SITEMAP   Physics Notes: Thermal energy 3.4 Investigating effective insulators

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Thermal energy - thermal conductivity: 3.4 Simple experiment to investigate the insulating effectiveness of various materials - relative thermal conductivity values

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3.4 A simple experiment to investigate the insulating effectiveness of various materials

• This is an investigation into thermal insulation - comparing, indirectly, the thermal conductivity of different materials..

• How can we get a simple comparative measure of how effective a material is in reducing the rate of heat transfer?

• Below is illustrated a simple experiment to get a comparative thermal conductivity value for a solid material.

• also to use heat02.gif

• The basic idea is to use a container e.g. a copper calorimeter filled with a fixed amount of water and covered with a lid to seal the system and minimise heat losses from the surface of the water.

• Its quite a good idea to use a copper container because it is a good conductor of heat - copper has a relatively high thermal conductivity - so its a good test of an insulating material to keep the temperature of the water as high as possible for as long as possible.

• The left side of the diagram represents the control i.e. no insulation around the copper calorimeter.

• The right side illustrates the insulated copper calorimeter.

• The lid must have a hole in it to allow the thermometer to be suspended in the bulk of the water.

• The lid is essential to avoid heat losses to the outside air, it can be just a thick piece of card.

• The copper vessel, water, thermometer and insulation constitutes the system and we are investigating heat loss from this system.

• The copper vessel is quite good to use because its made of a good thermal conductor just like an immersion cylinder in a house - which of course needs insulating to keep the water hot over long periods of time.

• Therefore any decent insulating material should be quite effective in reducing the rate of heat loss i.e. rate of temperature fall.

• You can experiment with materials like carpet, bubble wrap, cotton wool, polystyrene etc. which are poor heating conducting non-metallic materials that also trap air to a greater or lesser degree.

• When comparing the effectiveness of insulating materials, for a fair test you should use the same thickness of material.

• The hot water can be quickly produced in an electric kettle and measure out a fixed amount eg 100 cm3 into the copper vessel.

• You may need to use a plastic measuring cylinder for very hot water at 80-90oC to avoid risk of cracking the glass, or use water at say around 50oC with a Pyrex measuring cylinder, either way take great care!

• It is important not only to keep the water volume constant (constant mass), but also the same starting temperature.

• You can gather the temperature-time data in several ways.

• (i) A single temperature-time reading (which should be of course repeated): You take the initial temperature and then allow the system to cool for a fixed time for each material of e.g. 20 minutes and remeasure the temperature to get the temperature fall in a given time,

•  or for a fixed given temperature fall, measure the time taken to do so.

• (ii) Multiple readings for a given insulator: You can take intermediate readings of temperature versus time and draw graphs to measure the initial temperature gradient as a measure of the rate of cooling.

• (i) If using a single temperature-time measurement you must run the experiment for the same length of time or measure the time for the same temperature fall.

• The smaller the temperature fall the better the insulating properties of the material - the lower its thermal conductivity.

• Using a single time and temperature measurement, a data table might look like this for a specified fall in temperature.

 Thermal insulating material(must have a control too) Time for the temperature to fall e.g. by 20oC (keep constant) Relative rate of cooling= 1/time No insulation - control 5 mins 0.20 Cotton wool 15 mins 0.07 Polystyrene foam 17 mins 0.06 Paper 12 mins 0.08
• The smaller the 1/time value, the more effective the insulation.

• The lower the rate of cooling, the more effective the insulator.

• The above values are fictitious, but make the point about using insulation!

• Alternatively, for the middle column, you could measure the temperature fall e.g. after 10 minutes and keep this time as a constant.

• In this case, for the 3rd column, the relative rate of cooling would then be given directly by the value of the temperature fall (initial - final temperature reading).

• The greater the heat conductivity of the insulating material, the less effective the insulating capacity of the material.

