📚 IGCSE OCR Physics: Comparison of Key Concepts | IGCSE OCR 物理：知识点对比
In IGCSE OCR Physics, many fundamental ideas come in closely related pairs. Being able to compare and contrast these concepts – from scalars and vectors to series and parallel circuits – is essential for answering exam questions accurately and building a deep understanding of physical principles. This article walks through the key comparisons that appear throughout the OCR specification, giving clear English and Chinese explanations side by side.
在 IGCSE OCR 物理中，许多基本概念都是成对出现的。能清晰地比较和对比这些概念——从标量与矢量到串联与并联电路——是准确解答考题、建立深刻物理理解的关键。本文逐一解析 OCR 考试大纲中反复出现的关键对比，以配对的中英文解释帮助你巩固知识。
1. Scalar vs Vector | 标量与矢量
A scalar quantity is fully described by its magnitude (size) alone. The direction does not matter. Common examples in physics include distance, speed, mass, time and temperature.
标量只需用大小就可以完全描述，与方向无关。物理中常见的标量有路程、速率、质量、时间和温度。
A vector quantity has both magnitude and direction. Without specifying the direction, the description is incomplete. Examples are displacement, velocity, acceleration, force and momentum.
矢量既有大小又有方向。如果不指明方向，描述就不完整。常见的矢量有位移、速度、加速度、力和动量。
When adding scalars, ordinary arithmetic is used. Two masses of 3 kg and 4 kg simply add to 7 kg. Vectors, however, must be added using geometry or the tip‑to‑tail method because their directions affect the resultant.
标量相加使用普通算术。3 kg 和 4 kg 的质量相加就是 7 kg。但矢量相加必须用几何方法或三角形法则，因为方向会影响合量的大小和方向。
2. Distance vs Displacement | 路程与位移
Distance is the total length of the path travelled by an object. It is a scalar quantity and cannot be negative. In a long‑jump run‑up followed by a foul, an athlete may cover a large distance.
路程是物体运动轨迹的总长度。它是标量，不能为负值。例如跳远运动员助跑后犯规，仍可能跑过很长的路程。
Displacement is the straight‑line separation between the start and end points, together with its direction. It is a vector. If a runner completes one full lap on a 400 m track, the distance is 400 m but the displacement is zero.
位移是起点到终点的直线距离及其方向，是矢量。如果一名跑者在 400 m 跑道上跑完完整一圈，路程为 400 m，但位移为零。
In a graph of motion, distance can only increase with time, whereas displacement can increase, decrease or become negative depending on the direction taken.
在运动图像中，路程只会随时间增加，而位移可以增大、减小甚至变为负值，取决于选取的正方向。
3. Speed vs Velocity | 速率与速度
Speed is the rate of change of distance. It is a scalar, calculated as speed = distance ÷ time. The speedometer of a car displays instantaneous speed, with no information about direction.
速率是路程对时间的变化率，为标量，计算公式为速率 = 路程 ÷ 时间。汽车的速度表显示瞬时速率，不包含方向信息。
Velocity is the rate of change of displacement. It is a vector, given by velocity = displacement ÷ time. A car driving around a roundabout at constant speed still has a changing velocity because its direction changes.
速度是位移对时间的变化率，为矢量，计算公式为速度 = 位移 ÷ 时间。汽车以恒定速率绕转盘行驶时，速度仍在变化，因为方向在改变。
Average speed and average velocity for a journey can be very different. An object that returns to its starting point has an average velocity of zero, but its average speed is positive.
一趟行程的平均速率和平均速度可能完全不同。物体若回到起点，平均速度为零，但平均速率为正数。
4. Mass vs Weight | 质量与重量
Mass measures the amount of matter in an object. It is a scalar, measured in kilograms (kg) with a balance, and it stays the same everywhere in the universe.
质量衡量物体所含物质的多少，是标量，用天平测量，单位为千克 (kg)。无论在宇宙何处，质量保持不变。
Weight is the gravitational force acting on a mass. It is a vector that points toward the centre of the planet, measured in newtons (N) using a spring balance or force meter.
