Edexcel IGCSE物理 电流 电压 电阻 电功率
电学是 Edexcel IGCSE Physics 分值最高、也最容易拉开差距的模块。Paper 1 与 Paper 2 都会用计算题、电路图分析和实验设计题反复考查电流、电压、电阻、电功率与家庭用电。很多同学公式背得滚瓜烂熟，却在单位换算、串并联判断、图像斜率和实验题步骤上连续丢分，原因往往不是不懂原理，而是答题顺序和术语不规范。下面按考试出现的自然顺序梳理 8 个高频考点，每个考点给出定义、公式、带数字的例题、常见陷阱和中英对照术语，读完可以直接当作复习清单使用。
Electricity carries the heaviest weight in Edexcel IGCSE Physics and separates grades more than any other topic. Paper 1 and Paper 2 test current, potential difference, resistance, electrical power and household electricity through calculations, circuit analysis and practical questions. Students who know the theory still lose marks on unit conversion, series and parallel reasoning, graph gradients and experimental method, usually because of careless ordering and non-standard wording rather than lack of understanding. The eight points below follow the natural exam sequence, each with the definition, the formula, a worked example with real numbers, the common traps and bilingual terminology, so the article doubles as a revision checklist.
1 电流与电荷 Current and Charge
电流是电荷的定向移动，定义式 I = Q / t。I 的单位是安培 A，Q 的单位是库仑 C，t 的单位是秒 s。1 A 表示每秒有 1 C 电荷通过导体的某一横截面。带电粒子在金属中是自由电子，在电解液中是正负离子。计算前必须统一单位：把毫安换成安培（1 mA = 0.001 A），把分钟换成秒。例题：某灯泡工作时电流为 0.25 A，通电 4 分钟，通过灯丝的电荷量 Q = I x t = 0.25 x 240 = 60 C。再追问一步，若每个电子电荷量为 1.6 x 10 的负 19 次方库仑，则通过的电子数约为 3.75 x 10 的 20 次方个。
Current is the rate of flow of charge and is defined by I = Q / t, with I in amperes, Q in coulombs and t in seconds. One ampere means one coulomb passing a cross-section each second. The charge carriers are free electrons in metals and ions in electrolytes. Convert units first: milliamperes to amperes (1 mA = 0.001 A) and minutes to seconds. Example: a lamp carries 0.25 A for 4 minutes, so Q = 0.25 x 240 = 60 C. Extending the question, with an electron charge of 1.6 x 10 to the power minus 19 coulombs, about 3.75 x 10 to the power 20 electrons pass.
常见陷阱：第一，忘记把分钟换算成秒，答案会被判错整整 60 倍；第二，误以为电流会被"用掉"，事实上串联电路中电流处处相同，被转移的是电能而不是电流；第三，把电流方向说成电子移动方向，规定的电流方向与电子移动方向相反。
Traps: forgetting to convert minutes to seconds throws the answer off by a factor of 60; saying current is "used up" is wrong because the current is the same at every point of a series circuit and it is energy that is transferred; and conventional current is opposite to the drift of electrons, so never write that current flows in the direction the electrons move.
2 电压与电动势 Potential Difference and EMF
电压也叫电势差，定义为单位电荷所转移的能量，V = W / Q，单位伏特 V，且 1 V = 1 J/C。它描述的是电路中某两点之间每库仑电荷获得或释放多少能量，而不是"电路里有多少电"。电源的电动势描述电源内部把化学能、光能等其他形式的能量转化为电能、并为每库仑电荷提供的能量。真实电源都有内阻 r，电流越大，内阻上损耗越多，端电压越低。例题：某电源电动势 6.0 V，内阻 0.50 欧姆，外接 5.5 欧姆电阻，总电流 I = 6.0 / (5.5 + 0.50) = 1.0 A，端电压 V = 6.0 减 1.0 x 0.50 = 5.5 V。若把外电阻改成 0.50 欧姆，电流升到 6.0 A，内阻上就消耗了 3.0 V，端电压只剩 3.0 V。
Potential difference, also called voltage, is the energy transferred per unit charge: V = W / Q in volts, where 1 V = 1 J/C. It tells you how much energy each coulomb gains or loses between two points, not how much charge a circuit holds. The electromotive force of a supply describes the energy per coulomb converted from chemical, light or other forms into electrical energy inside the source. Real supplies have internal resistance r, so a larger current wastes more energy inside the source and the terminal voltage drops. Example: a 6.0 V supply with 0.50 ohm internal resistance drives 5.5 ohm, giving I = 6.0 / 6.0 = 1.0 A and a terminal voltage of 5.5 V. Reduce the external resistance to 0.50 ohm and the current rises to 6.0 A, the internal resistance takes 3.0 V, and the terminal voltage falls to 3.0 V.
