﻿ Exploring Fluid Pressure
By Jeffrey Hao-Chan Huang

# Exploring the Properties of Fluid Pressure

### Motivation

As secondary school students progress through the physics curriculum as prescribed by the British Columbia’s Ministry of Education, they are led through a series of activities and exercises designed to assist them in developing a solid understanding of the concept of force, how forces dictate motion as described by Newton’s Second Law, and the relationship between forces, work, and energy.

In contrast, the concept of pressure is often hastily presented to students as the “force per unit area”. Although this mathematical definition is a logical progression from the concept of force, it fails to demonstrate to students the fundamental role played by pressure in naturally-occurring phenomena and how the properties of pressure can be harnessed in man-made systems. As a result, students may develop the misconception that pressure is simply a product of mathematical convenience instead of a fundamental property of thermodynamics, whose laws govern all natural phenomena and are used extensively in diverse areas of engineering.

This series of demonstrations has been designed to reinforce the concept that pressure is a fundamental entity of nature while providing students with intuitive visual representations of its basic properties.

# A Series of Demonstrations

### Intuitive Visual Representations of the Fundamental Properties of Fluid Pressure

#### Learning Goals

This demonstration is designed to provide students with a simple visualization of the forces acting on solid surfaces as a result of atmospheric pressure.

Through in-class discussions, students will be led to make the following observations:

• Pressure is present at every point in the atmosphere.
• Atmospheric pressure exerts a force on solid surfaces. The direction of this force is normal to the solid surface.

#### Brainstorming Activity

This demonstration begins by the instructor asking students to brainstorm among themselves for possible solutions to the following problem:

Given a balloon, a plastic bottle, and a pair of scissors, is there a way to inflate the balloon by breathing air into your lungs instead of blowing air out of them?

#### Learning Goals

Over the course of this demonstration, students will be guided to make the following observations:

• The pressure of a gas, specifically an ideal gas, is proportional to the density of the gas.
• Pressure at a point in a fluid will exert a force normal to any surface at that point.

### Materials:

• A balloon
• A plastic bottle
• A pair of scissors/An X-Acto precision knife
• Rubbing alcohol
• Cotton balls

### Setup:

1. Using the pair of scissors or the X-Acto prevision knife, carefully and cleanly cut a small hole on the side wall of the plastic bottle, preferably as far away from the bottle's top opening as possible. Ensure that the hole does not have any sharp or jagged edges.

2. Sanitize the hole you have cut using cotton balls and rubbing alcohol.

3. Insert the balloon into the bottle through it's top opening, then secure the balloon by wrapping it around the lip of the bottle.

#### Learning Goals

This demonstration was designed to reinforce the concepts presented to students in the previous demonstration in a captivating and visual way.
In addition, students will be introduced to the relationship between the pressure of a gas, specifically an ideal gas, and its temperature.
Over the course of this demonstration, students will see clearly that:

• The pressure of a gas, specifically an ideal gas, is proportional to both the density of the gas as well as its temperature.
• Pressure at a point in a fluid will exert a force normal to any surface at that point.

### Materials:

• A balloon
• A transparent glass bottle with strong and thick walls
• A hot-plate
• A pair of heat-resistant gloves

### Setup:

1. Fill the glass bottle with a small amount of water.
For the approxiately 150mL bottle shown in the demonstration videos below, roughly 15mL of water was added in this step.
The amount of water can be increased proportionally for larger bottles.
2. Place the glass bottle onto a hot plate, then heat up the water until it begins to boil.
The hot steam produced by this process will rise, pushing air out of the bottle.
3. While wearing a pair of heat-resistant gloves, remove the glass bottle from the hot plate.
4. Keeping the heat-resistant gloves on, secure the balloon to the bottle's opening by wrapping it around the lip of the bottle.
5. Wait for the bottle to cool to room-temperature. As this occurs, the balloon will be pushed into the bottle and expands.

#### Demonstration - Video

1. Putting the glass bottle on the hot plate. 2. Removing the glass bottle from the hot plate.

As the hot plate heats up the glass bottle, the liquid water within the glass bottle begins to evaporate, turning into water vapor. Therefore, the density of gases within the bottle increases, increasing air pressure. In addition, as the hot plate supplies heat to the bottle, the temperature within the bottle increases as well, further increasing air pressure.

As the pressure difference between the inside of the bottle and the outside of the bottle increases, the balloon is gradually pushed out of the bottle and begins to expand.

After the glass bottle has been removed from the hot plate, the temperature within the bottle begins to decrease, causing the air pressure within the bottle to decrease. In addition, the water vapor begins to condense back into the liquid state. As a result, the density of gases within the bottle decreases, further decreasing air pressure.

As the pressure difference between the inside of the bottle and the outside of the bottle decreases, the balloon begins to contract and is gradually pushed back into the bottle. As the pressure difference approaches zero, the balloon returns to its original state.

#### Learning Goals

This demonstration shows students in a simple and intuitive manner the following properties of pressure in a liquid:

• For a liquid at rest, the pressure along a horizontal plane parallel to the surface of the liquid is constant.
• The magnitude of this pressure is proportional to the plane's vertical distance from the surface.

### Materials

• 2 plastic containers of different shapes and sizes
• A box of thumbtacks
• Food-colouring

### Setup:

#### Learning Goals

This demonstration provides a clear visualization of the principles of hydrostatic equilibrium. Over the course of this demonstration, students will discover:

• For a liquid in a container to be at rest and in hydrostatic equilibrium, the fluid pressure along a horizontal plane parallel to the surface of the liquid must be constant. If the system has been disturbed such that the pressure along this plane is no longer uniform, the liquid will flow until the pressure is constant again and hydrostatic equilibrium is re-established.

### Materials:

• 2 transparent glass bottles
• Plastic tubing of sufficient length to connect liquid in one glass bottle to the liquid in the other glass bottle
• Food-colouring
• A tub of water

#### Demonstration - Video

Model Hydralic Lift
This series of demonstrations have been presented to a local class of Science 10 students as an introduction to the physics unit of the course. Significant effort has been made to spark the students' curiosity by showing them that the properties of fluid pressure in the atmosphere and in water, two fluids which they are very familiar with, will result in suprising and sometimes dramatic phenomena occurring in simple demonstration setups. In addition, the presentation has been designed to give students an intuitive understanding of the relationship between pressure and forces.

The PowerPoint slides used to direct this presentation, and the worksheet handed out to students to guide in-class discussions are available to be viewed, downloaded, and modified for your specific needs.