CORE Physics (9223) | Kinetic theory

Welcome to the World of Kinetic Theory!

Hello there! If you’ve ever wondered why ice melts, why a balloon shrinks when it gets cold, or why smells travel across a room, you are about to find out! This chapter, Kinetic Theory, is one of the most fundamental ideas in physics.

It might sound complicated, but Kinetic Theory simply explains that all matter is made of tiny, constantly moving particles. Understanding this movement helps us explain temperature, pressure, and the states of matter. Let’s dive in and make these microscopic movements easy to understand!

Quick Review: The States of Matter

Before we look at the movement, let's quickly remember how particles are arranged in the three common states.

• Solids: Particles are held tightly together in fixed positions. They vibrate around these positions but cannot move past each other.
• Liquids: Particles are close together but are not in fixed positions. They can slide past one another.
• Gases: Particles are far apart and move randomly and rapidly in all directions. They frequently collide with each other and the walls of their container.

Section 1: The Core Idea of Kinetic Theory

What is Kinetic Theory?

The Kinetic Theory (sometimes called the Kinetic Model) is a way of describing matter based on the idea that the particles within it (atoms or molecules) are always in motion.

The word "Kinetic" means "relating to motion." Therefore, the kinetic theory is essentially the theory of moving particles.

Key Assumptions of the Kinetic Model (Gases)

When we talk about the movement of gas particles, we assume a few things:

1. Particles are constantly moving quickly and randomly.
2. Collisions between particles and the walls of the container are perfectly elastic (meaning no kinetic energy is lost during the collision, only transferred).
3. The volume of the particles themselves is negligible compared to the volume of the container.

Section 2: Evidence for Moving Particles – Brownian Motion

What is Brownian Motion?

If particles are invisible, how do we know they are constantly moving? The evidence comes from observing Brownian Motion.

Brownian Motion is the random, erratic (unpredictable) motion of visible particles suspended in a fluid (liquid or gas), caused by collisions with the much smaller, invisible particles of the fluid.

Visualising Brownian Motion (The Smoke Cell Experiment)

Imagine looking at tiny specs of smoke dust floating in the air under a microscope.

1. You see the larger smoke particles (which are visible) jittering and moving randomly.
2. The cause of this movement is the incredibly fast, invisible gas molecules (like Nitrogen and Oxygen) colliding randomly and unevenly with the smoke particles.
3. Because the collisions happen more strongly on one side than the other at any given moment, the smoke particle is constantly shoved in random directions.

Analogy: Think of a giant beach ball floating in a swimming pool. Invisible, tiny tennis balls (the gas particles) are constantly being thrown at the beach ball from all sides. If more tennis balls hit the left side at once, the beach ball lurches to the right! This random lurching is Brownian Motion.

Why is this important? Brownian Motion proves the core idea of Kinetic Theory: the particles making up the fluid (air/gas) are constantly moving randomly and possess kinetic energy.

Section 3: Temperature and Kinetic Energy

This is the most crucial part of the Kinetic Theory chapter! The connection between how hot something is and how fast its particles move is fundamental.

Temperature is a Measure of Kinetic Energy

In physics, temperature is not just how "hot" something feels; it is a direct measure of the average kinetic energy of the particles in the substance.

• High Temperature: The particles have high average kinetic energy; they are moving very fast.
• Low Temperature: The particles have low average kinetic energy; they are moving slowly (or vibrating slowly).

Internal Energy

When discussing the energy stored inside a substance, we talk about Internal Energy.

Internal Energy is the total energy stored by the particles within a system. It has two parts:

1. Kinetic Energy (E_k): The energy due to the movement/vibration of the particles. (This is related to temperature.)
2. Potential Energy (E_p): The energy stored due to the forces between the particles (their position relative to each other). (This is related to the state/phase, e.g., solid vs. liquid.)

Focus on Temperature: When the state of matter is not changing (e.g., heating up water but not boiling it), adding heat increases the kinetic energy, which increases the temperature.

Common Mistake to Avoid

Do not say "All particles in a hot substance are moving fast." While the average speed is high, particles move randomly. At any given moment, some particles will be moving faster than average, and some will be moving slower than average. Temperature measures the average speed/energy.

Key Takeaway for Section 3: Temperature tells us the average speed of the particles. Heating a substance increases the speed of its particles.

Section 4: The Kinetic Theory and Gas Pressure

The Kinetic Theory provides a perfect explanation for why gases exert pressure.

What is Pressure?

In simple terms, Pressure (\( P \)) is the amount of Force (\( F \)) applied over a specific Area (\( A \)).

$$ P = \frac{F}{A} $$

How Do Gas Particles Create Pressure? (The Mechanism)

Gas pressure is caused by the constant, rapid collisions of the gas particles with the walls of the container.

Here is the step-by-step process:

1. Gas particles move randomly and rapidly.
2. When a particle hits the wall of the container, it reverses direction (bounces back).
3. When a particle reverses direction, its momentum changes.
4. This change in momentum exerts a small force on the wall (remember Newton's Third Law!).
5. Because there are billions of particles hitting the walls every second, the sum of all these tiny forces creates a constant, overall force pushing outward on the container walls.
6. This total force spread over the area of the walls is the Gas Pressure.

Analogy: Think of rain hitting a metal roof. Each drop is small, but the constant drumming of millions of drops creates an audible, overall effect. Gas particles are like the rain—each collision is tiny, but the continuous stream creates measurable pressure.

Factors Affecting Gas Pressure (Using the Kinetic Model)

If you understand the mechanism of collision, you can easily explain how temperature and volume change pressure.

1. Increasing Temperature (Constant Volume)

If you heat a gas in a sealed container:

Process:

• Increased temperature means particles gain kinetic energy and move faster.
• Faster particles hit the walls harder (greater force per collision).
• They also hit the walls more frequently.
• Result: Pressure increases.

2. Decreasing Volume (Constant Temperature)

If you squash a gas into a smaller container (like pushing down on a bicycle pump):

Process:

• The particles are moving at the same speed (constant temperature).
• The same number of particles now have less space to move in.
• This means they hit the walls more frequently (they travel less distance between collisions).
• Result: Pressure increases.

Chapter Summary: The Kinetic Theory is the foundation of the particle model. It links the movement of invisible particles (shown by Brownian motion) to observable properties like temperature (average kinetic energy) and pressure (caused by particle collisions).

You’ve got this! Keep practicing visualizing those tiny moving particles!