Thermal Insulation

Introduction

Thermodynamics tells us that heat always flows from a hotter area to an adjacent cooler area. In general, different substances will allow heat to conduct through them at different rates. When we heat one side of a body, the energy will transfer through the body to the other side and then dissipate. There will be a temperature gradient through the body.

Where a fire inside a house is heating a window, the temperature on the inside of the window is relatively high when compared to the temperature on the outside of the window. Throughout the wall is a temperature gradient joining the inside temperature to the outside temperature. Heat is dissipated into the air on the outside.

The ease with which heat can transfer through a body is called its thermal conductivity.

This simple experiment allows you to determine how different materials and thicknesses of materials affect heat loss. We are all used to the idea of having heating on indoors during the winter when the outside temperature is low. We commonly use double glazing to make the insulation between the inside and outside of our houses stop as much heat loss as possible. All substances conduct heat at different rates, some are much better insulators than others. In the diagram we can see that the temperature drops from an inside warmer temperature to a colder temperature across the insulating substance.

The objective

To investigate the insulation properties of different substances.

The apparatus

A Bunsen burner, gauze, and stand
A thermometer
A beaker of water with a thick cork lid
A set of ‘wraps’ made of different material that can be placed over the beaker

Put the stand over the Bunsen burner and place the gauze and beaker of water on the stand as shown here.

The variables

In this first part of the experiment different materials will be investigated so, the independent variable is the material , and the temperature and time are the dependent variables.

In the second part of the experiment different thicknesses of paper will be investigated. Here the thickness of the insulation material is the independent variable, and the temperature and time are the dependent variables.

The Physics

In the at the top of the page, a fire inside a house is heating the window. The temperature on the inside of the window is relatively high when compared to the temperature on the outside of the window. Throughout the wall is a temperature gradient joining the inside temperature to the outside temperature. Heat is dissipated into the air on the outside.

The ease with which heat can transfer through a body is called its Thermal Conductivity. In general, the thicker the material, the slower the conduction from hot to cold.

The method - first experiment

Light the Bunsen burner (by clicking on the gas tap in the e-practical). The flame can be adjusted by rotating the air intake on the Bunsen burner. Heat the water in the beaker to 85ᵒC. Turn off the Bunsen burner. Record the temperature again at 5 minute intervals up to 20 mins. Now repeat the experiment for the different available materials (or different materials could be allotted to other groups doing the experiment).

The method – second experiment

Light the Bunsen burner. Select the paper 1mm insulation. Heat the water in the beaker to 85ᵒC. Turn off the Bunsen burner record the temperature. Record the temperature again at 5minute intervals up to 20 mins. Now repeat the experiment for the material cylinders of the other thicknesses (or different thicknesses should be allotted to other groups doing the experiment).

By far the easiest way of doing this is to use a spreadsheet program like Excel, then all the calculations can be done for you each time you change something. The spreadsheet is included with the full package.

The Video

Watch a video of the Specific Heat e-practical here.

This shows how to use it and how to collect the data.

The E-Practical

Perform the experiment yourself, collect your own data, make mistakes and be able to correct them. The e-practical requires that your browser can run WebGL 2 (usually found on Windows browsers, safari on iOS, and various Mobile browsers, test with https://get.webgl.org/webgl2/). This link is for students and evaluation only, schools should purchase a site licence.

The e-practical will run on laptops and desktops for PCs and Apple computers and will run on mid to high spec tablets and 'phones. All the e-practicals will run on devices as small as a mobile 'phone. However, the best experience is on a PC using a mouse which gives very precise control, but if you are limited to a small device, consider using a stylus or a blue tooth mouse.

On a portable device, make sure you click on 'Toggle onscreen controls'. The left joystick controls movement, the right joystick controls direction and where you are looking.

Light the Bunsen burner by clicking on the gas tap.

The materials are cork, newspaper, and wool. A material wrap can be used or removed by clicking on it.

Try it yourself

Further Discussion

Is the cooling completely linear, or does the cooling rate slow down? Can you do an experiment to confirm this? Are there any difficulties. Can you explain your result?

Exam Questions

AQA June 22 Foundation Paper 1

Thermometer B is clearly measuring to one tenth of a degree, so this is its resolution.

Taking a reading from a scale is always going to be harder to get right than simply reading a value from a digital display.

Read the drop in temperature of material X, multiply this by the specific heat capacity and the mass and you have the answer in Joules. You don't really need the Physics Equations Sheet as the clue is in the units of the specific heat capacity - Joules per kilogram degree.

The anomaly is the reading that is not on the best-fit line. i.e. this is a reading that is not in agreement with the other readings, so, we would be justified in inferring that this reading is incorrect.

The graph shows that material Y offers less insulation because it allow the liquid to cool faster.

The curves show that the rate of cooling slows as the temperature drops. This is an example of Newton's Law of Cooling.

If you have done the e-practical the answer will be obvious.

If you don't know the answer, try doing the e-practical.

This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.