Describe the pressure and the changes in pressure of a gas in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area

What is Pressure?

Pressure is the force that a gas exerts on the walls of its container, divided by the area of those walls.

Mathematically: \$P = \dfrac{F}{A}\$

Think of it like a crowd of people (particles) pushing against a door (surface). The more people pushing, the higher the pressure.

How Particles Create Pressure

Particles move randomly and collide with the walls of the container.

• Each collision transfers a small amount of momentum to the wall.
• The total force on the wall is the sum of all these tiny momentum transfers.
• Because the wall area is fixed, the pressure depends on how often and how hard the particles hit.

🔍 Analogy: Imagine a ping‑pong ball bouncing against a wall. Every bounce pushes the wall a little. If you have many balls bouncing quickly, the wall feels a stronger push.

Changing Pressure

Pressure can change when:

1. Temperature increases – particles move faster, collide more often and harder.
2. Volume decreases – particles are squeezed into a smaller space, so collisions per unit area rise.
3. Number of particles changes – more particles mean more collisions.

📈 Example: A sealed balloon in a hot room expands because the gas inside gets hotter, raising its pressure until the balloon stretches.

Below is a simple table summarising how pressure varies with temperature and volume (ideal gas assumption).

Exam Tips

📝 Remember:

• Use the formula \$P = \dfrac{F}{A}\$ when asked to calculate pressure.
• Explain pressure changes using particle collisions and the ideal gas law where appropriate.
• When describing how temperature or volume affects pressure, state whether the pressure increases or decreases and why.
• Use clear diagrams (drawn by hand) to illustrate particle motion and collisions.

Good luck! 🚀