OCR物理 电学 电路 电流 电压 电阻入门
OCR GCSE 物理（9-1）考试中，电学是权重最高的模块之一，通常在 Paper 1 里占到四分之一左右的分数。很多同学觉得电路抽象难懂，其实整个模块的核心只有三个概念：电流、电压、电阻。把三者的定义、单位、测量方法和相互关系弄清楚，绝大多数电路题都能迎刃而解。这篇文章用中英双语带你系统入门 OCR 电学考点，从电荷与电流讲到家庭用电安全，每个知识点都配上公式、数字例题和易错提醒，帮你搭起完整的电学知识框架。
Electricity is one of the highest-value topics in the OCR GCSE Physics (9-1) course, worth roughly a quarter of the marks on Paper 1. Many students find circuits abstract and confusing, but the whole topic rests on just three ideas: current, potential difference and resistance. Once you know their definitions, units, measuring instruments and how they connect, most circuit questions become straightforward. This bilingual guide walks through the OCR electricity syllabus from charge and current to mains safety, with formulas, worked examples and common pitfalls, so you can build a complete framework before the exam.
一、电荷、电流与电路 / Charge, Current and Circuits
电路的本质是电荷的定向移动。电荷的单位是库仑（C），一个电子带 1.6×10⁻¹⁹ C 的负电荷。电流是每秒流过导体横截面的电荷量，单位安培（A），定义式 I = Q/t。电流方向按惯例取正电荷移动的方向，与电子实际运动方向相反。测量电流用电流表（ammeter），必须串联在电路中。电路必须闭合才可能有电流，开关断开时电路开路，电流为 0。数字例题：某电路电流为 3 A，通电 10 秒，流过横截面的电荷量 Q = It = 3×10 = 30 C。易错点：电流不会在元件里"消耗"，串联电路中同一根导线处处电流相等，题目说"电流经过灯泡后变小"是错的。
A circuit is a closed loop that allows electric charge to flow. Charge is measured in coulombs (C), and each electron carries 1.6 x 10^-19 C of negative charge. Current is the rate of flow of charge through a cross-section of a conductor, measured in amperes (A), with I = Q/t. By convention, current direction is the direction positive charge would move, which is opposite to the actual electron flow. An ammeter measures current and must be connected in series. The loop must be complete for current to flow; an open switch makes the circuit open and current drops to zero. Worked example: a current of 3 A flowing for 10 s transfers Q = It = 3 x 10 = 30 C. A common mistake is thinking current gets used up inside components; in a series circuit the current is the same at every point, so "the lamp uses up some current" is wrong.
二、电压与电阻 / Potential Difference and Resistance
电压（电位差）是每库仑电荷在元件两端获得的电能，单位伏特（V），定义式 V = W/Q。它像水压一样推动电荷流动，没有电压就没有电流。电压表（voltmeter）测量电压，必须并联在元件两端。电阻是导体对电流的阻碍作用，单位欧姆（Ω）。欧姆定律 V = IR 把三者连在一起，是电学计算的核心工具。固定电阻的 I-V 特性曲线是过原点的直线，遵守欧姆定律；白炽灯丝的电阻随温度升高而增大，I-V 曲线弯曲，不再成正比。数字例题：6 V 电池接在电路上，电流为 2 A，则总电阻 R = V/I = 6/2 = 3 Ω。考试常考单位换算：1 kΩ = 1000 Ω，1 mA = 0.001 A，答题时先换算再代入。
Potential difference (p.d.) is the energy given to each coulomb of charge passing through a component, measured in volts (V), with V = W/Q. It acts like water pressure, pushing charge around the circuit; without p.d. there is no current. A voltmeter measures p.d. and is always connected in parallel across the component. Resistance opposes current, measured in ohms (Ω). Ohm's law V = IR links the three quantities and is the workhorse of every electricity calculation. A fixed resistor at constant temperature gives a straight I-V graph through the origin, obeying Ohm's law; a filament lamp has a curved I-V graph because its resistance rises with temperature. Worked example: a 6 V battery drives a current of 2 A, so the total resistance is R = V/I = 6/2 = 3 Ω. Always convert units first: 1 kΩ = 1000 Ω and 1 mA = 0.001 A.
