Density for OCR GCSE Physics
This page covers the following topics:
4. Particle model and density
The amount of matter that is in an object is measured by its mass. Mass is measured in grams (g) and kilograms (kg). Experimentally, mass is measured using scales.
Volume is the amount of three-dimensional space an object occupies and is measured in m³, cm³ or mm³. The volume of regular objects can be calculated using the following formula: Volume = length × height × width. The volume of an irregular object can be found using a measuring cylinder. The measuring cylinder is filled with water and the volume of the water is recorded. The object is then placed in the measuring cylinder and the new volume is recorded. The volume of the object is given by the difference between the two recorded volumes.
Density is the mass per unit volume of an object and is measured in kg/m³. Density can be calculated using the following formula: Density = mass/volume. The density of water is approximately 1000 kg/m³. Any object that sinks in water has a density greater than that of water. When liquids that don't mix together are placed in the same container, the denser liquid will sink to the bottom, while the least dense one will lie on top of it.
Different states of matter have differently arranged particles which move in different ways. The density of a substance will change with changing state due to the fact that the particles will be rearranged, causing them to occupy a different volume. When a material melts from a solid to a liquid, the particles will remain close together with only a small change in their arrangement, therefore there is a small change in the density of the material. When a material evaporates from a liquid to a gas, the particles will move far apart from each other, therefore there is a big change in density.
Find the volume of a cube of side 5 cm.
Volume = 5 cm × 5 cm × 5 cm = 125 cm³.
State the piece of equipment needed to experimentally find the mass of an object.
The mass of an object can be experimentally found using scales.
A cake recipe calls for 550 g of flour. State the mass of flour that is needed for the cake in kg.
Mass = 550/1000 kg = 0.55 kg.
A stone of mass 80 g is placed in a measuring cylinder with water of volume 40 cm³. Given that the volume rises to 80 cm³ when the stone is placed in the measuring cylinder, calculate the density of the stone.
Volume = 80 cm³ − 40 cm³ = 40 cm³ = 0.00004 m³.
Mass = 80 g = 0.08 kg.
Using the formula for density, density = 0.08 kg/0.00004 m³ = 2000 kg/m³.
Use the given diagram to explain why the density of a material in the gas state will be lower than in the liquid state.
In the gas state, the particles will be far apart from each other, therefore there will be less of them in a given volume, and thus the density decreases in the gas state.
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