常见陷阱：把电压表串联进电路，导致电路几乎断路；把电动势和端电压当成同一个量；在解释内阻时只说电流变大而不说明端电压下降的因果关系。答题时写清"电流增大，内阻上分得的电压增大，所以端电压减小"，因果链完整才拿到解释分。
Traps: connecting a voltmeter in series, which almost breaks the circuit; treating emf and terminal voltage as the same quantity; and describing internal resistance without the chain of cause and effect. Write that a larger current gives a larger voltage across the internal resistance, so the terminal voltage decreases, to collect the explanation marks.
3 电阻与欧姆定律 Resistance and Ohm's Law
电阻 R = V / I，单位欧姆，符号是希腊字母 Omega。电阻反映导体对电流阻碍的程度，取决于四个因素：材料、长度、横截面积与温度。同种材料、温度相同时，导线越长电阻越大，越粗电阻越小，横截面积加倍电阻约减半。欧姆定律指出，在温度不变的前提下，金属导体中的电流与其两端电压成正比，即 V 与 I 的图像是一条过原点的直线，斜率就是电阻。例题：某电阻两端电压 12 V 时电流为 0.40 A，则 R = 12 / 0.40 = 30 欧姆；若电压改成 6.0 V，电流降为 0.20 A，斜率不变。灯泡是例外，灯丝温度随电流升高而急剧上升，电阻变大，所以它的 V 与 I 图像是向上弯曲的曲线。
Resistance is R = V / I in ohms. It measures how strongly a conductor opposes current and depends on four factors: material, length, cross-sectional area and temperature. For the same material and temperature, a longer wire has more resistance and a thicker wire has less, so doubling the area roughly halves the resistance. Ohm's law states that, at constant temperature, the current through a metallic conductor is proportional to the voltage across it, so a graph of V against I is a straight line through the origin whose gradient is the resistance. Example: 12 V drives 0.40 A, so R = 12 / 0.40 = 30 ohms; at 6.0 V the current halves and the gradient is unchanged. A filament lamp is the exception: the filament gets much hotter, its resistance rises, and the graph curves upwards.
常见陷阱：把欧姆定律当成对所有元件都成立，遇到灯泡、二极管仍套用直线；图像题里把 I 与 V 的斜率直接当成电阻（应看 V 与 I 的斜率，或取 I 与 V 斜率的倒数）；描述电阻与横截面积关系时写"成正比"而不写"反比"。此外，温度升高使金属电阻变大，这一点在解释灯泡曲线时是必写点。
Traps: applying Ohm's law to every component, including lamps and diodes; reading the gradient of I against V as the resistance when it is actually the reciprocal; and writing that resistance is proportional to cross-sectional area instead of inversely proportional. Also remember that heating raises the resistance of a metal, which is the standard explanation for the curved lamp graph.
4 串联与并联电路 Series and Parallel Circuits
串联电路电流处处相同，电源电压被各元件分摊，总电阻等于各电阻之和，因此每增加一个电阻，总电流都会下降。并联电路各支路电压相同且都等于电源电压，干路电流等于各支路电流之和，总电阻的倒数等于各支路电阻倒数之和，所以并联总电阻一定小于其中任何一个支路电阻。例题：两个 6.0 欧姆电阻并联，1 / R = 1 / 6.0 + 1 / 6.0 = 1 / 3.0，得 R = 3.0 欧姆；串联则为 12 欧姆。若电源 12 V，串联时电流为 1.0 A，并联时干路电流为 4.0 A，说明并联让电路整体导流能力增强。家用电器全部并联，原因正是它们需要各自独立开关，并且都需要完整的 220 V 电压。
In a series circuit the current is the same everywhere, the supply voltage is shared between the components and the total resistance is the sum of the individual resistances, so adding a resistor always reduces the current. In a parallel circuit every branch has the same voltage, equal to the supply voltage; the main current is the sum of the branch currents; and 1 / R = 1 / R1 + 1 / R2, so the combined resistance is smaller than any single branch. Example: two 6.0 ohm resistors in parallel give 3.0 ohm while in series they give 12 ohm. With a 12 V supply the series current is 1.0 A whereas the parallel main current is 4.0 A, showing that parallel branches carry more total current. Household appliances are connected in parallel precisely because each needs its own switch and the full mains voltage.