三、串联与并联电路 / Series and Parallel Circuits
串联电路：电流处处相等；总电压等于各元件电压之和；总电阻等于各电阻之和，R = R1 + R2。并联电路：每条支路两端电压相等且等于电源电压；干路电流等于各支路电流之和；总电阻满足 1/R = 1/R1 + 1/R2，计算结果总是小于最小的支路电阻。判断方法很简单：看电流会不会分岔，会分岔就是并联。数字例题一：两个 6 Ω 电阻串联，总电阻 6 + 6 = 12 Ω。数字例题二：同样两个 6 Ω 电阻并联，1/R = 1/6 + 1/6 = 1/3，R = 3 Ω，比单个还小。串联增加总电阻，并联减小总电阻，这也解释了为什么两盏同规格灯泡并联时都比串联时亮。
In a series circuit, current is the same everywhere, p.d. adds up around the loop, and total resistance is R = R1 + R2. In a parallel circuit, every branch has the same p.d., equal to the supply voltage; the main current splits between the branches, and the total resistance follows 1/R = 1/R1 + 1/R2, giving a value smaller than the smallest branch. The quick test is whether the current splits: if it splits, the circuit is parallel. Example one: two 6 ohm resistors in series give 6 + 6 = 12 Ω. Example two: the same two resistors in parallel give 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω, less than either alone. Adding series resistors raises total resistance, while adding parallel branches lowers it, which is why two identical lamps in parallel are brighter than the same lamps in series.
四、光敏电阻与热敏电阻 / LDR and Thermistor Circuits
LDR（光敏电阻）的光照越强，电阻越小；热敏电阻（thermistor）的温度越高，电阻越小。它们通常与固定电阻串联组成分压电路，把电阻变化转换成电压变化，供控制电路使用。串联分压的核心规律：电压按电阻大小成比例分配，电阻大的元件分到更多电压。OCR 高频题：LDR 与固定电阻串联接在 6 V 电源上，白天 LDR 电阻 100 Ω，夜晚升到 1000 Ω，固定电阻为 200 Ω。白天固定电阻分压 6×200/300 = 4 V，夜晚 6×200/1200 = 1 V，控制电路检测到电压大幅下降，就触发继电器接通路灯。热敏电阻同理：温度升高电阻减小，分压变化，恒温器据此切断加热器。
An LDR (light-dependent resistor) has a smaller resistance in brighter light, while a thermistor has a smaller resistance at higher temperature. Each is connected in series with a fixed resistor to form a potential divider, converting a change in resistance into a change in p.d. for a control circuit. The key rule: in series, p.d. is shared in proportion to resistance, so the larger resistor takes the larger share. Favourite OCR question: an LDR and a 200 Ω fixed resistor are in series across 6 V. By day the LDR is 100 Ω and the fixed resistor takes 6 x 200/300 = 4 V; at night the LDR rises to 1000 Ω, so the fixed resistor takes only 6 x 200/1200 = 1 V. The control circuit senses this drop and switches on the street lamp. A thermistor works the same way: hotter means lower resistance, and the thermostat cuts the heater when the divided voltage crosses its threshold.
五、电功率与电能 / Electrical Power and Energy
电功率表示用电器做功的快慢，P = VI，单位瓦特（W）。结合欧姆定律可推出 P = I²R 和 P = V²/R，按已知条件选用。电能 E = Pt，单位焦耳（J）。家庭用电按千瓦时（kWh）计费，1 kWh = 3.6×10⁶ J。算电费两步走：功率化成 kW，时间化成 h，相乘得 kWh，再乘以单价。数字例题：2 kW 电水壶工作 1.5 小时，耗电 2×1.5 = 3 kWh，若每度电 30 便士，电费 90 便士。易错点：用 P = V²/R 时，V 必须是该元件两端的实际电压，不能拿电源电压硬套；额定功率与实际功率是两回事，灯泡标 60 W 指 230 V 电压下的功率。
Electrical power is the rate at which energy is transferred, P = VI, measured in watts (W). Combining with Ohm's law gives P = I²R and P = V²/R; choose the form that matches the quantities you know. Energy transferred is E = Pt in joules (J). Domestic bills use kilowatt-hours: 1 kWh = 3.6 x 10^6 J. To find the cost, convert power to kW and time to hours, multiply to get kWh, then multiply by the unit price. Worked example: a 2 kW kettle running for 1.5 hours uses 2 x 1.5 = 3 kWh; at 30 pence per unit that costs 90 pence. Common trap: in P = V²/R, V must be the actual p.d. across that component, not the supply voltage. Also distinguish rated power from actual power: a lamp labelled 60 W draws 60 W only when connected to 230 V.
六、静电与电场 / Static Electricity and Electric Fields
摩擦起电的本质是电子转移：两种不同材料摩擦时，电子从一种材料转移到另一种材料，失去电子的带正电，得到电子的带负电，电荷总量守恒。带电体周围存在电场，电场对放入其中的电荷施力：同种电荷相斥，异种电荷相吸。静电应用与危害并存：静电除尘器利用带电金属板吸引烟尘，静电喷漆让漆雾均匀吸附；而油罐车拖铁链、医院手术室防静电，是因为摩擦积累的电荷一旦放电产生火花，可能引燃易燃物。OCR 常考：解释为什么用毛皮摩擦过的橡胶棒带负电。关键答出"电子从毛皮转移到橡胶棒上"，而不是"产生了电荷"，后者违反电荷守恒。
Static electricity comes from the transfer of electrons, not their creation. When two different materials rub together, electrons move from one to the other: the material losing electrons becomes positively charged and the one gaining electrons becomes negatively charged, with total charge conserved. Around any charged object there is an electric field, which exerts forces on other charges: like charges repel, unlike charges attract. Applications include electrostatic precipitators that attract smoke particles to charged plates and paint spraying that coats surfaces evenly; dangers include sparks near fuel tankers and in operating theatres, where a discharge could ignite fumes. OCR often asks why a rubber rod rubbed with fur becomes negatively charged. The key answer is that electrons transfer from the fur onto the rod; saying that charge was created breaks the law of conservation of charge.
