Kinetic Particle Model of Matter
Section: Thermal Physics | Syllabus: Cambridge IGCSE Physics (0625)
Particle Arrangement, Separation and Motion
- Solids
- arrangement: particles are arranged in a regular, fixed pattern
- separation: particles are close together, touching their neighbours
- motion: particles vibrate about a fixed position but cannot move from place to place
- Liquids
- arrangement: particles are arranged randomly, with no fixed pattern
- separation: particles are close together, about as close as in a solid
- motion: particles move around each other, in random directions, at moderate speed
- Gases
- arrangement: particles are arranged randomly
- separation: particles are far apart, with large spaces between them
- motion: particles move quickly in random directions, colliding with each other and with the walls of their container
Simple particle diagrams for a solid, a liquid and a gas
Forces, Separation and Properties
Extended Only
- the forces of attraction between particles, their separation, and their motion together determine the properties of a solid, liquid or gas
- in a solid, strong forces hold closely-spaced particles in fixed positions, giving a fixed shape and fixed volume
- in a liquid, weaker forces allow the closely-spaced particles to move around each other, giving a fixed volume but no fixed shape
- in a gas, forces between the widely-spaced particles are very weak, so they move independently and fill their container, giving no fixed shape and no fixed volume
Particle Motion and Temperature
- the higher the temperature of a substance, the greater the average kinetic energy of its particles, and the faster they move
- the lower the temperature, the less kinetic energy the particles have, and the slower they move
- there is a lowest possible temperature, called absolute zero, at which particles have the least possible kinetic energy
- absolute zero = -273\,^
Gas Pressure
- gas particles are in constant, random motion, travelling in straight lines until they collide with another particle or with the walls of their container
- each time a particle collides with a surface, it exerts a tiny force on that surface
- billions of collisions happen every second - the combined effect of all these collisions on the container walls is the pressure of the gas
Gas particles colliding with the walls of a sealed container produce gas pressure
Pressure as Force per Unit Area
Extended Only
- gas pressure can be described in terms of the forces exerted by particles colliding with a surface, creating a force per unit area
- p = (F/A)
Brownian Motion
- the random motion of microscopic particles suspended in a liquid or gas is evidence for the kinetic particle model of matter
- this random, jerky movement is called Brownian motion
Explaining Brownian Motion
- a classic example: pollen grains suspended in water are observed under a microscope to follow a random, zig-zag path
- the pollen grains are too large to be moved by a single collision, but they are constantly bombarded from all directions by the much smaller, faster-moving molecules of the water
- these collisions are unequal from moment to moment, so the pollen grains are knocked first one way, then another, producing the random zig-zag motion observed
- since the pollen grains cannot move on their own, their random motion is caused by random collisions with the invisible, moving water molecules - this is evidence that the molecules of a liquid or gas are in constant, random motion
A visible particle follows a random zig-zag path due to collisions with fast-moving molecules
Correct Use of Terms
Extended Only
- the term microscopic particles is used for the larger particles whose random motion is observed (e.g. pollen grains, smoke particles)
- the terms atoms or molecules should be used for the much smaller, lighter, fast-moving particles of the surrounding liquid or gas that collide with and move the microscopic particles
Gas Pressure, Temperature and Volume
For a fixed mass of gas, the pressure changes if the temperature or the volume changes.
Effect of Temperature at Constant Volume
- heating a fixed mass of gas at constant volume:
- the particles gain kinetic energy and move faster
- the particles collide with the walls of the container more frequently
- the particles also collide with the walls with greater force
- the pressure of the gas increases
- cooling a fixed mass of gas at constant volume has the opposite effect: the particles move slower, and collide with the walls less frequently and with less force, so the pressure decreases
Effect of Volume at Constant Temperature
- decreasing the volume of a fixed mass of gas at constant temperature (compressing it):
- the particles still move at the same average speed, since the temperature has not changed
- the particles are confined to a smaller space, so they reach the walls of the container more often
- the more frequent collisions increase the pressure of the gas
- increasing the volume at constant temperature has the opposite effect: collisions with the walls become less frequent, so the pressure decreases
| Change | Particle speed | Collision frequency | Pressure |
|---|---|---|---|
| Increase temperature (constant volume) | Increases | Increases | Increases |
| Decrease temperature (constant volume) | Decreases | Decreases | Decreases |
| Decrease volume (constant temperature) | No change | Increases | Increases |
| Increase volume (constant temperature) | No change | Decreases | Decreases |
The Kelvin Temperature Scale
- the Kelvin scale is a temperature scale that starts at absolute zero (0 K)
- a change of 1 K is equal to a change of 1°C, so the two scales are related by:
- Formula:
- T = θ + 273
- Where:
- T = temperature in kelvin (K)
- θ = temperature in degrees Celsius (°C)
- Formula:
- key reference points:
- 0 K = -273\,^ (absolute zero)
- 273 K = 0°C (water freezes)
- 373 K = 100°C (water boils)
The Celsius and Kelvin temperature scales
Worked Example
- Muumbe measures the temperature of a beaker of water as 27°C. What is this temperature in kelvin?
Muumbe's beaker of water at 27°C
- T = θ + 273 = 27 + 273 = 300 K
The Pressure-Volume Relationship (Boyle's Law)
Extended Only
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