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The Kinetic Theory of Gases

A fully inflated balloon stays rigid because of billions of invisible impacts happening inside it every second. The Kinetic Theory of Gases models a gas as consisting of small, hard spheres that are constantly moving at high speeds.

Gas particles exist in a state of random motion. This means they travel with a wide distribution of speeds and have no predictable path, undergoing sudden changes in direction whenever they collide with each other or the internal walls of their container.

We can see evidence for this through Brownian motion, where larger, visible particles (like smoke) randomly jerk around in the air. This movement is caused by the constant, unpredictable collisions from invisible, fast-moving gas molecules.

The Mechanism of Gas Pressure

Why does a bicycle tyre feel hard when you pump it up? The answer lies in the mechanical collisions of the gas particles inside.

Every time a gas particle collides with the internal wall of its container, it bounces off and exerts a tiny outward force. Because there are billions of particles colliding every fraction of a second, these individual microscopic forces add up to produce a significant net force.

Crucially, this net force always acts as a force normal to a surface—meaning it pushes outward at exactly a 90-degree angle (perpendicular) to the container walls. Gas pressure is simply the measurement of this total force distributed across the internal surface area of the container.

Calculating Gas Pressure

To calculate the pressure a gas exerts on its container, we use the following equation:

P = \frac{F}{A}

Students frequently lose marks by writing that "particles expand" when a gas is heated. The individual particles (the 'hard spheres') never change size; it is the space between them that increases.

When an exam question asks you to 'Explain' how gases exert pressure for 3 or 4 marks, examiners are looking for a specific four-step sequence: 1) random motion at high speeds, 2) collisions with walls, 3) the exertion of a force during impact, and 4) that the force acts at right angles to the surface.

Always check the units of area in pressure calculations. A classic Edexcel trap is giving the surface area in $\text{cm}^2$ ; you must convert this to $\text{m}^2$ by dividing by 10,000 before substituting it into the $P = F/A$ equation.

For Higher Tier students: be prepared to link gas pressure to momentum. The force exerted on the wall during a collision is equal to the particle's rate of change of momentum ( $F = \Delta p/t$ ) as it reverses direction.

A model describing gas particles as small, hard spheres in constant, rapid, and random motion.

Movement where particles have no predictable path, exhibiting a wide distribution of speeds and sudden changes in direction upon collision.

The random, jerky movement of larger visible particles (like smoke) caused by collisions with invisible, fast-moving fluid molecules.

The overall resultant force produced by the sum of billions of individual particle collisions against a surface.

The component of a force that acts at exactly a 90-degree angle (perpendicular) to a surface.

The total force exerted by gas particles per unit area of the container walls.

The standard unit of pressure, equivalent to a force of 1 Newton acting over an area of 1 square metre ( $1\text{ N/m}^2$ ).