📚 Year 11 Cambridge Physics: Common Misconceptions and How to Correct Them | 剑桥Year 11物理常见误区与纠正方法
Many Year 11 students find Cambridge Physics challenging not because the concepts are extremely difficult, but because subtle misunderstandings can lead to consistent errors. This article tackles ten of the most frequent misconceptions in mechanics, electricity, waves, thermal physics, and radioactivity, and provides clear, exam-friendly corrections.
许多Year 11学生觉得剑桥物理有挑战性，并非因为概念极其困难，而是细微的误解往往会导致重复犯错。本文针对力学、电学、波、热物理和放射性中最常见的十个误区，提供清晰、适合考试的纠正方法。
1. Mass vs Weight | 质量与重量
A very common mistake is using 'mass' and 'weight' as if they were the same quantity. You will hear students say 'my weight is 50 kilograms' or write the unit of weight as kg.
一个极常见的错误是把“质量”和“重量”当成同一个量。你会听到学生说“我的重量是50千克”，或者用kg作为重量的单位。
Mass is the amount of matter in an object and is measured in kilograms (kg). It does not change with location. Weight is the force of gravity acting on that mass, so it is measured in newtons (N). On Earth the relationship is given by W = m x g, with the gravitational field strength g approximately 9.8 N/kg near the surface. An astronaut has the same mass on the Moon, but her weight is only about one sixth because the Moon's g is smaller.
质量是物体所含物质的多少，单位为千克(kg)，不随位置改变。重量是作用在该质量上的重力，因此单位为牛顿(N)。在地球上两者的关系是W = m x g，地表附近引力场强度 g 约为 9.8 N/kg。宇航员在月球上质量不变，但重量只有约六分之一，因为月球的 g 较小。
W = m x g
In exam answers, always write weight in newtons, and never say an object 'weighs 5 kg'. Remember that a spring balance measures weight, whereas a beam balance compares masses.
在考试答案中，永远用牛顿表示重量，绝不能说某物体“重5公斤”。记住弹簧秤测量的是重量，而天平比较的是质量。
2. Speed vs Velocity | 速率与速度
Many learners treat speed and velocity as identical, forgetting that direction matters. When they calculate 'velocity' for a round trip, they often divide total distance by time and call it average velocity.
许多学习者把速率和速度当成一回事，忘记了方向的重要性。当他们计算往返行程的“速度”时，经常用总路程除以时间，并称之为平均速度。
Speed is a scalar: it tells you how fast something moves, but not where it is going. Velocity is a vector: it specifies both how fast and in which direction. The formula for average speed is total distance / total time, while average velocity is total displacement / total time. Displacement is the straight-line distance in a given direction. If a car travels 400 m around a track and returns to the start, its displacement is zero, so the average velocity is 0 m/s, even though its average speed may have been high.
速率是标量：它告诉你运动的快慢，但不指明方向。速度是矢量：它同时给出快慢和方向。平均速率的公式是总路程/总时间，而平均速度是总位移/总时间。位移是带有方向的直线距离。如果一辆车绕跑道行驶400米并回到起点，位移为零，因此平均速度是0 m/s，尽管平均速率可能很高。
average speed = total distance / total time
average velocity = total displacement / total time
When drawing vector diagrams, always add an arrow to represent velocity; never use 'speed' arrows for vector addition.
画矢量图时，永远用箭头表示速度；不要用“速率”箭头进行矢量相加。
3. You Need a Force to Keep Moving | 需要力才能维持运动
One of the oldest misconceptions in physics is the belief that a constant force is required to maintain constant velocity. Students often think that if you stop pushing a moving object, it will instantly stop because the driving force is gone.
物理学中最古老的误解之一，就是认为维持恒定速度需要一个恒力。学生常以为如果停止推动一个运动的物体，物体就会因为失去驱动力而立即停下。
Newton's First Law states that an object will remain at rest or move with constant velocity unless a resultant force acts on it. If you see constant velocity, the forces are balanced (or no net force is present). Newton's Second Law F = m x a tells us that a resultant force causes acceleration, not steady speed. A cyclist moving at a constant speed on a flat road is experiencing a forward pedalling force exactly balanced by air resistance and friction, giving zero resultant force.
