Kinetic Theory (Edexcel GCSE Physics: Combined Science)

Motion of Particles in a Gas

• Molecules in a gas are in constant random motion at high speeds
• Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:
  • With the walls of its container
  • With other molecules

• The random motion of tiny particles in a fluid is known as Brownian motion

Random motion of gas molecules in a container

Pressure

• Molecules of gas in a container will collide with the container walls
• Pressure is defined as the force exerted per unit area 

Pressure, force, area formula triangle

• Imagine molecules of gas that are free to move around in a box
• The molecules in the gas move around randomly at high speeds, colliding with surfaces and exerting pressure upon them
• The temperature of a gas is related to the average speed of the molecules:
  • The hotter the gas, the faster the molecules move and vice versa
  • Hence, the molecules collide with the surface of the walls more frequently and with more force
  • This increases the pressure

Gas molecules hit the sides of the container which creates pressure

• The motion of molecules in a gas changes according to the temperature
• As the temperature of a gas increases, the average speed of the molecules also increases
• Since the average kinetic energy depends on their speed, the kinetic energy of the molecules also increases if its volume remains constant
  • The hotter the gas, the higher the average kinetic energy
  • The cooler the gas, the lower the average kinetic energy

• If the gas is heated up, the molecules will travel at a higher speed
  • This means they will collide with the walls more often
  • This creates an increase in pressure

• Therefore, at a constant volume, an increase in temperature increases the pressure of a gas and vice versa
• Diagram A shows molecules in the same volume collide with the walls of the container more with an increase in temperature
• Diagram B shows that since the temperature is proportional to the pressure, the graph against each is a straight line

At constant volume, an increase in the temperature of the gas increases the pressure due to more collisions on the container walls