2.1.4P2. Thermal Physics
Pressure Changes — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 13 of 14 Unit quiz
Part 1 of 5Background: what gas pressure is
Collisions with the walls
- Collisions with the walls The particles of a gas move randomly at high speed.
- Whenever one hits the wall of its container and rebounds, it exerts a tiny force on the wall.
- Pressure is the total force of all these collisions divided by the area of wall they act on.
Fixed mass of gas
- Fixed mass of gas In every case here the amount of gas is fixed, so no particles enter or leave the container.
- Only temperature, volume and pressure can change.
Temperature and particle speed
- Temperature and particle speed The temperature of a gas is linked to the average kinetic energy of its particles.
- Heating the gas makes the particles move faster on average, and cooling it makes them slower.
Part 2 of 5Pressure and volume at constant temperature (Extended tier)
Constant temperature, fixed mass
Compressing the gas
- Compressing the gas Reducing the volume squeezes the same particles into a smaller space.
- They are closer to the walls, so they hit them more often, which creates a bigger overall force and a higher pressure.
Expanding the gas
- Expanding the gas Increasing the volume gives the particles further to travel between collisions,
- so each part of the wall is hit less often and the pressure falls.
Average speed unchanged
- Average speed unchanged Because the temperature has not changed, the particles still move at the same average speed.
- The change in pressure comes only from how frequently collisions happen.
Pressure changing volume
- Pressure changing volume The effect works in reverse too.
- If the pressure outside a sealed, flexible container drops,
- such as a tied balloon placed under a vacuum pump, the gas inside pushes outwards and the balloon expands.
Part 3 of 5Pressure and temperature at constant volume (Extended tier)
1Temperature rises
- Temperature rises The container is rigid, so the volume cannot change, but the particles gain kinetic energy.
2Faster particles
- Faster particles The average speed of the particles increases, so they cross the container more quickly.
3More frequent collisions
- More frequent collisions Each part of the wall is hit more often every second.
4Harder collisions
- Harder collisions Faster particles change their momentum by more when they rebound, so each impact exerts a larger force.
5Pressure rises
- Pressure rises A greater total force on the same area means a higher pressure.
- Cooling the gas causes the opposite chain of events.
Part 4 of 5Summary of the gas relationships
| Condition kept constant | Change made | Effect on pressure | Reason |
|---|---|---|---|
| Temperature | Volume decreased | Increases | More frequent collisions with the walls |
| Temperature | Volume increased | Decreases | Less frequent collisions with the walls |
| Volume | Temperature increased | Increases | Faster particles hit the walls more often and harder |
| Volume | Temperature decreased | Decreases | Slower particles hit the walls less often and more gently |
Part 5 of 5Reading the graph
Pressure against temperature
- Pressure against temperature When the pressure of a gas at constant volume is plotted against its temperature in kelvin, the points lie on a straight line through the origin,
- so pressure is directly proportional to temperature.
- On a Celsius scale the line is still straight and rising, but it does not pass through the origin.
Describing the trend
- Describing the trend A good answer states the relationship and then explains it with particles, for example: 'as temperature increases, the particles move faster, collide with the walls more often and with greater force,
- so pressure increases'.
Key terms
| Term | Meaning |
|---|---|
| Pressure | Force acting per unit area; for a gas it is caused by particles colliding with the container walls. |
| Fixed mass of gas | A sample where no gas enters or leaves, so the number of particles stays constant. |
| Compression | Reducing the volume of a gas, which usually raises its pressure if the temperature is kept the same. |
| Collision frequency | The number of collisions per second between the particles and a given area of wall. |
| Directly proportional | Two quantities that increase together in the same ratio, giving a straight line through the origin when plotted. |
| Kelvin | The absolute temperature scale, where 0 K is the lowest possible temperature. |
Avoid these mistakes
1
Saying that compressing a gas makes the particles move faster. At constant temperature their average speed stays the same; only collision frequency changes.
2
Stating that the particles 'expand' or 'get bigger' when heated. The particles stay the same size; they move faster.
3
Writing 'more collisions' without adding 'with the walls per second'. Pressure depends on collisions with the container, not with other particles.
4
Forgetting that heating at constant volume increases both the frequency and the force of collisions, and mentioning only one.
5
Quoting a direct proportion for pressure and Celsius temperature. The proportional link only holds when temperature is in kelvin.
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