15.8. Shadow of a flame 2

Visualization of heat flow

Author:	Freek Pols
Time:	5 minutes
Age group:	13-15y
Concepts:	Density, heat, convection

Introduction

This is a demonstration that must be seen by everyone because it showcases such beautiful, quickly overlooked physics. Depending on the purpose you choose you can emphasize different aspects.

Fig. 15.22 The density current is visible

Equipment

• Tea lights

• Dark room

• Strong light source (e.g. phone flashlight)

• Ice cream

Preparation

None

Procedure

Start this demo with the rotating spiral (https://www.nvon.nl/leswerk/proevenboek-51a-de-draaiende-spiraal) and let the students explain what is happening. You can also feel the heat flow yourself. Just keep your hand at various places, always the same distance from the flame. You will naturally notice where you feel the heat the most. Then, make the flow visible by darkening the room and holding a bright lamp close to the flame, projecting the flow onto the wall.

Provide an explanation for why the flow is visible.

It’s nice to attach an ice cream to a stand. It also creates an airflow that you can feel by holding your hand close

What direction does the flow have?

Fig. 15.23 The shadow of a flame

Physics background

The flame heats the air. The density of the warm air is now lower than that of the surrounding, causing it to rise. Due to the density difference in the air, light passes through different media and undergoes refraction. This can be seen as ‘the shadow of the flame’. The density differences and fluctuations in the flow are made visible here. Initially, there is laminar flow, but it becomes turbulent at a certain point. Gently wave your hand and you’ll see the laminar flow immediately become turbulent.