4.2 Thermal physics - Particle model and gasses

Density (ρ) is a measure of how much "stuff" (mass) is packed into a certain space (volume). It is usually measured in kg/m³ or g/cm³.

Density = Mass / Volume

ρ = m / V

Where:

• ρ (rho) is density in kilograms per cubic metre (kg/m³)
• m is mass in kilograms (kg)
• V is volume in cubic metres (m³)

In general, solids are denser than liquids, and liquids are denser than gases. A key exception is water, which is one of the few substances where the solid form (ice) is less dense than its liquid form.

Gas particles are far apart and move randomly and at high speeds.

Gas pressure is caused by the particles colliding with the walls of their container. Each collision exerts a tiny force, and billions of these collisions add up to create the overall pressure.

Pressure = Force / Area

P = F / A
P = pressure in Pascals (Pa)
F = force (N)
A = Area (m²)

If you heat a gas (at constant volume), the particles gain kinetic energy and move faster. This means they hit the walls harder and more often, so the pressure increases.

If you compress a gas (at constant temperature) by making the volume smaller, the particles will hit the walls more often, so the pressure increases.

A fluid is any substance that can flow (a liquid or a gas).

The pressure in a fluid (like water) increases with depth. This is because the weight of the fluid column above pushes down.

Pressure = Height of Column × Density of Fluid × Gravitational Field Strength

P = h × ρ × g

This pressure difference with depth is what causes upthrust. Because the pressure at the bottom of a submerged object is greater than the pressure at the top, there is a net upward force on the object.

Archimedes' Principle states that the upthrust on an object is equal to the weight of the fluid it displaces.

• If an object's weight is greater than the upthrust, it sinks.
• If an object's weight is less than the upthrust, it floats.

Atmospheric pressure is the pressure caused by the weight of the air in the atmosphere. Pressure is highest at sea level and decreases as you go up in altitude, as there is less air above you.