WJEC物理 运动 能量 电荷 ALevel基础入门
WJEC 是威尔士的考试局，其 A-Level 物理课程结构清晰，AS 阶段第一单元叫 Motion, Energy and Matter，把力学与能量基础放在一起考。这篇文章用中文和英文对照，带你把运动、能量、电荷三个核心板块一次理清，适合刚开始学 WJEC Physics 的同学建立知识框架。
WJEC is the Welsh exam board, and its A-Level Physics is neatly structured. The first AS unit, Motion, Energy and Matter, combines the foundations of mechanics and energy in one paper. This bilingual guide walks you through the three core areas, motion, energy and charge, so you can build a solid framework from day one.
运动学 Motion: 描述运动需要哪些量
描述运动首先要把标量和矢量分开。距离 distance 是标量，只有大小；位移 displacement 是矢量，既有大小又有方向，是起点到终点的直线距离。速度 velocity 和速率 speed 的区别同理，速度带方向，速率不带。WJEC 考卷上经常给一段情境，让你判断某个量是标量还是矢量，这类送分题要确保不失分。
To describe motion you must first separate scalars from vectors. Distance is a scalar with magnitude only, while displacement is a vector with both magnitude and direction, measured in a straight line from start to finish. Velocity is a vector but speed is a scalar, and WJEC often asks you to classify quantities in a given scenario, so master this early.
匀加速直线运动用 suvat 方程处理。s 是位移，u 是初速度，v 是末速度，a 是加速度，t 是时间。最常用的三条是 v = u + at、s = ut + ½at²、v² = u² + 2as。解题时先写已知量和未知量，再选方程，最后统一单位。加速度的单位是 m/s²，位移的单位是 m，时间必须是秒。
Constant acceleration along a straight line is handled with the suvat equations. Here s is displacement, u is initial velocity, v is final velocity, a is acceleration and t is time. The three you will use most are v = u + at, s = ut + ½at² and v² = u² + 2as. Write down what you know, choose the right equation, then check units before substituting.
图像题是 WJEC 的常客。位移时间图上的斜率是速度，速度时间图上的斜率是加速度，速度时间图曲线下的面积是位移。看清横纵轴的物理量和单位，比急着计算更重要，读图能力本身就能帮你拿不少分。
Graph questions appear constantly in WJEC papers. On a displacement-time graph the gradient gives velocity; on a velocity-time graph the gradient gives acceleration and the area under the curve gives displacement. Reading the axes carefully often earns marks before any calculation begins.
动力学 Dynamics: 力与运动的关系
牛顿第一定律说，物体在不受合外力时保持静止或匀速直线运动。第二定律给出定量关系 F = ma，合外力等于质量乘以加速度，单位是牛顿 N，1 N = 1 kg m/s²。第三定律强调作用力与反作用力大小相等、方向相反，作用在不同物体上。
Newton's first law states that an object stays at rest or moves at constant velocity unless acted on by a resultant force. The second law gives the quantitative link F = ma, where the resultant force in newtons equals mass times acceleration. The third law reminds us that forces always come in equal and opposite pairs acting on different bodies.
应用 F = ma 时，先画受力分析图，把重力、支持力、摩擦力、拉力全部标出来，再取运动方向为正方向列方程。物体匀速运动时合外力为零，加速运动时合外力等于 ma。常见陷阱是忘记把质量换算成 kg，或者把重量 weight 和质量的单位混用。
When applying F = ma, always sketch a free-body diagram first, labelling gravity, normal contact force, friction and any applied force. Take the direction of motion as positive, then write the equation. Common traps include leaving mass in grams and confusing weight, a force in newtons, with mass in kilograms.
动量 momentum 定义为 p = mv，单位是 kg m/s，是矢量。在 WJEC 课程里，动量守恒用于分析碰撞：系统不受外力时，碰撞前后总动量不变。弹性碰撞动能守恒，非弹性碰撞有一部分动能转化为内能。记住动量守恒在两类碰撞中都成立。
Momentum is defined as p = mv, a vector measured in kg m/s. When no external force acts, total momentum is conserved in any collision. Elastic collisions also conserve kinetic energy, while inelastic collisions convert some kinetic energy into internal energy. Momentum conservation applies to both, so use it first.
