🔌 Electric Circuits — The Foundation | 电路基

🔌 Electric Circuits — The Foundation | 电路基础

An electric circuit is a closed loop that allows charge to flow continuously. For current to flow, you need three things: a source of potential difference (like a cell or battery), a complete conducting path (wires), and a component that uses the energy (like a lamp or resistor). Without a complete loop, no current flows.

电路是电荷持续流动的闭合回路。电流流动需要三个条件：电势差源（如电池）、完整的导电路径（导线）和消耗能量的元件（如灯泡或电阻）。没有完整回路就没有电流——这是考试中最基本的知识点。OCR 考试常考电路图的识别与绘制，务必掌握标准电路符号，尤其是电池、电阻、可变电阻、二极管、LED 和保险丝的符号。

⚡ Current, Voltage & Resistance | 电流、电压与电阻

Current (I) is the rate of flow of charge. It is measured in amperes (A) using an ammeter connected in series. In a single closed loop, current is the same everywhere. Potential difference (V), often called voltage, is the energy transferred per unit charge. It is measured in volts (V) using a voltmeter connected in parallel across the component. Resistance (R) is anything that opposes the flow of current, measured in ohms (Ω).

电流 (I) 是电荷流动的速率，单位安培 (A)，使用串联的电流表测量。在单一闭合回路中，各处电流相等。电势差 (V) 即电压，是单位电荷转移的能量，单位伏特 (V)，使用并联在元件两端的电压表测量。电阻 (R) 是阻碍电流流动的因素，单位欧姆 (Ω)。OCR 考试高频考点：电流表串联、电压表并联——这是实验题中反复出现的送分点。

📐 Ohm's Law & I-V Characteristics | 欧姆定律与 I-V 特性

Ohm's Law states that for an ohmic conductor at constant temperature, the current through it is directly proportional to the potential difference across it: V = IR. The I-V graph is a straight line through the origin. However, not all components obey Ohm's Law. A filament lamp shows a curve that flattens at higher voltages because its resistance increases as it heats up. A diode only allows current to flow in one direction — its I-V graph shows zero current in reverse and a sharp upward curve in forward bias above the threshold voltage (~0.6V).

欧姆定律指出，对于恒温下的欧姆导体，电流与电势差成正比：V = IR。I-V 图是一条过原点的直线。但并非所有元件都遵循欧姆定律。灯丝灯泡的 I-V 曲线在高电压区趋于平缓，因为温度升高导致电阻增大。二极管只允许单向导电——反向时电流为零，正向电压超过阈值（约0.6V）后电流急剧上升。OCR 常考这三种元件的 I-V 特性曲线识别与解释，务必能画出并描述每条曲线的形状及原因。

🔗 Series vs Parallel Circuits | 串联电路与并联电路

This is the single most-tested topic in OCR GCSE Electricity. You must know the rules cold.

Series Circuits: Current is the same everywhere (I₁ = I₂ = I₃). The total potential difference is the sum of individual p.d.s (V_total = V₁ + V₂ + V₃). The total resistance is the sum of individual resistances (R_total = R₁ + R₂ + R₃). Adding more resistors in series increases total resistance and decreases current.

Parallel Circuits: The total current from the source equals the sum of currents in each branch (I_total = I₁ + I₂ + I₃). The potential difference across each branch is the same (V₁ = V₂ = V₃). Total resistance decreases as you add more parallel branches because there are more paths for current — total resistance is less than the smallest individual resistance.

