- 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
Kinetic Theory (Edexcel GCSE Physics)
Revision Note
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AshikaExpertise
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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
- 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
Exam Tip
You are required to be able to describe the link between temperature and pressure qualitatively. This means that the correct use of terms such as 'collision', 'kinetic energy' and 'frequency', will be really important.
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