Aug 2020

Kinetic Model Of Matter: Brownian Motion And Gas Pressure

August 13, 2020

Kinetic Model of Matter

Take a moment to feel the mouse or phone in your hand right now. It may not feel like it is moving, but the molecules inside of it are constantly vibrating. The same goes for all matter in the universe, from the smallest speck of dust to enormous galaxies.

This is defined in the Kinetic Model of Matter, which states that all matter is made of tiny particles, such as atoms, molecules, and ions, which are in continuous and random motion. In each state of matter, i.e. solid, liquid, and gas, the particles behave differently. However, all of them share this defining trait: the particles are always moving.

Internal energy

Let us first discuss how this constant movement relates to the temperature of a system. To comprehend this relationship, we must first understand what ‘internal energy’ is. Internal energy refers to the total energy possessed by particles in a system. It is a combination of the kinetic energy (energy possessed due to its movement) and potential energy (energy possessed by virtue of its position or state) of the particles.

Consider a solid. According to the Kinetic Model of Matter, solids contain particles held together by very strong bonds, which vibrate about their fixed positions. As the internal energy of particles in a solid increase, so too does the temperature. This is because a higher internal energy suggests that the particles have more kinetic energy, and are vibrating more vigorously. This would then lead to an increase in temperature. Potential energy, on the other hand, is related to the space between the particles. When potential energy increases, the particles become spaced further apart.

Brownian motion

Brownian motion refers to the observed continuous and random motion of particles in a fluid, which includes liquids and gases. This occurs due to each particle being bombarded by the other particles in motion throughout the fluid.

The term is named after Robert Brown. He was a botanist who, when observing pollen grains under a microscope, found that the pollen would move randomly through the water. It was later determined by none other than the famous physicist Albert Einstein, that the pollen grains were being moved around by the individual water molecules.

You can see this effect for yourself using the smoke chamber experiment. Also known as Brownian Motion Apparatus, this setup consists of a metal chamber, with an inlet and a rubber bulb on opposite sides of the chamber. There is also a glass viewing window on the top of the chamber, and a lens on its side.

Smoke from a match or burning rope is drawn through the inlet by squeezing the rubber bulb. A laser light can then be shone into the chamber through the lens, concentrating the light in the chamber’s centre. When the chamber is observed under a microscope through the viewing window, the smoke particles will appear as bright spots that are moving around randomly.

This movement is due to the air molecules colliding with the smoke particles. Due to the air molecules colliding unevenly with the smoke particles in different directions, the movement appears to be irregular.

Properties of gas

Due to the random motion of particles in gases, as described by the Kinetic Model of Matter, gas molecules collide with the walls of any container they are kept in. These numerous collisions result in an overall force being applied on the walls. The average force applied per unit area is referred to as the gas pressure. Here are some important relationships connecting the pressure, volume, and temperature of gases.

  • Pressure is directly proportional to temperature (when volume is constant): As temperature increases, the kinetic energy of gas particles increases. This causes them to collide with the walls with a greater force and more frequently.
  • Pressure is inversely proportional to volume (when temperature is constant): When the volume of gas increases, the number of particles per unit volume decreases. This results in a lower frequency of collision and a decrease in pressure.
  • Volume is directly proportional to temperature (when pressure is constant): As temperature increases, gases expand. This results in fewer particles per unit volume.


The Kinetic Model of Matter gives several insights into the concepts in the field of thermodynamics. As such, it is imperative that you have a strong understanding of this topic as you continue on your physics journey. However, if you need any assistance, you can consider joining a quality physics tuition class. Here at Best Tuition Physics, our classes are taught by a competent and experienced physics teacher. Let us help you excel in your O Level and A Level physics.

WhatsApp chat