WJEC物理 运动学 能量守恒 电荷 电流
WJEC 的 AS 物理把第一年的内容切成两个单元，一个讲运动、能量与物质，另一个从电荷谈起，一路讲到电流、电阻与直流电路。很多学生以为这两块分属两个世界，做题时把公式分成两摞背，遇到传送带、电动机或者带电滑块就接不上。真正把两部分连起来的是能量这一个量：运动学描述位置怎么变，动力学解释为什么变，能量给出变多少的账本。电荷只是换了记账的载体，公式的形状没有变。这篇按试卷的出题顺序把三块拆开，方向约定、公式选用和容易丢分的地方都写清楚，读完可以直接对着历年试卷练。
Unit 1 of WJEC A level Physics deals with motion, energy and matter, while Unit 2 opens with charge and current; students who keep the two sets of equations in separate mental boxes often freeze when a conveyor belt or a charged slider turns up in the question. Energy is the quantity that links them, because kinematics describes how position changes, dynamics explains why it changes, and energy accounting states how much changes; charge simply carries the same accounting into a circuit. What follows tracks the order in which the papers ask for it, with sign conventions, equation choice and the places where marks quietly disappear.
Unit 1 与 Unit 2 的分工：运动、能量、电荷落在哪一章
WJEC 的 A level 物理资格代码是 1420，AS 的两个单元分别叫 Motion, Energy and Matter 与 Electricity and Light。第一个单元的前四个主题是基本物理、运动学、动力学与能量概念，第五个主题讲固体受力，后面才是恒星辐射与粒子核结构。第二个单元的第一个主题专门处理电荷与电流的传导，第二与第三个主题接着讲电阻和直流电路。按 WJEC 官网公布的安排，Unit 1 是 1 小时 30 分钟的笔试，占整个 A level 的 20%。很多学生以为电荷只属于第二个单元，其实运动与能量那套账本在电路里同样成立，只是把物体换成了载流子。把两章的公式写在同一张纸上，做题时才知道该调用哪一套。2015 年起实施的 specification（2023 年 10 月版本）里，energy concepts 要求你能算功、动能与功率，conduction of electricity 要求你解释电流的微观图像。两处都考单位，单位写错扣的分常常比算错还多。
WJEC splits A level Physics into five units, and the mechanics and electricity content that Year 12 students meet sits in the first two; Unit 1 is a 1 hour 30 minute paper worth 20% of the qualification, and its opening four topics cover basic physics, kinematics, dynamics and energy concepts. The same conservation bookkeeping reappears in Unit 2, where charge carriers take the place of blocks on a slope, so the quantities change while the rules do not. In practice, examiners see far more marks lost to missing units than to faulty algebra, and that is the cheapest thing to fix tonight.
位移、速度与加速度的方向约定
位移是矢量，从起点指向终点，与走了多远的路程不是同一个量。速度是位移随时间的变化率，平均速度用总位移除以总时间，瞬时速度看的是某一时刻。加速度描述速度变化的快慢，减速时加速度方向与速度相反，代入公式时取负值。常见的错误是把路程直接代进 v = s/t，遇到折返运动就会算出双倍的速度。阅卷时能看到大量这样的答案，数字算对了，符号和方向错了，只能拿过程分。单位也要统一，千米每小时换成米每秒要除以 3.6，这一步每年都有人漏。竖直方向的重力加速度取 9.81 米每二次方秒，这个值印在 WJEC 的数据表里，答卷上写 9.8 会被判精度不符。正方向一旦选定，整道题都要守住它，中途改方向是最容易被扣分的地方。
Displacement is a vector running from start to finish, while distance simply adds up every step taken, so a runner who returns to the start has travelled a long way and moved nowhere. Velocity is the rate of change of displacement, and acceleration is the rate of change of velocity, which means a decelerating object carries a negative acceleration in the chosen direction. Many candidates lose marks by dropping a minus sign when a ball reverses, and examiners report that sign errors rather than arithmetic account for most of the losses here.