重量是作用在物体上的重力，是矢量，方向指向地心，用弹簧秤或测力计测量，单位为牛顿 (N)。
The relationship between weight and mass is:
W = m × g
质量与重量之间的关系是：
W = m × g
where g is the gravitational field strength. On Earth g ≈ 9.8 N/kg, so a 1 kg mass has a weight of about 9.8 N. On the Moon g is about 1.6 N/kg, so the mass stays 1 kg but the weight drops to about 1.6 N.
其中 g 为引力场强度。地球表面 g ≈ 9.8 N/kg，所以 1 kg 物体的重量约为 9.8 N。月球上 g ≈ 1.6 N/kg，质量仍为 1 kg，但重量仅约 1.6 N。
5. Series vs Parallel Circuits | 串联与并联电路
In a series circuit, components are connected one after another in a single loop. The current is the same at every point. If one lamp fails, the circuit is broken and all lamps go out.
在串联电路中，元件首尾相连形成单一回路。电流处处相等，若一盏灯损坏断路，所有灯都会熄灭。
In a parallel circuit, components are connected on separate branches. The current from the source splits at a junction, with the sum of the branch currents equalling the main current. If one branch breaks, the other branches still work.
在并联电路中，元件连接在不同的支路上。干线电流在节点处分流，各支路电流之和等于干路电流。一条支路断开时，其他支路仍可工作。
For potential difference: in series, the supply voltage is shared across components. In parallel, each branch gets the full supply voltage.
对于电压：串联电路中，电源电压被各元件分担。并联电路中，每条支路两端的电压等于电源电压。
Resistance behaviour also differs. For two resistors in series:
R = R₁ + R₂
电阻的表现也不同。两个电阻串联时：
R = R₁ + R₂
For two resistors in parallel:
1/R = 1/R₁ + 1/R₂
两个电阻并联时：
1/R = 1/R₁ + 1/R₂
6. Direct Current vs Alternating Current | 直流电与交流电
Direct current (DC) flows in one fixed direction. The voltage across a DC source is constant. Batteries and solar cells produce DC, making it the standard for portable electronics.
直流电 (DC) 沿单一方向流动，其电压保持恒定。电池和太阳能电池产生直流电，因此便携式电子设备普遍使用直流。
Alternating current (AC) periodically reverses its direction and changes its magnitude. In the UK, mains electricity supplies AC at 230 V with a frequency of 50 Hz, meaning the current changes direction 100 times per second.
交流电 (AC) 会周期性地改变方向和大小。英国市电为 230 V、50 Hz 的交流电，意味着电流每秒改变方向 100 次。
On an oscilloscope trace, DC appears as a straight horizontal line, while AC shows a wave pattern, typically a sine wave. AC can be transformed to different voltages using a transformer, which is crucial for efficient long‑distance power transmission.
在示波器图形中，直流显示为一条水平直线，交流则显示为波形，通常是正弦波。交流电可以通过变压器改变电压，这对长距离输电的高效性至关重要。
7. Transverse vs Longitudinal Waves | 横波与纵波
In a transverse wave, the oscillations are perpendicular to the direction of energy transfer. Examples include light, water ripples and all electromagnetic waves. Transverse waves can travel through a vacuum.
横波的振动方向与能量传播方向垂直。例子有光、水波和所有电磁波。横波可以在真空中传播。
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. They consist of compressions and rarefactions. Sound waves in air and seismic P‑waves are longitudinal. Longitudinal waves require a medium.
纵波的振动方向与能量传播方向平行，由疏部和密部组成。空气中的声波和地震 P 波是纵波。纵波需要介质才能传播。
The wavelength and frequency of both wave types follow the wave equation:
v = f × λ
两种波的波长和频率都遵循波动方程：
v = f × λ
However, particle motion is completely different: transverse waves have crests and troughs, while longitudinal waves show regions of high and low pressure.
