断；断路器电磁跳闸。双重绝缘电器（方框套方框符号）无需接地。

Safety: never touch appliances with wet hands, do not overload sockets, ensure proper earthing.

安全：切勿湿手触摸电器，不使插座过载，确保正确接地。

Static Electricity / 静电

Static charge accumulates on insulators via friction — electrons transfer between materials. Like charges repel; opposite charges attract. Conductors allow charge flow; insulators trap charge. Electric field lines point away from positive and toward negative charges.

静电通过摩擦在绝缘体表面积累——电子在材料间转移。同种电荷相斥，异种电荷相吸。导体允许电荷流动，绝缘体困住电荷。电场线远离正电荷、指向负电荷。

Applications: electrostatic precipitators, spray painting, photocopiers. Dangers: sparks igniting flammable gases, damage to electronics. Always describe charging as electron transfer, never proton transfer.

应用：静电除尘器、喷涂、复印机。危害：火花点燃易燃气体、损坏电子元件。始终用电子转移而非质子转移来描述起电。

Exam Tips / 考试技巧

Edexcel IGCSE Electricity is worth about 20-25% of total marks. Common errors: mixing up series vs parallel rules, using wrong power equation, forgetting unit conversions (mA→A, kJ→J, min→s), and describing charging as proton transfer. Always show full working in calculations — Edexcel awards method marks even if the final answer is wrong.

爱德思IGCSE电学约占20-25%分数。常见错误：混淆串并联规则、使用错误功率公式、忘记单位换算、用质子转移描述起电。计算题始终展示完整步骤——爱德思即使在最终答案错误时也为正确方法给分。

Master the core equations, practice past papers systematically, and you will be well prepared for the examination.

掌握核心公式，系统练习历年真题，你将为考试做好充分准备。

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