Kinetic Theory Unveiled From frantic particles to the calm laws they obey.

Atomic Hypothesis

Atoms in Perpetual Motion

All matter is made of tiny particles called atoms. These atoms move continuously, attract when slightly apart, and repel when forced together.

19th-century chemist John Dalton first used this idea to explain gas laws. Quiz: choose Dalton, Maxwell, Einstein, or Feynman.

Gas Molecules in Motion

Random motion and collisions build up gas pressure.

Diagram: Molecules travel straight, collide elastically, and hit walls to create pressure.

What the image shows

Straight-line motion is interrupted by collisions; wall hits transfer momentum as tiny pushes.

  • Molecular motion is random but constant.
  • Collisions are elastic—speed changes, energy conserved.
  • Each wall collision exerts force; many together give pressure \(P\).
  • Pressure formula: \(P = \frac{F}{A}\).

Tip: Hotter gas → faster molecules → more frequent, harder hits → higher pressure.

Ideal Gas Equation

\[PV = \mu R T = k_B N T\]

Variable Definitions

\(P\) Pressure of gas
\(V\) Volume of gas
\(\mu\) Amount of gas in moles
\(R\) Universal gas constant
\(T\) Absolute temperature (K)
\(k_B\) Boltzmann constant
\(N\) Number of molecules

Applications

Predict Gas Behaviour

Find new pressure or volume when a tyre heats up.

Cylinder Storage Design

Calculate volume needed to store industrial gases safely.

Micro-Macro Link

Relate molecular count \(N\) to macroscopic \(P\) and \(V\).

Boyle’s Law Snapshot

Pressure × Volume = constant (temperature fixed)

Gas pressure vs volume illustration
Diagram: Compressing gas halves volume and doubles pressure.

What the image shows

Keeping temperature steady, volume shrinks and pressure rises in exact inverse proportion.

  • Temperature \(T\) remains constant.
  • Volume ↓ ⇒ molecules hit walls more often.
  • Law: \(P \propto \frac{1}{V}\) or \(PV = k\).

Tip: Double the volume and pressure halves—an easy way to spot Boyle’s Law in action.

Multiple Choice Question

Question

This concept check assesses your grasp of kinetic theory basics.
According to the kinetic theory of gases, the temperature of a gas measures the ____ of its molecules.

1
Potential energy of molecules
2
Average kinetic energy of molecules
3
Total molecular mass
4
Volume occupied by molecules

Hint:

Think about how fast the particles move when temperature increases.

Kinetic Theory — Key Takeaways

Six bullet reminders of the theory.

1

Particle model

Matter consists of tiny, widely spaced particles.

2

Constant motion

Particles move randomly and collide elastically.

3

Temperature link

Average kinetic energy is proportional to \(T\): \(E_k=\tfrac{3}{2}k_B T\).

4

Pressure origin

Gas pressure results from particle impacts on container walls.

5

Absolute zero

At 0 K (−273 °C) particle motion would theoretically stop.

6

Gas laws

Boyle’s & Charles’s laws emerge naturally from the model.