牛顿第一定律指出，除非受到合外力，物体将保持静止或匀速直线运动。如果看到恒定速度，说明力是平衡的（或者没有净力）。牛顿第二定律 F = m x a 告诉我们，合外力产生的是加速度，而不是稳定速度。在平路上匀速骑行的自行车手，向前的蹬踏力正好与空气阻力和摩擦力平衡，合外力为零。
F = m x a
In free fall without air resistance, the only force is weight, so the object accelerates at g. Once terminal velocity is reached, weight equals air resistance; the resultant force is zero and the velocity stops changing.
在无空气阻力的自由落体中，唯一的力是重力，所以物体以加速度 g 下落。一旦达到终极速度，重力等于空气阻力，合外力为零，速度不再变化。
4. Action-Reaction vs Balanced Forces | 作用力与反作用力 vs 平衡力
Students frequently think that the action and reaction forces in Newton's Third Law cancel each other out, so a horse can never pull a cart, or a rocket cannot accelerate in space. This happens because they treat these forces as a pair acting on the same object.
学生常认为牛顿第三定律中的作用力和反作用力会互相抵消，因此马拉不动车，或者火箭在太空中无法加速。这是因为他们把这对力当成作用在同一个物体上。
Newton's Third Law says: if body A exerts a force on body B, then body B exerts an equal and opposite force on body A. These two forces act on different bodies, so they never cancel each other. Balanced forces, however, are forces that act on the same body. For a horse pulling a cart, the horse pulls the cart forward (force on cart), and the cart pulls the horse backward (force on horse). The cart will accelerate if the forward pull on the cart is greater than the friction acting on the cart.
牛顿第三定律说：若物体A对物体B施加一个力，则物体B同时对物体A施加一个等大反向的力。这两个力作用在不同物体上，因此绝不会抵消。平衡力则是作用在同一个物体上的力。马拉车时，马向前拉车（力作用在车上），车向后拉马（力作用在马上）。如果马车受到向前的拉力大于作用在车上的摩擦力，车就会加速。
A classic exam question shows a man pushing against a wall. The man exerts a force on the wall; the wall exerts an equal force on the man. These do not cancel because they act on different objects. The man may accelerate backward if the force from the wall is unbalanced by other forces on him.
一个经典考题是人推墙壁。人对墙施加力，墙对人施加等大的力。它们不抵消，因为作用在不同物体上。如果人受到墙的力未被其他力平衡，人可能向后加速。
5. Current is 'Used Up' in a Circuit | 电流在电路中被“消耗”
It is extremely common to hear students say that current is 'used up' as it passes through a lamp, so the current leaving the lamp must be less than the current entering it.
极其常见的是，学生说电流在经过灯泡时被“用掉”了，因此流出灯泡的电流一定小于流入的电流。
In a series circuit, the current is the same at every point. Charge is conserved: the number of coulombs per second entering a component equals the number leaving it. What is 'used' or transferred is energy, not current. The potential difference (voltage) across the component shows how much energy each coulomb delivers to the component. An ammeter placed before and after a resistor will read exactly the same value.
在串联电路中，各处电流相同。电荷是守恒的：每秒进入元件的库仑数等于每秒离开的库仑数。被“用掉”或转移的是能量，而不是电流。元件两端的电势差（电压）表明每库仑电荷向元件传递了多少能量。放在电阻前和电阻后的电流表会读出完全相同的数值。
I = I₁ = I₂ (in series)
Energy is converted to heat and light in the lamp, and the energy transferred per second (power) can be calculated using P = I x V. The current remains constant; it is the voltage that drops across each component.
能量在灯泡中转化为热和光，每秒转移的能量（功率）可用 P = I x V 计算。电流保持恒定；降落的是各元件两端的电压。
6. Voltage and Current Relationships | 电压与电流的关系混淆
Pupils frequently say 'voltage flows through the bulb' or assume that if you increase the voltage, the current always doubles, regardless of the component. They also confuse the roles of voltage and current in parallel circuits.
