# 3.3.1 Kinetic Theory

### Temperature & Kinetic Energy

• Molecules in a gas are in constant random motion at high speeds
• The temperature of the gas is related to the average kinetic energy of the molecules
• The hotter the gas, the higher the average kinetic energy
• The cooler the gas, the lower the average kinetic energy
• 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

### Temperature & Pressure

• The motion of molecules in a gas changes according to the temperature and pressure of the gas

#### 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

#### Pressure

• Molecules of gas in a container will collide with the container walls
• Pressure is defined as the force exerted per unit area of the container • Therefore a gas at high pressure has more frequent collisions on the container walls and therefore overall more force per unit area
• 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

#### Exam Tip

You are required to be able to describe the links between pressure & volume and pressure & temperature qualitatively. This means that the correct use of terms such as ‘collision’, ‘kinetic energy’ and ‘frequency’, will be really important. ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close Close