📚 Year 8 WJEC Physics: Formula & Theorem Quick-Reference Guide | 8年级WJEC物理：公式定理速查手册
This handbook brings together the essential formulas, laws and principles you will meet in Year 8 WJEC Physics. Use it to check definitions, practise calculations and build confidence with the key ideas that underpin your course.
本手册汇集了你在8年级WJEC物理课程中将遇到的基本公式、定律和原理。用它来查阅定义、练习计算并建立对核心概念的信心。
1. Speed, Distance and Time | 速度、距离与时间
To find the average speed of an object, divide the total distance travelled by the total time taken. If the speed changes, you can still work with average values.
计算物体的平均速度时，用总距离除以总时间。即使速度发生变化，你仍然可以使用平均量来处理。
v = s / t
Where v is average speed (metres per second, m/s), s is distance travelled (metres, m) and t is time taken (seconds, s).
式中 v 为平均速度（米/秒，m/s），s 为所行距离（米，m），t 为所用时间（秒，s）。
Remember: when an object moves at a steady speed in a straight line, its instantaneous speed equals its average speed. The gradient of a distance–time graph also tells you the speed.
记住：当物体沿直线匀速运动时，它的瞬时速度等于平均速度。距离-时间图线的斜率也能告诉你速度的大小。
mmon units include m/s, km/h and mph. You can convert between them using 1 m/s = 3.6 km/h.
常见的单位有m/s、km/h和mph。你可以利用 1 m/s = 3.6 km/h 在它们之间进行换算。
2. Density | 密度
Density describes how much mass is packed into a given volume. Materials with tightly packed particles have a high density.
密度描述的是单位体积内含有多少质量。粒子堆积紧密的材料具有较高的密度。
ρ = m / V
Here ρ (Greek letter ‘rho’) is density (kg/m³ or g/cm³), m is mass (kg or g) and V is volume (m³ or cm³).
式中 ρ（希腊字母“rho”）为密度（kg/m³ 或 g/cm³），m 为质量（kg 或 g），V 为体积（m³ 或 cm³）。
The density of water is 1 g/cm³ (or 1000 kg/m³). An object floats if its density is less than the density of the fluid it is placed in.
水的密度是 1 g/cm³（即 1000 kg/m³）。若物体的密度小于其所浸入流体的密度，它就会漂浮。
When you are given mass in grams and volume in cm³, density naturally comes out in g/cm³. For kg/m³, use mass in kg and volume in m³.
当质量的单位是克、体积的单位是立方厘米时，密度自然以 g/cm³ 表示。要得到 kg/m³，需要使用以千克为单位的质量和以立方米为单位的体积。
3. Work and Energy | 功与能量
Work is done when a force moves an object in the direction of the force. Energy is transferred mechanically during this process.
当力使物体沿力的方向移动时，力就做了功。在此过程中能量通过机械方式发生了转移。
W = F × d
W is work done (joules, J), F is the force applied (newtons, N) and d is the distance moved in the direction of the force (metres, m).
W 为做功大小（焦耳，J），F 为施加的力（牛顿，N），d 为沿力方向移动的距离（米，m）。
One joule is the work done when a force of 1 N moves an object 1 m. Lifting a book from the floor onto a table requires you to do work against gravity.
1 J 相当于 1 N 的力使物体移动 1 m 所做的功。把书从地板上拿起放到桌上，你需要克服重力做功。
Work and energy are both measured in joules. Whenever work is done, energy is transferred from one store to another.
功和能量都以焦耳为单位。无论何时做功，能量都会从一个储存体转移到另一个储存体。
4. Gravitational Potential Energy and Kinetic Energy | 重力势能与动能
Objects lifted against gravity store gravitational potential energy (GPE). The faster an object moves, the more kinetic energy (KE) it has.
物体被举高克服重力时会储存重力势能（GPE）。物体运动越快，它所具有的动能（KE）就越大。
Eₚ = m g h
Eₚ is the change in gravitational potential energy (J), m is mass (kg), g is gravitational field strength (≈ 10 N/kg on Earth) and h is the change in height (m).
Eₚ 为重力势能的变化量（J），m 为质量（kg），g 为重力场强度（地球表面约为 10 N/kg），h 为高度变化（m）。
Eₖ = ½ m v²
Eₖ is kinetic energy (J), m is mass (kg) and v is speed (m/s). Notice that KE depends on the square of the speed.
Eₖ 为动能（J），m 为质量（kg），v 为速度（m/s）。注意动能与速度的平方成正比。
In many situations, such as a falling object (ignoring air resistance), GPE is converted into KE. The total energy remains constant according to the principle of conservation of energy.
在很多情况下（如忽略空气阻力时下落的物体），重力势能会转化成动能。根据能量守恒定律，总能量保持不变。
5. Pressure | 压强
Pressure is the force acting per unit area. A sharp knife cuts easily because the force is concentrated over a small area, producing a high pressure.
压强是作用在单位面积上的力。锋利的刀容易切割，是因为力集中在很小的面积上，产生了很大的压强。
P = F / A
P is pressure (pascals, Pa or N/m²), F is force (N) and A is the area over which the force acts (m²).
P 为压强（帕斯卡，Pa或N/m²），F 为力（N），A 为力作用的面积（m²）。
uces a greater pressure on the ground than standing on two feet, because the contact area is smaller.
1 Pa = 1 N/m²。单脚站立对地面产生的压强大于双脚站立，因为接触面积更小。
equation may appear in extension work), but the core idea remains: force spread over area.
