Temperature and Pressure

• Molecules in 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 equation:

  
• • 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
• 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. A higher temperature increases the average speed of the molecules, increasing the 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:

  
• At constant volume, an increase in the temperature of the gas increases the pressure due to more collisions on the container walls: (part 2)