常见陷阱：判断电路时画出等效图，先确定哪些元件真的串联；并联计算中忘记取倒数，把 1/R 直接当答案；比较亮度时只算电流不算功率。串联中电流相同但电压不同，并联中电压相同但电流不同，用电压乘电流得到功率再比较亮度才严谨。
Traps: redraw the circuit and identify which components are genuinely in series before calculating; do not forget to invert 1/R; and compare lamp brightness by power, not current alone. In series the current is shared equally but the voltages differ, while in parallel the voltages match and the currents differ, so P = V x I is the reliable comparison.
5 电功率与电能 Electrical Power and Energy
电功率是电能转化为其他形式能的速率，公式有三条等价形式：P = V x I，P = I 的平方 x R，P = V 的平方 / R，单位瓦特 W。电能 E = P x t，单位焦耳 J；家庭电费使用千瓦时 kWh，1 kWh 表示功率 1 kW 的电器工作 1 小时所消耗的电能，1 kWh = 3.6 x 10 的 6 次方焦耳。例题：一个额定功率 2.0 kW 的电水壶工作 15 分钟，消耗电能 E = 2.0 x 0.25 = 0.50 kWh；若电价为每千瓦时 0.6 元，本次烧水花费 0.30 元。反过来，若要计算电费总额，先求每个电器的 E = P t，把所有电器相加得到总 kWh，再乘电价。电表读数题用本次读数减去上次读数，单位通常就是 kWh。
Electrical power is the rate of energy transfer, and the three equivalent forms are P = V x I, P = I squared x R and P = V squared / R, in watts. Energy transferred is E = P x t in joules, while household bills use the kilowatt-hour: 1 kWh is the energy used by a 1 kW appliance in one hour, equal to 3.6 x 10 to the power 6 joules. Example: a 2.0 kW kettle runs for 15 minutes, using E = 2.0 x 0.25 = 0.50 kWh, which costs 0.30 yuan at 0.6 yuan per kWh. To estimate a bill, find E = P t for each appliance, add the kilowatt-hours and multiply by the unit price. For a meter reading, subtract the previous reading from the current one, since the meter already reads in kWh.
常见陷阱：把功率当成电能，忽略时间；在 220 V 的电路里用额定功率计算时未核对电压；单位混用，把瓦特直接乘分钟却写成焦耳或千瓦时。选择题里"功率越大越费电"是典型错误选项，耗电多少还取决于使用时间。若要计算电能，先统一到瓦特与秒，或统一到千瓦与小时，中途切勿混用。
Traps: confusing power with energy by ignoring time; using rated power without checking the supply voltage; and mixing units, for example multiplying watts by minutes and quoting joules or kilowatt-hours. The statement that a higher power appliance always uses more energy is a classic distractor, because time matters. Convert everything to watts and seconds, or everything to kilowatts and hours, and never mix the two sets midway.
6 家庭用电与安全 Household Electricity and Safety
家庭电路采用并联，火线与零线之间提供约 220 V 电压，开关必须接在火线上，这样断开开关后灯头不带电。保险丝与断路器也装在火线上，其作用是当电流超过额定值或发生短路时切断电路，防止导线过热引发火灾。三脚插头中的地线连接到电器的金属外壳，一旦内部火线接触外壳，电流直接入地并触发保险丝熔断或断路器跳闸，从而保护使用者。触电的原因通常是人体成为电路的一部分：手触火线时，若人体与地之间形成通路，就会有电流通过人体。安全原则是不同时接触火线与零线、不接触裸露导体、湿手不碰电器。计算保险丝规格时用 P = V x I 求电流，选择略大于正常工作电流的规格。
Household circuits are wired in parallel, with about 220 V between the live and neutral wires. Switches must be placed in the live wire so that the lamp holder is not live when the switch is open. Fuses and circuit breakers also sit in the live wire and cut the supply when the current exceeds its rated value or a short circuit occurs, preventing wires from overheating and starting a fire. The earth wire in a three-pin plug connects to the metal case, so if the live wire touches the case a large current flows to earth and blows the fuse or trips the breaker, protecting the user. Electric shock happens when the body becomes part of a circuit: touching the live wire with a path to earth drives current through the body. Safe practice means never touching the live and neutral wires together, never handling bare conductors, and keeping wet hands away from appliances. To choose a fuse rating, find the normal current from P = V x I and pick a value just above it.
