📚 Year 8 WJEC Science: Formula & Theorem Quick Reference Guide | 八年级 WJEC 科学公式定理速查手册
Welcome to your Year 8 WJEC Science formula and theorem quick reference guide. This handbook collects the essential equations, principles and laws you will meet in physics, chemistry and biology. Use it to check your recall before tests and to support your homework.
欢迎使用八年级 WJEC 科学公式定理速查手册。这本手册汇集了你在物理、化学和生物中将遇到的核心公式、原理与定律。用它可以在考试前检验记忆，也能辅助你完成家庭作业。
1. Speed, Distance and Time | 速度、距离与时间
Average speed tells you how fast an object is moving. It links the distance covered to the time taken.
平均速度告诉你物体运动的快慢，它将行驶的距离与所用的时间联系起来。
You can calculate speed using:
你可以用下式计算速度：
speed = distance ÷ time
In symbols, v = d / t, where v is speed, d is distance and t is time. The standard unit of speed is metres per second (m/s).
用符号表示为 v = d / t，其中 v 代表速度，d 代表距离，t 代表时间。速度的标准单位是米每秒(m/s)。
Example: A cyclist travels 300 m in 20 s. Her speed is 300 ÷ 20 = 15 m/s.
示例：一名自行车手在20秒内行驶300米，她的速度为300 ÷ 20 = 15 m/s。
nd distance (d = v × t) or time (t = d / v) when the other two quantities are known.
变形公式可在已知另外两个量时求距离(d = v × t)或时间(t = d / v)。
2. Density | 密度
Density describes how much mass is packed into a certain volume. It helps to identify materials and predict whether they will float.
密度描述了单位体积内所含的质量，可用来鉴别物质并预测其沉浮。
The formula for density is:
密度公式为：
density = mass ÷ volume
Often written with the Greek letter rho: ρ = m / V. Mass is measured in kilograms (kg) or grams (g), volume in cubic metres (m³) or cubic centimetres (cm³). The unit of density is kg/m³ or g/cm³.
常用希腊字母 ρ 表示：ρ = m / V。质量以千克(kg)或克(g)为单位，体积以立方米(m³)或立方厘米(cm³)为单位。密度单位是 kg/m³ 或 g/cm³。
mass of 100 g and a volume of 20 cm³. Its density is 100 ÷ 20 = 5 g/cm³;.
示例：一块金属的质量为100 g，体积为20 cm³，其密度为100 ÷ 20 = 5 g/cm³;。
Water has a density of about 1 g/cm³. Objects with density less than water float; those with greater density sink.
水的密度约为1 g/cm³。密度小于水的物体会浮起，密度大于水的物体会下沉。
3. Pressure | 压强
Pressure is a measure of how concentrated a force is on a surface. It increases if the force is larger or if the area is smaller.
压强衡量力在表面上的集中程度。力越大或受力面积越小，压强就越大。
The pressure equation is:
压强公式为：
pressure = force ÷ area
In symbols: P = F / A quare metres (m²). The unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m²;.
符号公式：P = F / A。力以牛顿(N)为单位，面积以平方米(m²)为单位。压强的单位是帕斯卡(Pa)，1 Pa = 1 N/m²;。
A force of 50 N acts on an area of 0.5 m². The pressure is 50 ÷ 0.5 = 100 Pa.
示例：一个50 N的力作用在0.5 m²的面积上，压强为50 ÷ 0.5 = 100 Pa。
Sharp objects like knives have small contact areas, producing high pressure and making them effective at cutting.
像刀子这样锋利的物体接触面积小，能产生高压强，因此切割很有效。
4. Moments | 力矩
A moment is the turning effect of a force about a pivot. It depends on both the size of the force and the distance from the pivot.
力矩是力绕支点产生的转动效果，其大小取决于力的大小以及力到支点的距离。
The moment of a force is calculated with:
力矩计算公式为：
moment = force × perpendicular distance from pivot
Symbol equation: M = F × d, where M is the moment in newton metres (Nm), F is force in newtons (N), and d is distance in metres (m).
符号方程：M = F × d，其中 M 为力矩，单位牛顿米(Nm)，F 为力(N)，d 为垂直距离(m)。
used with a force of 20 N applied 0.3 m from the nut. The moment is 20 × 0.3 = 6 Nm.
示例：用扳手拧螺母，在距螺母0.3 m处施加20 N的力，力矩为20 × 0.3 = 6 Nm。
For an object to be balanced, the total clockwise moments must equal the total anticlockwise moments about the pivot.
要使物体平衡，绕支点的顺时针力矩之和必须等于逆时针力矩之和。
5. Work Done | 做功
Work is done whenever a force moves an object in the direction of the force. The amount of work tells you how much energy has been transferred.
每当一个力使物体沿力的方向移动时，就做了功。功的多少代表转移了多少能量。
Use this formula to find work done:
计算做功的公式为：
work done = force × distance moved in the direction of the force
In symbols: W = F × d. Work is measured in joules (J), force in newtons (N) and distance in metres (m). 1 joule is the work done when a force of 1 N moves an object 1 m.
符号公式：W = F × d。功的单位是焦耳(J)，力的单位是牛顿(N)，距离的单位是米(m)。1焦耳就是1 N的力使物体移动1 m所做的功。
a box with a force of 30 N over a distance of 4 m. The work done is 30 × 4 = 120 J.
示例：一个人用30 N的力推动箱子移动了4 m，做功为30 × 4 = 120 J。
6. Efficiency and Energy Conservation | 效率与能量守恒
The law of conservation of energy states that energy cannot be created or destroyed, only transferred, stored or dissipated. This is true in all processes.
能量守恒定律指出，能量既不能被创造也不能被消灭，只能被转移、储存或耗散。这在所有过程中都成立。
When energy is transferred in a device, some is always converted into less useful forms (often heat). Efficiency describes how good a device is at converting input energy into useful output.
当能量在设备中转移时，总有一部分会转化为不太有用的形式（通常是热）。效率描述了一个设备将输入能量转换为有用输出能量的能力。
The efficiency formula is:
效率公式为：
iciency = (useful output energy transfer ÷ total input energy transfer) × 100%
Efficiency can also be written as a decimal between 0 and 1, but multiplying by 100 gives a percentage. A perfect device would have 100% efficiency, but in reality some energy is always wasted.
效率也可写成0到1之间的小数，但乘以100可得到百分比。完美的设备效率为100%，但实际上总有一些能量被浪费。
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