能量 Energy: 做功与能量转化
做功 work done 的定义是力沿位移方向的分量与位移的乘积，W = Fs，力的方向与位移方向一致时取最大。做功的单位是焦耳 J，1 J = 1 N m。物体在力的方向上移动了距离，力才对它做功；推墙不动不算做功，这是概念题的高频考点。
Work is done when a force moves an object in the direction of the force, given by W = Fs, measured in joules where 1 J = 1 N m. Pushing a wall that does not move does no work, a classic concept question that tests whether you really understand the definition.
动能 kinetic energy 的公式是 Ek = ½mv²，重力势能 gravitational potential energy 是 Ep = mgh。物体下落时势能转化为动能，动能大小与速度平方成正比，所以速度加倍动能变为四倍，这类比例推理题几乎每年都出现。
Kinetic energy is Ek = ½mv² and gravitational potential energy is Ep = mgh. As an object falls, potential energy transfers into kinetic energy. Since kinetic energy is proportional to the square of speed, doubling the speed quadruples the kinetic energy, a ratio question that WJEC asks almost every year.
能量守恒 principle of conservation of energy 是物理学的基石：能量既不能凭空产生也不能凭空消失，只能从一种形式转化为另一种形式。解题时把初状态的总能量和末状态的总能量列等式，注意摩擦会把一部分机械能转化为内能，算出来的差值就是损耗。
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred between forms. Set the total energy at the start equal to the total energy at the end, remembering that friction converts some mechanical energy into internal energy, so the difference shows up as a loss.
功率 power 是做功的快慢，P = W/t，也等于 P = Fv，单位是瓦特 W，1 W = 1 J/s。效率 efficiency 是有用输出能量与总输入能量之比，通常写成百分比。汽车爬坡时需要更大牵引力，功率一定时速度下降，这个关系用 P = Fv 一算就明白。
Power is the rate of doing work, P = W/t, which for a constant force becomes P = Fv, measured in watts. Efficiency is the ratio of useful output energy to total input energy, usually given as a percentage. A car climbing a hill needs more driving force, so at fixed power its speed falls, a result that follows directly from P = Fv.
电荷与电流 Charge and electric current
电荷 charge 的单位是库仑 C。电流 current 被定义为电荷的流动速率，I = Q/t，即每秒通过导体横截面的电荷量，单位是安培 A，1 A = 1 C/s。电路中电子实际流动方向与习惯规定的电流方向相反，电流方向规定为正电荷移动的方向。
Charge is measured in coulombs. Current is the rate of flow of charge, I = Q/t, the charge passing a cross-section of the conductor per second, measured in amperes where 1 A = 1 C/s. In metals the charge carriers are electrons, which flow opposite to conventional current, defined as the direction positive charge would move.
电势差 potential difference 是单位电荷获得的能量，V = W/Q，1 V = 1 J/C。欧姆定律给出 V = IR，电阻 R 的单位是欧姆 Ω。做电路计算时先判断串联并联：串联电流相同电压分配，并联电压相同电流分配。WJEC 的电学题经常把电流、电压、电阻三个量交叉设问。
Potential difference is the energy transferred per unit charge, V = W/Q, and Ohm's law gives V = IR with resistance in ohms. In circuit problems identify series and parallel first: series branches share current and split voltage, parallel branches share voltage and split current. WJEC often asks you to move fluently between I, V and R.
一个小技巧是把电流类比成水管里的水流，电势差类比成水压差，电阻类比成管道变窄处。电流不会凭空增多也不会凭空消失，在串联电路中处处相同，很多同学的错误都来自忘记这一点。
A helpful analogy: current behaves like water flow, potential difference like pressure difference, and resistance like a narrow section of pipe. Current is not created or destroyed, so it is the same everywhere in a series circuit, a point many students forget under exam pressure.
综合例题 Worked example
一辆质量 800 kg 的汽车从静止开始匀加速，10 s 内速度达到 20 m/s。求加速度，再求 10 s 内汽车行驶的距离，并计算此时汽车的动能。第一问用 v = u + at，20 = 0 + a × 10，得 a = 2 m/s²。第二问用 s = ut + ½at²，s = 0 + ½ × 2 × 10² = 100 m。第三问用 Ek = ½mv²，Ek = ½ × 800 × 400 = 160000 J = 160 kJ。
A car of mass 800 kg accelerates uniformly from rest and reaches 20 m/s in 10 s. Find the acceleration, the distance travelled and the kinetic energy at that moment. Using v = u + at gives a = 2 m/s². Then s = ut + ½at² = 100 m. Finally Ek = ½mv² = ½ × 800 × 20² = 160000 J = 160 kJ.