匀加速运动的四个公式与选用顺序
匀加速直线运动只有四个常用关系式，每个式子都缺一个量。v = u + at 不含位移，s = (u + v)t/2 不含加速度，s = ut + at²/2 不含末速度，v² = u² + 2as 不含时间。做题时先把已知的三个量列出来，再看缺哪一个，缺什么就挑不含这个量的式子。WJEC 的题常把两段运动接在一起，第一段匀加速、第二段匀速，连接点的速度是唯一的桥梁。很多学生以为分段就要换一套公式，其实公式没变，只是初速度换了。一个球以 12 米每秒竖直上抛，到最高点用 1.22 秒，上升 7.34 米，这两个数出自同一个 g。式子里 a 与 v 方向相反时符号要跟着变，别把减速当成加速再取绝对值。答案写上单位与有效数字，评分方案对有效数字是单独给分的。
Only four equations are needed for uniform acceleration, and each one leaves out a different quantity: time, displacement, acceleration or final velocity. A reliable routine is to list the three known values first and then pick the equation that avoids the missing one, which saves the minutes otherwise spent solving simultaneous equations. In WJEC papers the two-stage problem is common, with a smoothly accelerating first part joined to a steady second part, and the velocity at the join is the only quantity the two stages share.
自由落体与抛体：把二维运动拆成两个一维
竖直下落的物体只受重力，加速度是 9.81 米每二次方秒，题目里通常忽略空气阻力。水平抛出的物体，水平方向匀速，竖直方向匀加速，两个方向共用同一个下落时间。轨迹是抛物线，速度方向沿轨迹的切线，落地速度要用两个分量合成。很多学生以为抛出速度越大下落越慢，其实两个方向互不影响，落地时间只由高度决定。水平速度只决定落点多远，竖直高度只决定在空中待多久，抓住这一条能省下不少时间。若题目要求速度与水平方向的夹角，先算分量再取反正切，别把角度写成弧度。从 20 米高处水平抛出，落地约用 2.0 秒，水平 15 米每秒就落到 30 米外。反向也要检查，抛出点若在斜坡上，落地时间要联立轨迹方程与斜面方程。
An object released from rest accelerates at 9.81 m/s², and the value printed in the WJEC data booklet should be the one you quote. Horizontal projection separates cleanly into a steady horizontal velocity and a uniformly accelerating vertical fall, both governed by the same clock; from 20 m the fall lasts about 2.0 s, which at 15 m/s puts the landing point some 30 m away. Candidates often believe a faster throw keeps the object airborne longer, yet the two motions are independent, so only the height sets the flight time.
牛顿三定律与受力图
牛顿第一定律说的是合外力为零时速度不变，静止与匀速直线运动在物理上没有区别。第二定律写成 F = ma，这里的 F 是合力，不是某一个拉力。第三定律讲作用力与反作用力，两个力作用在两个不同的物体上，永远不会互相抵消。画受力图时按接触点逐个检查，重力竖直向下，支持力垂直接触面，摩擦力沿接触面，绳子只能拉不能推。常见的错误是让支持力直接等于重力，斜面上一放就错。阅卷时如果受力图少画一个力，后面的方程通常整段不给分。电梯题要分加速、匀速、减速三段，支持力在每段都不同，体重计读数跟着变。把合力写成各力的矢量和再代数求和，方向用正负号表达，比画一堆箭头快得多。
Newton first law says that zero resultant force means unchanged velocity, so rest and steady straight-line motion are physically the same state. The second law, F = ma, uses the resultant force rather than any single pull, and the third law pairs forces acting on two different bodies, which is exactly why they never cancel. Free-body diagrams are marked strictly: weight downwards, normal contact force perpendicular to the surface, friction along it, tension only pulling, and one missing arrow usually costs the whole equation that follows.
动量与冲量：碰撞题的两条守恒式
动量 p = mv 是矢量，方向要与选定的正方向一致。冲量等于力乘时间，也等于动量的变化量，写成 FΔt = Δp。碰撞与爆炸时外力远小于内力，动量守恒；动能是否守恒要看碰撞类型。完全弹性碰撞动能也守恒，非弹性碰撞动能有损失，完全非弹性碰撞是两个物体粘在一起同速运动。做题时先画碰撞前后的速度箭头，再写守恒式，最后才代数。常见的错误是把动能守恒用在所有碰撞上，题目没说弹性就不能用那条式子。一辆 1200 千克的车以 20 米每秒行驶，动量是 24000 千克米每秒；6000 牛的制动力作用 4 秒刚好让它停下。题目给出接触时间时，多半是要你求平均力，用动量变化除以时间即可。
Momentum is mass times velocity and carries direction, so every term in a conservation statement must use the same positive sense. Impulse equals force multiplied by contact time and also equals the change in momentum, which is why a question supplying a contact time is really asking for an average force. Kinetic energy survives only in a fully elastic collision; where the wording says nothing about elasticity, momentum alone is safe, and a 1200 kg car at 20 m/s carrying 24000 kg m/s needs 6000 N for 4 s to stop.