这是 OCR GCSE 电学中考频最高的知识点，必须滚瓜烂熟。串联电路：各处电流相等（I₁ = I₂ = I₃）；总电压等于各元件电压之和（V_total = V₁ + V₂ + V₃）；总电阻等于各电阻之和（R_total = R₁ + R₂ + R₃）。串联更多电阻会增大总电阻、减小电流。并联电路：干路电流等于各支路电流之和（I_total = I₁ + I₂ + I₃）；各支路电压相等（V₁ = V₂ = V₃）；并联更多支路会减小总电阻——因为电流路径增多，总电阻小于最小的单个电阻值。

🔋 Power, Energy & the National Grid | 功率、能量与国家电网

Power (P) is the rate of energy transfer, measured in watts (W). Two key equations you must memorise: P = IV (power = current × potential difference) and P = I²R (power = current² × resistance). Energy transferred (E) uses E = Pt (energy = power × time) or E = QV (energy = charge × potential difference), where Q = It. Exam questions often combine these: calculate current first, then power, then energy over a given time.

功率 (P) 是能量转移的速率，单位瓦特 (W)。两个关键公式：P = IV 和 P = I²R。能量 (E) 公式：E = Pt 或 E = QV（其中 Q = It）。考试常组合考查：先算电流，再算功率，最后算给定时间内的能量。关于 国家电网：电力以高电压、低电流传输以减少热损耗（P_loss = I²R）。升压变压器提高电压、降压变压器降低电压供家庭使用——这是 OCR 必考的能源话题。

🧲 LDRs, Thermistors & Sensing Circuits | 光敏电阻、热敏电阻与传感电路

An LDR (Light Dependent Resistor) has high resistance in the dark and low resistance in bright light. A thermistor has high resistance when cold and low resistance when hot (NTC type — the one tested in OCR). These components appear in potential divider circuits: when the sensing component's resistance changes, the output voltage (V_out) changes, and this can trigger a switch or alarm. You must be able to explain what happens to V_out when light intensity or temperature changes, and why.

LDR（光敏电阻）：暗处电阻高，亮处电阻低。热敏电阻：低温电阻高，高温电阻低（NTC型，OCR考查类型）。这些元件常用于分压电路：当传感元件的电阻变化时，输出电压 (V_out) 随之改变，触发开关或报警器。考试要求解释光照或温度变化时 V_out 如何变化及其原因。常见情景：路灯自动控制（LDR + 固定电阻分压）、火灾报警器（热敏电阻 + 固定电阻分压）。

📝 Common Exam Mistakes & OCR Tips | 常见失分点与应试技巧

Mistake 1: Confusing series and parallel rules. Write a quick summary table on your exam paper as soon as you start. Mistake 2: Forgetting units. OCR deducts marks — always write A, V, Ω, W, J. Mistake 3: Saying "current is used up" in a series circuit. Current is conserved. Mistake 4: Mixing up ammeter and voltmeter connections — ammeter in series, voltmeter in parallel. Draw a diagram to check. Mistake 5: In calculation questions, not showing working. OCR awards method marks even if the final answer is wrong — always write the formula, substitution, and answer with units.

失分点1：混淆串并联规律。开考后立即在草稿纸上画出对比表。失分点2：忘记单位——OCR 扣分毫不手软，答案必须带 A、V、Ω、W、J。失分点3：说串联电路中的电流被"消耗"了——电流守恒，各处相等。失分点4：电流表电压表接法混淆——电流表串联、电压表并联，画图确认。失分点5：计算题不写步骤——OCR 按步骤给分，即使最终答案错误也能得分。务必写出：公式 → 代入数值 → 计算结果 + 单位。

📋 Key Formula Summary | 核心公式汇总

• Charge: Q = It  |  • Ohm's Law: V = IR  |  • Power: P = IV = I²R
• Energy: E = Pt = QV  |  • Series: R_total = R₁ + R₂, I same, V splits
• Parallel: I_total = I₁ + I₂, V same, 1/R_total = 1/R₁ + 1/R₂
• Energy transferred (with time): E = IVt

以上是 OCR GCSE Physics 电学部分的核心知识体系。建议打印贴在书桌前，每周默写一遍公式，考前重点复习 I-V 特性曲线和分压电路。坚持系统复习，电学部分满分可期。

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