• (ii) You can be more elaborate with the experimental measurement by taking a series of readings every minute or longer time period and then plotting temperature versus time to produce cooling curve graphs.

 Thermal insulating material Temperature readings every 10 minutes Time (minutes) 0 10 20 30 40 50 60 No insulation - control 80 60 45 33 30 28 25 Cotton wool 80 70 62 56 51 47 44 Polystyrene foam etc.! Paper
• Again, these are fictitious results!

• A graph of the results - a series of cooling curves.

• The graph lines level out as you get nearer and nearer the ambient laboratory temperature.

• The rate of cooling always slows down with time because the temperature difference between the calorimeter contents and the outside air is decreasing - the thermal temperature gradient is steadily decreasing.

• The initial steepness of the graph line (a negative gradient) gives you a measure of the thermal energy flow from the system.

• Typical results from taking multiple temperature measurements with time are shown in the diagram above.

• I haven't shown the tangent lines to show how to measure the gradient - I'm assuming you know how to do this.

• With no insulation the temperature falls quite rapidly - steep gradient.

• With any insulation, however poor, the graph falls less steeply - smaller gradient.

• With a good insulating material, the negative gradient is the smallest, showing it to be the most effective insulation.

• The graph line gradients indicate that the good insulating material was about twice as effective as the poor insulating material because its temperature gradient is about half of that of the poor insulating material.

• the greater the heat conductivity of the insulating material, the less effective the insulating capacity of the material.

• By doing the experiment without an insulating layer you get a sort of baseline value.

• You then repeat the experiment with different solid materials, and, although not always possible or convenient, ideally each insulating layer should be of the same thickness.

• You will have to poor the water away each time and allow the container to cool down before repeating the experiment with the same amount of water and start temperature.

• You can attempt to cover all the surface of the 'system' but at the very least wrap the insulating material around the curved side of the copper container, even just this will give valid comparative results.

• You can also try out different thicknesses of the same material.

• In this experiment you can use sheets of polystyrene foam, thick plastic sheeting (not expanded), sheets of newspaper, layer carpet, wool, cotton wool, bubble wrap, felt, fibre glass etc. i.e. anything you can conveniently wrap around the copper container!

• You should find that materials trapping air e.g. wool or foam will show smaller temperature falls, better heat insulators than bulk materials like newspaper or thick plastic sheeting.

• You should also find that the greater the thickness of the material the better the heat retention, so the temperature fall should be roughly inversely to the thickness.

• Here are some typical relative thermal conductivity values for a variety of materials.

• aluminium 205, brass 109, cast iron 58, steel alloys 16 to 43, stone 1.7, dense brick 1.3, concrete 0.4 to 1.7, common brick 0.6 to 1.0, glass 0.8 to 0.9, water 0.58, lightweight concrete 0.1 to 0.3, non-expanded plastics 0.1 to 0.5, wood 0.14 to 0.19 (timber dependent), insulating brick 0.15, balsa wood 0.05, wool/felt insulation 0.04 to 0.07, fibre glass 0.04, glass wool insulation 0.04, wool blankets 0.04, cotton wool insulation 0.03, plastic foams 0.02 to 0.03

• I've included a wide variety of materials, even though they obviously will not be used as heat insulators, but a high thermal conductivity is important if you do want to transfer heat energy efficiently e.g. a metal radiator.

• ====> the lower the thermal conductivity value, the better the insulating effectiveness of the material

• Note ...

• the wide difference between metals and non-metals in thermal conductivity

• the increased insulation effectiveness when air is trapped e.g. glass and glass wool, bulk plastic and expanded plastic eg plastic foams

Keywords, phrases and learning objectives for thermal conductivity and insulation

Be able to describe an experiment to investigate the insulating effectiveness of various materials. Outline the method and apparatus and typical materials to investigate.

Be able to draw graphs of data, calculations of relative heat energy loss and be able to relate this to relative thermal conductivity values of the materials under investigation.

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