Edexcel IGCSE Physics · Spec 5.10-5.12
Gas Pressure & Temperature
How the pressure of a gas changes with temperature and volume.
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Gas Pressure & Temperature
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Explained
Where gas pressure comes from
A gas has no shape of its own, yet it pushes outwards on every surface of its container. The particle model explains why, and the explanation is worth building carefully because it is marked in separate steps.
The chain of reasoning
Gas molecules are in continuous motion, travelling in all directions and at a range of speeds.
Each molecule that reaches a wall collides with it and bounces off. Its momentum changes direction, and a change of momentum means a force has acted, so the molecule exerts an equal force on the wall.
An enormous number of these collisions happen every second, spread over the whole area of the wall. Pressure is force divided by area, so the total force from all those collisions divided by the area is the gas pressure.
Three steps, three marks: collisions with the walls, a force produced, and force over an area giving pressure. Answers that stop at the collisions have done a third of the work.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks what is meant by the random motion of gas particles, for two marks. It credits the idea that the particles move in different or unpredictable directions, and the idea that they move at different or unpredictable speeds.
Then it instructs the examiner to ignore random directions and to ignore random speeds. The word random is in the question, so repeating it explains nothing. You have to unpack what it means, and the two things it means are the directions and the speeds.
On the next part, about how the particles produce pressure, the mark scheme credits the idea that a force is produced on the walls, and that pressure is force on an area. It instructs the examiner to ignore pushing on walls. Push is not a physics word here; force is.
An earlier paper is more generous on vocabulary, allowing atoms or molecules for particles and allowing bounce or hit for collide. So the vocabulary of the collision is flexible; the vocabulary of the physics is not.
Temperature and particle energy
Temperature is a measure of the average kinetic energy of the particles. Heat a gas and the particles move faster.
Faster particles affect the pressure twice over. They reach the walls more often, so there are more collisions each second, and each collision involves a greater change of momentum, so each exerts a greater force. Both effects push the pressure up.
The kelvin scale and absolute zero
If you keep cooling a gas, the particles move more and more slowly. Absolute zero is the temperature at which they have the minimum possible kinetic energy and their motion is as slow as it can be. Nothing can be colder.
That point is minus 273 degrees Celsius, and it is zero on the kelvin scale. The two scales rise in equal steps, so you convert by adding 273 to a Celsius temperature or subtracting 273 to go the other way. The unit is written K, with no degree symbol.
The reason the scale exists is that the average kinetic energy of the particles is directly proportional to the absolute temperature. Double the kelvin temperature and you double the average kinetic energy. That statement is simply untrue in degrees Celsius, because the Celsius scale starts at an arbitrary point rather than at zero energy.
Volume at constant temperature
Squeezing a gas into a smaller space raises the pressure without changing the temperature at all.
The particles are moving at the same average speed as before, and each impact is no harder. They simply have less distance to travel between one wall and the next, so they arrive more often. More collisions per second on each unit of area is the whole of the pressure increase, and saying that the particles move faster describes heating rather than compression.
Spec 5.10-5.12
What you need to know
- Explain gas pressure using particles
- Link temperature to particle energy
- Understand the Kelvin scale and absolute zero
Active recall
Quick check
Answer each question before opening the answer.
Explain gas pressure using the particle model.
Gas particles collide with the container walls; the force of these collisions per unit area is the pressure.
Why does gas pressure increase with temperature at constant volume?
The particles move faster and hit the walls harder and more often.
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