| Matter is made up of a large number of particles (atoms or molecules) which are in continuous random motion. |
-
- This motion includes vibrations as well as translational motion.
- Collisions occur due to this motion
Brownian Motion
Brownian motion is the constant random motion seen (under a microscope) of very small particles such as smoke or dust particles or grains of pollen.
| Robert Brown (1773 – 1858) |
|---|
| Brownian Motion is named after Robert Brown who in 1827 observed pollen suspended on the surface of water exhibited random, continuous motion.
The explanation for the movement was only given in 1905 by Albert Einstein. |
Why did the particles move continuously? Why was the motion seemingly random?
The explanation was that there were even smaller particles pushing the pollen particles around. These were too small to be seen (even with a microscope). These are of course atoms (and molecules) that make up the water the pollen on.
| Bronian Motion |
|---|
| Smoke particles in air were observed under a microscope to be under constant random motion. What would be the effect on these particles if the temperature of the air was increased?
The smoke particles would still move continuously and in random directions. However, they would move a a faster speed, and change direction more frequently. This can be explained by the faster moving speed of the air molecules. |
Solids, Liquids and Gases
The different properties of solids, liquids and gasses can be explained in terms of particles.
Based on the state of the system (solid, liquid or gaseous states), the arrangement of the particles and the type of motion exhibited by the particles will be different.
Solids
Solids have the highest densities.
Solids have fixed volume and fixed shape.
Arrangement: Particles are closely packed and arranged in a regular pattern. The particles in solids have the least energy among the three states of matter.
Motion: Particles vibrate about fixed positions. They are held in positions by strong forces between the particles.
Liquids
Liquids generally have a slightly lower density than solids.
Liquids have fixed volume but no fixed shape.
Arrangement: Particles are less closely packed than in solids and arranged in an irregular pattern.
Motion: The particles slide over each other throughout the liquid without fixed positions. The forces holding the particles are weaker than in a solid.
Gases
Gases have the lowest densities.
Gases have no fixed volume and no fixed shape. Gases take the volume and shape of their container.
Arrangement: Particles are very far apart from one another in a irregular pattern. The particles in gases have the most energy among the three states.
Motion: The particles can move freely in any direction. The attractive forces between the particles are very weak.
The intermolecular forces between particles also vary with the state.
Solids: Particles and closely packed. Forces between particles are very strong.
Gases: Particles are very far apart. Forces between particles are very low.
- Particles have energies associated with this random motion. This energy is called the internal energy of the system (also see Section 20.4).
- When a system changes state, the spacing (or distance) between the particles will be different. The larger the spacing between the particles, the higher the potential energy of the particles.
- In any given system, there will be a distribution of kinetic energies of all the atoms or molecules.
- Average kinetic energy of the particles is directly proportional to the temperature of the system.
| Using Kinetic Theory to Understand Pressure
The kinetic theory can be used to explain how a gas exerts pressure on the walls of its container:
|
| << Back | Thermal Physics | Next >> |
| Links |
|---|
| https://en.wikipedia.org/wiki/Brownian_motion |
| YouTube video on Brownian Motion (Steve Mould) |