Kinetic Theory (Edexcel GCSE Physics)

The Pressure of a Gas

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 is measured in the units Pascals (Pa)

• The area should always be the cross-sectional area of the object

  • This means the area where the force is at right angles to it

• This equation can be rearranged with the help of a formula triangle:

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

Temperature & 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