功、动能与机械能守恒
功等于力乘沿力方向的位移，写成 W = Fs cosθ，力与位移垂直时做功为零。动能 ½mv² 与速度的平方成正比，速度加倍动能就是四倍。重力做功只与高度差有关，与路径无关，因此可以定义重力势能 mgh。机械能守恒的条件是只有重力或弹力做功，有摩擦时要把摩擦生热单独列一项。很多学生以为匀速运动就没有能量转化，其实拉力做的功正好补上摩擦消耗的部分。解能量题先写账本，左边是初始动能加势能，右边是末态动能加势能再加损失。0.5 千克的球以 12 米每秒运动，动能是 36 焦耳；把它竖直抬到 7.34 米高，势能正好回到同样的数。账本不平衡时先回去查有没有漏掉弹簧，别急着改数字。
Work is the product of force and the displacement along it, so a force at right angles to the motion transfers no energy at all. Kinetic energy grows with the square of speed, while gravitational work depends only on the height difference and not on the route taken, which is what allows a potential energy term to be defined. A 0.5 kg ball moving at 12 m/s carries 36 J, and lifting it through 7.34 m stores the same amount, so the balanced statement is initial energy plus work done equals final energy plus losses.
功率与效率：W/t 与 Fv 两个入口
平均功率是功除以时间，瞬时功率用力乘速度，匀速时两个式子给出同一个数值。WJEC 的题干常放电动机、水泵或汽车，问的是输出功率与输入功率的比值。效率没有单位，写成百分数时要看清题目要求保留几位有效数字。常见的错误是把输入与输出搞反，效率算出大于 100%，这在答卷上等于自曝概念错误。给出额定电压与电流时，输入功率就是电压乘电流；只给牵引力与速度时，用 Fv。牵引力 6000 牛配 15 米每秒的速度，输出功率是 90 千瓦，跑十小时就是 900 度电。两个入口算出的量不同，一个是电学量一个是力学量，别混着代。若电压、电流与速度同时给出，先用 VI 求输入再用 Fv 求输出，两式相除就是效率。
Average power divides work by time, while instantaneous power multiplies force by velocity, and the two agree for steady motion. Efficiency compares useful output with total input and carries no unit, so check how many significant figures the paper asks for before writing a percentage. Candidates who swap input and output end up reporting efficiencies above 100%, which reads as a conceptual error rather than a slip, and the safer route is to compute the electrical input with VI and the mechanical output with Fv before dividing.
固体受力：应力、应变与杨氏模量
应力是力除以横截面积，单位是帕斯卡；应变是伸长量除以原长，没有单位。杨氏模量等于应力除以应变，只在比例极限以内是常数，所以取的是应力应变曲线直线段的斜率。WJEC 的题常给直径，面积要用 πr² 算，半径不是直径。直径 0.50 毫米、长 2.0 米的钢丝挂 100 牛的砝码，应力约 5.1 乘 10 的 8 次方帕，伸长约 5.1 毫米，模量落在 200 吉帕附近。实验题常要求画力与伸长量的图，斜率乘原长除以截面积才是杨氏模量。很多学生以为钢丝越粗就越硬，模量其实是材料属性，与尺寸无关。超过比例极限后应力与应变不再成正比，题目一旦画出弯曲的曲线，就不能再用应力除以应变。这一节的换算是高发区，毫米要换成米，平方毫米要换成平方米，差一千倍就是全错。
Stress divides force by cross-sectional area and is measured in pascals, while strain divides extension by original length and has no unit at all. The Young modulus is their ratio, valid only within the limit of proportionality, so the gradient of the straight part of a stress-strain graph is what the calculation targets. A 0.50 mm steel wire of length 2.0 m carrying 100 N feels a stress near 5.1 x 10^8 Pa and stretches about 5.1 mm, which puts the modulus close to 200 GPa; examiners see a diameter used as a radius more often than any other slip in this topic.