常见陷阱：把开关画在零线上；忘记说明断路器比保险丝更方便可复位；解释地线作用时只写"安全"而没有说明电流入地导致电路切断的机制。解释题要求因果完整，写全"外壳带电，电流经地线入地，电流过大使保险丝熔断或断路器跳闸，人体免于触电"。
Traps: drawing the switch in the neutral wire; forgetting that a circuit breaker can be reset while a fuse must be replaced; and answering "for safety" without the mechanism. Full-credit explanations state that the case becomes live, a large current flows through the earth wire, the fuse blows or the breaker trips, and the user is therefore protected from electric shock.
7 实验与图像题技巧 Practical Work and Graph Questions
电学实验的标准接法是：电流表串联在被测元件所在支路，电压表并联在被测元件两端；滑动变阻器接成限流式以保护电路并改变电流读数。测量电阻时至少取六组不同的电流与电压数据，并改变变阻器阻值分散取点，避免数据集中在原点附近。作图时先标明坐标轴与单位，用光滑直线或曲线连接趋势，不要机械连点成折线，再在直线上取点求斜率。例题：某实验得到一组数据，电压 1.0、2.0、3.0、4.0 V 对应电流 0.10、0.20、0.30、0.39 A，作图后斜率取 4.0 / 0.39 约等于 10 欧姆，注意最后一组数据略偏小说明灯丝温度升高使电阻变大。读数时视线要与表盘垂直，注意量程对应的最小分度值：0 到 0.6 A 量程最小分度通常为 0.02 A，0 到 15 V 量程最小分度通常为 0.5 V。
The standard practical setup places the ammeter in series with the component under test and the voltmeter in parallel with it, while a variable resistor connected in series protects the circuit and varies the readings. When measuring resistance, take at least six pairs of current and voltage values, spreading them by adjusting the variable resistor instead of clustering near the origin. On the graph, label both axes with units, join the trend with a smooth line rather than a zigzag of straight segments, and read the gradient from the line itself. Example: readings of 1.0, 2.0, 3.0 and 4.0 V giving 0.10, 0.20, 0.30 and 0.39 A produce a gradient near 4.0 / 0.39, about 10 ohms, and the slightly low final current shows the filament resistance rising with temperature. Read meters with your eye directly in front of the scale and note the smallest division for the range: 0.02 A on the 0 to 0.6 A range and 0.5 V on the 0 to 15 V range.
常见陷阱：电流表与电压表接反；滑动变阻器接成短路使读数不变；只测两三组数据导致图像无法定斜率；单位写成 A 与 V 却把电阻写成毫欧。写实验题时按"装置、操作、读数、作图、结论"五步表述，每一步都给出可检验的细节，评分点就齐了。
Traps: swapping the ammeter and voltmeter positions; wiring the variable resistor so that it does nothing; collecting only two or three readings so the gradient cannot be found; and misquoting the resistance unit. Structure practical answers as apparatus, procedure, readings, graph and conclusion, with a checkable detail in each step, and every marking point is covered.
8 高频陷阱清单与冲刺建议 Revision Checklist
把上面八个考点浓缩成一份自查清单：一是所有计算先统一单位，安培对秒，千瓦时对小时；二是写公式、代数据、给单位三步分开，绝不跳步；三是串联求电流相同，并联求电压相同，先判断再计算；四是区分额定功率与实际功率，功率与电能不能混用；五是解释题写完整因果链，尤其是内阻、地线与灯泡曲线三类题型；六是图像题先看斜率代表什么量，再看趋势是否受温度影响。最后两周的复习建议是每天限时做两道电学计算题和一道实验设计题，做完立刻对照评分标准圈出漏写的关键词，比重复看书有效得多。考前把 1 kWh 等于 3.6 乘 10 的 6 次方焦耳、1 mA 等于 0.001 A、欧姆定律三种功率形式默写一遍，进入考场就能直接落笔。
Condense the eight points into a self-check list: convert units first, amperes with seconds and kilowatt-hours with hours; separate formula, substitution and unit into three visible steps; decide that series shares current and parallel shares voltage before calculating; distinguish rated from actual power and never mix power with energy; write complete causal chains for internal resistance, the earth wire and the filament lamp curve; and in graph questions identify what the gradient represents before judging whether heating has changed the trend. For the final two weeks, solve two electricity calculations and one practical design question under timed conditions each day, then mark your own work against the mark scheme and highlight the missing key words, which beats rereading notes. Before the exam, write out that 1 kWh equals 3.6 x 10 to the power 6 joules, that 1 mA equals 0.001 A, and the three forms of the power equation, so you can start writing the moment the paper opens.
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