14.2.3Particle Model

Pressure and Volume

5 min read 5 parts Lesson 14 of 15 Unit quiz

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Section 1 of 5Gas Pressure

A gas in a sealed syringe is a simple system for studying how pressure depends on volume.

Cause

A gas exerts pressure because its particles collide with the container walls and each collision applies a small force.

Many collisions

There are billions of collisions every second, so the overall effect is a steady pressure on the walls.

Direction

The force acts at right angles to the surface, and the pressure is the same in all directions in the gas.

Equation

Pressure = force ÷ area, so more force on each square metre of wall means a higher pressure.

Fixed conditions

Here the mass of gas and its temperature are held constant, so only the volume and pressure change.

The effect can be explained step by step with the particle model.

Push the plunger of a sealed syringe to reduce the volume of the gas.

The same number of particles now moves in a smaller space.

Particles travel a shorter distance between collisions with the walls, so they hit the walls more often.

The force per unit area of the wall increases, so the pressure of the gas increases.

Pulling the plunger out gives the opposite result: the volume increases and the pressure decreases.

Relationship

Inverse relationship

When volume goes down, pressure goes up, so pressure and volume are inversely related at constant temperature.

Doubling

If the volume is halved, the pressure is doubled; if the volume is doubled, the pressure is halved.

Same speed

The particles move at the same average speed because the temperature has not changed, so the change is due to collision rate only.

Not directly proportional

A graph of pressure against volume is a curve, not a straight line through the origin.

Volume	Particle behaviour	Pressure
Smaller	More frequent collisions	Higher
Larger	Less frequent collisions	Lower

Slow compression

Pushing slowly lets the gas stay at room temperature, so the temperature stays constant during the investigation.

Sealed system

The gas must not leak, otherwise the mass of gas changes and the results are unreliable.

Real uses

Bicycle pumps, scuba tanks and car tyres all rely on increasing pressure by squeezing gas into a smaller volume.

Gas pressure The force per unit area exerted by gas particles colliding with a surface.
Volume The space taken up by the gas, equal to the volume of its container.
Inversely proportional A relationship where one quantity falls by the same factor as the other rises.
Collision rate The number of collisions with the walls per second.

Saying that the particles get faster when the gas is compressed slowly;
- the temperature, and so the speed, is constant.
Claiming that the particles get bigger or more numerous in a smaller volume.
Describing the pressure change as being due to harder collisions rather than more frequent ones.
Using direct proportion instead of inverse proportion for pressure and volume.