这类综合题把 suvat 方程和动能公式串在一起，是 AS 阶段最典型的送分大题。注意三问之间层层递进，第一问的结果是第二问的输入，任何一步算错都会连累后面，所以写完每步都回头核对单位和数量级。
This linked question chains the suvat equations into the kinetic energy formula, a very typical AS-level structure. Each part feeds the next, so check the units and the size of every intermediate answer before moving on, since one slip cascades through the whole question.
易错点清单 Common misconceptions
第一个高频错误是把加速度为零当成静止。物体匀速运动时加速度为零，但速度不为零，牛顿第一定律描述的正是这种状态。第二个错误是忘记 suvat 方程只适用于匀加速直线运动，遇到变加速运动必须回到图像或者用定义式计算。
The first classic mistake is equating zero acceleration with being at rest. An object moving at constant velocity has zero acceleration but is clearly moving, exactly the state described by Newton's first law. The second is forgetting that the suvat equations only apply to uniform acceleration; for changing acceleration you must return to graphs or first principles.
第三个错误出现在做功题里，力与位移方向垂直时做功为零，例如物体在水平面上匀速移动时，重力方向竖直向下，与位移垂直，重力不做功。第四个错误是电路题里把电荷和电流混为一谈，电荷是以库仑计量的物理量，电流是电荷流动的速率，应该说电流流过灯泡，不能说电荷流过灯泡。
The third error appears in work questions: a force perpendicular to displacement does no work, so gravity does no work on an object sliding horizontally at constant speed because gravity acts vertically while the displacement is horizontal. The fourth is confusing charge with current. Charge is a quantity measured in coulombs while current is its rate of flow, so we say current flows through a bulb, never charge flows through it.
第五个错误是忽略欧姆定律的适用条件，只有金属导体这类欧姆器件在温度不变时，V 与 I 才成正比。钨丝灯泡、二极管是非欧姆元件，它们的 I-V 图像不是过原点的直线，工作在不同电压下电阻不同。判断一个元件是否欧姆器件，看它的 I-V 图是不是直线即可。
The fifth trap is ignoring the condition behind Ohm's law. Only ohmic components such as metal conductors at constant temperature show V proportional to I. Filament lamps and diodes are non-ohmic, their I-V graphs are not straight lines through the origin, and their resistance changes with the operating voltage. If the graph curves, the component is non-ohmic.
WJEC 考试技巧 Exam technique
WJEC 物理考试以简答题、计算题和拓展解释题为主，实验设计与数据分析也占一定比例。作答时注意三点。第一，定义题必须写完整定义，例如动量要写质量与速度的乘积，并说明是矢量。第二，计算题展示全部步骤，公式、代入、结果三步缺一不可，单位换算写在代入之前。
WJEC Physics papers mix short answers, calculations and extended explanations, with some data analysis and experimental design. Three habits pay off. First, write full definitions, stating whether each quantity is a vector or a scalar. Second, show every step of a calculation: formula, substitution and answer, converting units before you substitute.
第三，解释题用因果链展开，因为所以一步步写清楚，例如电流增大，单位时间通过导线的电荷变多，因此产生的热量增加。避免只写结论不给理由，WJEC 的评分看重解释过程的逻辑完整性。答题前先看分值，2 分题通常需要两个要点或两步推理。
Third, build explanation answers as causal chains: current increases, so more charge passes each second, therefore more heat is produced. A conclusion with no reasoning earns little credit because WJEC rewards the logic of the explanation. Check the mark value before writing, a 2-mark question usually needs two points or two steps.
考前把三个板块的公式写在同一张纸上反复默写：suvat 方程组、F = ma、p = mv、W = Fs、Ek = ½mv²、Ep = mgh、P = Fv、I = Q/t、V = IR。公式之间其实是相通的，运动学描述运动，动力学解释运动的原因，能量把两者用做功联系起来，电荷与电流则是把力学思维迁移到电学。理解这条主线，WJEC 物理的基础部分就牢牢握在手里了。
Before the exam, write every core formula on one sheet and test yourself: the suvat set, F = ma, p = mv, W = Fs, Ek = ½mv², Ep = mgh, P = Fv, I = Q/t and V = IR. They all connect: kinematics describes motion, dynamics explains why it happens, energy links the two through work, and charge and current extend the same thinking to electricity. Follow that thread and the foundation of WJEC Physics is yours.
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