Density (AQA GCSE Physics): Revision Notes
Density
What is density?
Density tells us how much matter is packed into a space. It measures the amount of stuff in each unit of volume.
Think of it like this - a sponge and a brick might be the same size, but the brick has much more matter packed into that space. The brick is more dense.
How density, mass and volume work together
Density depends on two things:
- Mass - how much matter something contains
- Volume - how much space it takes up
When you change the amount of material:
- Both mass and volume change
- But the density stays the same for that material
The density of a material doesn't change much when it's in the same state. This is because the particles stay roughly the same distance apart.
Density in different states of matter
The way particles are arranged affects density:
- Solids - particles are packed closely together, so solids have high density
- Liquids - particles are less tightly packed than solids, so liquids usually have lower density
- Gases - particles are spread far apart, so gases have very low density
The spacing between particles is the key factor that determines density in different states of matter.
Calculating density
You can work out density using this equation:
Or using symbols:
Where:
- (rho) = density
- = mass
- = volume
Units for density
The most common units are:
- kg/m³ (kilogrammes per cubic metre)
- g/cm³ (grammes per cubic centimetre)
Always check your units match! If mass is in kg and volume is in m³, then density will be in kg/m³.
Simple examples
Worked Example 1: Finding Density
A material has a mass of 84 kg and volume of 0.075 m³
Step 1: Use the density formula
Step 2: Substitute the values
Worked Example 2: Finding Mass
Aluminium has a density of 2700 kg/m³. A block has volume 0.05 m³
Step 1: Rearrange the formula to find mass
Step 2: Substitute the values
Key facts about density
- Changing the volume of something changes its mass, but density stays constant
- Different materials have different densities
- Copper has a density of 8.96 g/cm³ - this means every 1 cm³ of copper has a mass of 8.96 g
Understanding that density is a characteristic property of materials helps explain why some objects float while others sink.
Remember!
Key Points to Remember:
- Density measures how much matter is packed into a space
- Formula: (density = mass ÷ volume)
- Common units: kg/m³ or g/cm³
- Solids are most dense, gases are least dense
- Always check your units match in calculations