3.3 Particle model and pressure Flashcards
Solids
A solid has a fixed shape because of the strong forces between its particles.
Solids cannot be compressed because their particles are already very close together and cannot flow.
The particles in a solid are arranged in an ordered pattern.
The particles in a solid move around a fixed point.
Solids are the densest state of matter.
The particles are packed tightly together.
Liquids
Liquids cannot be compressed because their particles are already very close together.
In a liquid, the particles are in contact with one another, but they can still move. This allows a liquid to flow and take the shape of its container.
Particles in a liquid are arranged in a disordered pattern.
Liquids are less dense than solids but denser than gases.
The particles in liquids can move around each other.
Gases
Gases can be compressed because their particles are very far apart.
When water evaporates to become steam (gas), its volume increases by 1000x.
The particles in a gas are free to move in any direction. Because of this, a gas can flow, has no fixed shape and completely fills its container.
The particles in a gas move randomly and are not organised in any way.
Gases are the least dense state of matter.
The particles are free to move with negligible forces between particles.
Gas pressure
A gas exerts pressure on the walls of its container.
There are lots of gas particles colliding with the container each second.
When a gas particle collides with the wall of its container, its momentum changes and it bounces back off the wall.
This exerts a force on both the particle and the wall.
The pressure exerted on the wall is equal to the force per unit area (of the wall being hit).
Boyle’s law
For a fixed mass of gas at a constant temperature, pressure x volume = constant.
pV = constant or p_1 V_1 = p_2 V_2
This means that gas pressure is inversely proportional to volume if temperature remains constant.
As volume increases pressure decreases as the number of air particles is the same in a higher volume leading to less frequent collisions with the walls of the container so the force decreases.
Charle’s law
Charles’ Law states that the volume of a gas is directly proportional to its absolute temperature when pressure is kept constant. Mathematically, V/T = k, where V is volume, T is temperature, and k is a constant.
As temperature increases the particles have more kinetic energy leading to more frequent collisions so a greater force so as temperature increases pressure also increases.
Temperature increase in a gas
The pressure of a gas produces a net force (the sum of all the forces) at right angles to the wall of a container. As you increase the temperature of a gas, its particles get more kinetic energy and move faster.
If we increase the temperature of the gas in a fixed volume, then the particles will collide with (hit) the container walls with more force.
Therefore the pressure will increase.
Work done on a gas
If a force compresses a gas, work has been done on the gas (there is a transfer of energy to the gas).
when a gas is compressed its temperature increases as the gas molecules gain KE. for example when air is compressed into a bike tyre from a bike pump it becomes hotter.
when a gas expands its temperature decreases. for example the gas from a deodorant can feels cold.
Pressure in a column of liquid
As you move deeper in a liquid, the pressure increases due to the greater weight of liquid above you. the increase in pressure caused due to a column of liquid can be calculated with the equation:
Pressure = density x gravitational field strength x height
p = rho x gh
Atmospheric pressure
Air molecules colliding with a surface creates pressure.
As the altitude increases the distance between molecules increases so the frequency of collisions with a surface decreases.
Therefore as altitude increases the pressure decreases.
Compressed gas
As the gas is compressed the volume of gas decreases.
This leads to more frequent collisions of gas particles with the container walls and each particle collision with the wall causes a force so there is a greater force on the walls.
Therefore when gas is compressed the pressure increases.
Inflating a tyre
When a tyre is inflated work is done on the air in the tyres so the temperature of the air increases. This causes the particles to have more kinetic energy leading to more frequent collisions on the walls of the container leading to a greater force.
So as temperature increases the pressure inside the tyre also increases.