七、家庭电路与用电安全 / Mains Electricity and Safety
英国市电是 230 V、50 Hz 的交流电。家庭电路三根线：火线（live）、零线（neutral）和地线（earth）。火线提供高压，零线构成回路，地线连接电器金属外壳与大地：如果外壳意外带电，电流经地线流入大地，防止人体触电。保险丝（fuse）串联在火线上，电流超过额定值会熔断，切断电路；双层绝缘的电器不需要地线。选保险丝的原则：额定电流略大于用电器正常工作电流，常见规格 3 A、5 A、13 A。数字例题：2300 W 电水壶接在 230 V 电源上，正常工作电流 I = P/V = 2300/230 = 10 A，应选 13 A 保险丝，选 3 A 会频繁误断。人体安全电流在 10 mA 以下，潮湿环境电阻小，更危险。
UK mains is 230 V alternating current at 50 Hz. The three wires in a domestic circuit are live, neutral and earth. Live carries the high voltage, neutral completes the circuit, and earth bonds the metal case of an appliance to the ground: if the case becomes live by fault, current flows to earth instead of through a person. A fuse is connected in series with the live wire and melts when current exceeds its rating, cutting the circuit; double-insulated appliances need no earth. Choose a fuse rated just above the normal working current, such as 3 A, 5 A or 13 A. Worked example: a 2300 W kettle on 230 V draws I = P/V = 2300/230 = 10 A, so a 13 A fuse is correct; a 3 A fuse would blow every time. Body currents below about 10 mA are generally safe, and wet skin lowers resistance, making electric shocks more dangerous.
常见问题解答 / Frequently Asked Questions
问：为什么串联电路中电阻越大的灯泡反而更亮？答：串联电路电流处处相等，功率 P = I²R，电阻大的灯泡发热功率大，所以更亮；并联时电压相等，P = V²/R，电阻小的反而更亮。问：电压表为什么不能串联在电路中？答：电压表内阻极大，串联会使电路电流几乎降为零，读数失去意义，所以必须并联。问：开关为什么必须接在火线上？答：如果开关装在零线上，断开时用电器仍与火线相连，外壳一旦漏电仍有触电风险；开关接火线才能彻底断电。这三问是 OCR 电学主观题的常客，建议把原理用自己的话写一遍。
Q: Why is the lamp with the larger resistance brighter in a series circuit? A: Current is identical everywhere in series, and power is P = I²R, so the larger resistor dissipates more power; in parallel the reverse holds, since p.d. is identical and P = V²/R makes the smaller resistor brighter. Q: Why must a voltmeter never be connected in series? A: A voltmeter has a very large resistance, so in series it would cut the circuit current to almost zero and the reading would be meaningless; it must be connected in parallel. Q: Why must the switch be placed on the live wire? A: If the switch is on the neutral wire, the appliance stays connected to live even when switched off, so a fault can still deliver a shock; switching the live wire disconnects the appliance completely. These three questions appear again and again in OCR written answers, so practise writing out the reasoning in your own words.
学习建议 / Study Tips
1. 先把 I、V、R 的定义、单位和测量方法背熟，V = IR 是一切推导的起点。2. 画图分析：拿到电路题先标出串并联关系，再决定用哪组规律，不要凭感觉。3. 培养分压思维：串联电压按电阻比例分配，电阻大分压多，LDR 和热敏电阻的题几乎全靠这条。4. 单位换算要敏感：mA 与 A、kΩ 与 Ω、kW 与 W 每年必考，换算错整题白算。5. 整理错题本，重点收录两类题：分压分析题，以及 P = V²/R 中电压代错的题，考前重做一遍，比刷新题更有效。
1. Memorise the definitions, units and measuring instruments of I, V and R first; V = IR is the starting point for every other derivation. 2. Draw the circuit: label the series and parallel parts before choosing which rules apply, and never answer from intuition alone. 3. Think in shares: in series, p.d. is shared in proportion to resistance, so the larger resistor takes the larger share; nearly every LDR and thermistor question depends on this. 4. Stay alert to units: mA to A, kΩ to Ω and kW to W conversions appear every year, and one conversion error can waste the whole question. 5. Keep a mistake log focused on two categories, potential divider analysis and wrong V in P = V²/R, and redo those questions before the exam instead of grinding new ones.
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