学生常说“电压流过灯泡”，或认为无论什么元件，增大电压电流总会翻倍。他们也容易混淆电压和电流在并联电路中的角色。
Voltage (potential difference) is not a substance that flows; it is the energy transferred per unit charge between two points. Current is the flow of charge. Ohm's Law V = I x R applies only if resistance is constant, which is true for a metal wire at constant temperature. In a filament lamp, the resistance increases as the wire heats up, so doubling the voltage does not double the current. In parallel circuits, the voltage across each branch is the same as the source voltage, but the current splits between branches. Saying 'the voltage splits in parallel' is a very serious mistake.
电压（电势差）不是流动的物质，它是两点间每单位电荷转移的能量。电流才是电荷的流动。欧姆定律 V = I x R 仅在电阻恒定时成立，例如温度不变的金属导线。在灯丝灯泡中，随着灯丝变热电阻增大，因此电压翻倍时电流并不会翻倍。在并联电路中，各支路两端的电压与电源电压相同，但电流在各支路间分流。说“电压在并联电路中分配”是一个严重错误。
V = I x R
Always remember that a voltmeter is connected in parallel and measures the energy difference per coulomb between two points. An ammeter is in series and measures the rate of charge flow.
永远记住，电压表并联连接，测量两点间每库仑电荷的能量差。电流表串联连接，测量电荷流动的速率。
7. Temperature vs Thermal Energy | 温度与热能的区别
Many students think that temperature is a measure of how much 'heat' an object contains, or that a hot cup of coffee must contain more thermal energy than a huge iceberg. They also expect the temperature to keep rising when a solid melts.
许多学生认为温度是物体含有多少“热”的量度，或者一杯热咖啡的热能一定比一座大冰山多。他们还认为固体融化时温度会持续上升。
Temperature measures the average kinetic energy of the particles. Thermal energy (internal energy) is the sum of the kinetic and potential energies of all the particles. So a massive iceberg has enormous thermal energy even though its temperature is near 0°C, because it contains a vast number of particles. During melting or boiling, the added energy goes into breaking bonds (potential energy), not increasing kinetic energy, so the temperature stays constant. This is why specific latent heat is needed.
温度衡量粒子平均动能。热能（内能）是所有粒子的动能与势能之总和。因此巨大的冰山虽然温度接近0°C，却拥有巨大的热能，因为它包含了极多的粒子。在融化或沸腾过程中，输入的能量用于破坏粒子间的键（增加势能），而不是增加动能，所以温度保持不变。这就是需要比潜热的原因。
energy = mass x specific latent heat (Q = m x L)
In IGCSE questions, always distinguish between 'temperature' (degree of hotness, in °C or K) and 'heat' or 'thermal energy' (energy transferred, in J). Using the correct vocabulary gains marks.
在IGCSE答题中，永远要区分“温度”（冷热程度，单位为°C或K）和“热量”或“热能”（传递的能量，单位为J）。使用准确词汇能得分。
8. Wave Motion: Do Particles Travel? | 波动：粒子会不会随波移动？
When asked about water waves, many learners imagine that water molecules travel across the ocean from the source to the shore. They draw diagrams with arrows showing the particles moving horizontally with the wave.
当被问到水波时，许多学习者设想水分子从源头随波一起横穿海洋到达岸边。他们画图时用箭头表示粒子随波水平移动。
In both transverse and longitudinal waves, the particles of the medium oscillate about a fixed equilibrium position; they do not travel with the wave. It is energy that is transferred, not matter. A cork floating on water waves bobs up and down (or in a small circle) but does not move forward with the wave. In sound waves, air particles vibrate backwards and forwards in the direction of energy transfer, creating compressions and rarefactions, but they return to their original positions.
在横波和纵波中，介质的粒子都在一个固定的平衡位置附近振动，并不随波移动。传递的是能量，不是物质。浮在水波上的软木塞上下浮动（或做小圆周运动），但不会随波向前移动。声波
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