电荷、电流与 Q = It
电荷量的单位是库仑，一个电子的电荷量是 1.60 乘 10 的负 19 次方库仑，带负号。电流是电荷流动的速率，写成 I = Q/t，方向规定为正电荷流动的方向。金属导线里实际移动的是电子，方向与常规电流相反，画图时箭头仍按常规电流画。WJEC 从 Unit 2 的第一个主题就要求分清这两者，答题时把电子流方向当成电流方向会被判概念错误。很多学生以为电流在导线上被一点点用掉，其实串联电路各处电流相同，被消耗的是能量。给出电子数目与时间时，先乘电子电荷量再除以时间，单位是安培。毫安换成安培要除以 1000，这一换算在电路题里年年出现。电荷守恒是这一节隐含的前提，节点处流入的电荷量总和等于流出的总和。
Charge is measured in coulombs, and a single electron carries 1.60 x 10^-19 C with a negative sign, which is why conventional current points opposite to the electron flow in a metal. Current is the rate at which charge passes a point, written I = Q/t, and in a series loop the same current passes every component because charge is conserved at each junction. Marks are lost when candidates describe current as being used up along a wire; what is transferred and then dissipated is energy, and the current stays the same all the way round.
漂移速度 I = nAve：电子走得比电流慢
电流的微观表达式是 I = nAve，n 是单位体积的自由电子数，A 是横截面积，v 是漂移速度，e 是电子电荷量。按教材常用的铜自由电子数密度 8.5 乘 10 的 28 次方每立方米估算，1 平方毫米的导线通过 1 安培时，漂移速度只有 7.4 乘 10 的负 5 次方米每秒。这个速度比蜗牛慢得多，台灯一按却立刻亮，原因是电场以接近光速在导线中建立，整条导线上的电子几乎同时开始移动。常见的错误是把漂移速度当成电信号的速度，答题时写电子跑得很快，概念分就没了。横截面积加倍，漂移速度减半，这个反比关系常被做成选择题。电流加倍时漂移速度也加倍，两处都用同一个式子推，别背两个结论。半导体里 n 小得多，同样的电流需要更大的漂移速度，载流子浓度随温度变化的问题这时不能忽略。式子里的 A 是有效横截面积，多股导线要按总截面算，只算一根会低估电流。
The microscopic form of current is I = nAve, where n counts free electrons per cubic metre, A is the cross-sectional area, v the drift velocity and e the electronic charge. Using the familiar copper value of about 8.5 x 10^28 m^-3, a 1 mm² wire carrying 1 A gives a drift velocity near 7.4 x 10^-5 m/s, slower than a snail, yet the lamp lights at once because the electric field establishes itself along the wire at close to the speed of light. Doubling the area halves the drift velocity, and confusing drift velocity with signal speed is the commonest conceptual slip in this topic.
电路中的能量与电功率
电荷从高电势流向低电势时释放能量，一段导体上做的功写成 W = VQ。两边同除以时间就得到 P = VI，结合欧姆定律还能写成 I²R 与 V²/R。三个写法等价，选哪个看题目给的是电流还是电压，给电流用 I²R 更省一步。电源内部也有电阻，电动势描述电源把其他形式能量转化为电能的能力。放电时端电压小于电动势，差值就是内阻上的电压降，测电源特性实验里要把这一点画进图里。常见的错误是把额定功率当成实际功率，灯泡接在低于额定电压的电路里，实际功率按 V²/R 的比例下降。能量账本在电路里同样成立，电源提供的总能量等于外电路消耗加上内阻发热。计算前统一单位，毫安、千欧要换到安培与欧姆，量级错一次的代价是整题零分。
Charge falling through a potential difference releases energy, and the work done on a length of conductor is W = VQ, which becomes P = VI once divided by time. Combining that with Ohm law gives I²R and V²/R, and the choice between them is a matter of which quantity the question supplies. Internal resistance explains why the terminal voltage falls below the emf while current flows, and the same energy balance holds in circuits: the source supplies what the external components and the internal resistance together dissipate.
Cambridge International AS ALevel Physics Coursebook 正版剑桥国际AS ALevel考试物理教材带线上账号 英文原版¥557.0📱 去京东购买京东精选
需要一对一辅导？Need One-on-One Tutoring?
A-Level / GCSE / IB / KS2-KS3 全科辅导
16621398022 同微信
查看 WJEC 物理单元页更多信息见 aleveler.com
或截图后用「扫一扫 → 相册」选择本图