但质点运动完全不同：横波有波峰和波谷，纵波则表现为高压区和低压区交替出现。
8. Conduction vs Convection vs Radiation | 热传导、对流与辐射
Conduction is the transfer of thermal energy through a solid (or between touching objects) without any bulk movement of the material. It occurs mainly by lattice vibrations and free‑electron diffusion. Metals are excellent conductors; non‑metals and gases are insulators.
热传导是热量通过固体（或相互接触的物体）传递，材料本身不发生整体移动。主要靠晶格振动和自由电子扩散传热。金属是优良导体，非金属和气体是绝缘体。
Convection occurs in fluids (liquids and gases). Warmer, less dense fluid rises, while cooler, denser fluid sinks, creating a convection current. Convection is responsible for sea breezes, central‑heating circulation and mantle movement.
对流发生在流体（液体和气体）中。温度较高、密度较小的流体上升，温度较低、密度较大的流体下沉，形成对流。海陆风、暖气循环和地幔运动都由对流引起。
Radiation transfers energy by electromagnetic waves, mostly infrared. It does not require any particles and can travel through a vacuum. All objects emit and absorb thermal radiation; hot, dark, matt surfaces are the best emitters and absorbers, while shiny, light surfaces are poor at both.
辐射通过电磁波（主要是红外线）传递能量。它不需要介质，可以在真空中传播。所有物体都会发射和吸收热辐射；温度高、颜色深、表面粗糙的物体是最好的辐射体和吸收体，而光亮浅色的表面则相反。
9. Reflection vs Refraction | 反射与折射
Reflection occurs when a wave bounces off a surface. The law of reflection states that the angle of incidence equals the angle of reflection (∠i = ∠r), measured from the normal. The incident ray, reflected ray and normal all lie in the same plane.
反射是波在界面上弹回的现象。反射定律指出，入射角等于反射角 (∠i = ∠r)，均从法线量起。入射线、反射线和法线在同一平面内。
Refraction happens when a wave passes from one medium into another at an angle, causing a change in speed and direction. If light enters a denser medium (for example from air to glass), it slows down and bends toward the normal.
折射是波倾斜地穿过不同介质时因波速变化而改变方向的现象。光从空气进入玻璃等更密介质时，速度减小，光线偏向法线。
Snell’s law links the angles of incidence and refraction with the refractive index n:
n = sin i / sin r
斯涅尔定律将入射角和折射角与折射率 n 联系起来：
n = sin i / sin r
Total internal reflection can occur when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle.
当光从光密介质射向光疏介质，且入射角大于临界角时，会发生全内反射。
10. Nuclear Fission vs Nuclear Fusion | 核裂变与核聚变
Nuclear fission is the splitting of a large, unstable nucleus (such as uranium‑235 or plutonium‑239) into two smaller daughter nuclei after absorbing a neutron. This process releases a huge amount of energy along with two or three more neutrons, which can trigger a chain reaction.
核裂变是指一个大的不稳定核（如铀‑235 或钚‑239）吸收一个中子后分裂为两个较小的子核。该过程释放巨大能量，并放出两到三个中子，从而可能引发链式反应。
Nuclear fusion is the joining together of two light nuclei (for example hydrogen isotopes deuterium and tritium) to form a heavier nucleus (such as helium). Fusion releases even more energy than fission per unit mass, but it requires extremely high temperature and pressure to overcome electrostatic repulsion.
核聚变是两个轻核（例如氢的同位素氘和氚）结合成一个较重核（如氦）的过程。每单位质量聚变释放的能量比裂变还大，但需要极高的温度和压力来克服静电排斥力。
Fission is used in nuclear power stations with controlled chain reactions, while the main challenge for fusion power is sustaining the extreme conditions long enough to produce a net energy gain. Fusion is the energy source of stars, including the Sun.
裂变已用于核电站，通过控制链式反应实现。聚变发电的主要挑战在于长时间维持极端条件以获取净能量增益。聚变是恒星（包括太阳）的能量来源。
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