在流体中，压强随深度增加，公式为 P = hρg（这个公式可能在拓展内容中出现），但核心概念始终是：力分布在面积上。
6. Moments | 力矩
A moment is the turning effect of a force. It explains how levers, seesaws and spanners work.
力矩是力的转动效应，它解释了杠杆、跷跷板和扳手的工作原理。
M = F × d
M is the moment (newton-metres, N m), F is the force (N) and d is the perpendicular distance from the pivot to the line of action of the force (m).
M 为力矩（牛顿·米，N m），F 为力（N），d 为支点到力作用线的垂直距离（m）。
If an object is balanced, the total clockwise moment equals the total anticlockwise moment. This is the principle of moments.
若物体处于平衡状态，总顺时针力矩等于总逆时针力矩。这就是力矩平衡原理。
You can increase the moment by applying a larger force or by using a longer lever, which is why a longer spanner makes it easier to undo a tight nut.
你可以通过施加更大的力或使用更长的杠杆来增大力矩，这就是为什么长扳手更容易拧松紧固的螺母。
7. Electric Current, Voltage and Resistance | 电流、电压与电阻
Electric current is the flow of electric charge. Voltage provides the ‘push’ that drives the current, while resistance opposes the flow.
电流是电荷的流动。电压提供驱动电流的“推力”，而电阻则阻碍流动。
I = Q / t
I is current (amperes, A), Q is electric charge (coulombs, C) and t is time (s). This shows that current is the rate of flow of charge.
I 为电流（安培，A），Q 为电荷（库仑，C），t 为时间（s）。这表明电流是电荷流动的速率。
V = I R
Ohm’s law states that the voltage V (volts, V) across a conductor is equal to the product of the current I (A) and the resistance R (ohms, Ω), provided temperature remains constant.
欧姆定律指出，在温度保持不变的情况下，导体两端的电压 V（伏特，V）等于电流 I（A）与电阻 R（欧姆，Ω）的乘积。
A resistor that obeys Ohm’s law gives a straight-line graph of current against voltage. The steeper the slope, the lower the resistance.
遵守欧姆定律的电阻器，其电流-电压图线为一条直线。斜率越大，电阻越小。
8. Series and Parallel Circuits | 串联与并联电路
Components can be joined in different ways, and the behaviour of current and voltage follows simple patterns.
电路元件可以按不同方式连接，而电流和电压的行为遵循简单的规律。
Series circuits: The current is the same at all points. The total voltage supplied by the battery is shared across the components. Adding more bulbs increases the total resistance, so the current falls.
串联电路：电路中各处的电流相等。电池提供的总电压被分配到各元件上。增加更多灯泡会使总电阻增大，因此电流减小。
Parallel circuits: The current from the supply splits at junctions and rejoins later. Each branch receives the full supply voltage. Adding more branches reduces the overall resistance, increasing the total current drawn from the battery.
并联电路：从电源流出的电流在节点处分开，之后再汇合。每条支路都获得完整的电源电压。增加更多支路会降低总电阻，使从电池流出的总电流增大。
These patterns help you predict how circuits behave and are central to understanding electrical safety and design.
这些规律能帮助你预测电路的行为，也是理解电气安全和设计的核心。
9. Wave Speed, Frequency and Wavelength | 波速、频率与波长
All waves—whether sound waves in air, water ripples or light—share common mathematical relationships.
所有波——无论是空气中的声波、水波还是光波——都具有共同的数学关系。
v = f λ
v is wave speed (m/s), f is frequency (hertz, Hz) and λ (lambda) is wavelength (m). Frequency is the number of complete waves passing a point per second.
v 为波速（m/s），f 为频率（赫兹，Hz），λ（lambda）为波长（m）。频率是指每秒通过某点的完整波的数目。
If the frequency of a wave increases (shorter period), the wavelength must decrease if the wave speed stays the same. This applies to waves on a string, sound waves and light travelling through a medium.
如果波的频率增大（周期变短），而波速保持不变，则波长必然减小。这一定量关系适用于弦上的波、声波以及在介质中传播的光。
nd time: v = s / t, but v = f λ is more useful when you know the frequency and wavelength.
波速也可以通过距离和时间求得：v = s / t，但当已知频率和波长时，v = f λ 更为常用。
10. Energy Efficiency and Conservation | 能量效率与守恒
Energy cannot be created or destroyed, only transferred, stored or dissipated. This is the law of conservation of energy.
能量不能被创造或消灭，只能被转移、储存或耗散。这就是能量守恒定律。
Efficiency = (Useful energy output / Total energy input) × 100%
Efficiency is often expressed as a percentage. A machine that wastes a lot of energy as heat or sound has a low efficiency.
效率通常以百分比来表示。将大量能量以热或声音形式浪费掉的机器效率较低。
The useful energy output is the energy that does the job you intend; the total energy input is the energy supplied. The difference is dissipated into the surroundings, often as thermal energy.
有用输出能量是完成目标任务的能量；总输入能量是所供给的能量。两者之差通常以热能的形式耗散到周围环境中。
Reducing unwanted energy transfers, for example by using lubrication or insulating materials, increases efficiency and reduces running costs.
减少不必要的能量转移（例如通过使用润滑剂或保温材料）可以提高效率并降低运行成本。
Both the GPE and KE equations from Section 4 are linked to the conservation of energy: in a closed system, total energy remains constant, even as it changes form.
第4节中的重力势能和动能公式都与能量守恒相关：在一个孤立系统中，即使能量形式发生变化，总能量仍保持不变。
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