Dr Yan shows you how to investigate double glazing
|Time/effort: wait for it...||Takes at least 15 minutes|
|Hazard level: low||Take care with hot water and scissors|
SAFETY: Children may need an adult to help them cut the plastic bottles safely.
Be careful with hot water. Sometimes hot taps are very hot indeed!
Two empty 1 litre plastic drinks bottles
One empty 2 litre plastic drinks bottle
Two identical glasses or paper cups - anything without a handle that is narrow enough to fit inside the smaller bottle
A jug or something to fill the glasses from
Hot water from the tap
Two thermometer strips (sold at chemists and supermarkets) and some sticky tape
Alternative: a normal 'dip' thermometer
Optional: a heatproof mat
Safety: Adult help required to cut plastic bottles
Carefully cut the tops off both the 1 litre plastic drinks bottles. You should be able to turn the remaining bottom sections upside down so that they completely cover the two glasses (or paper cups).
Then cut the top off the 2 litre drinks bottle, making sure that the remaining bottom section is a bit taller than the bottoms of the two 1 litre containers you cut in the step above.
If you have strip thermometers, tape the ends of them so that they’re held firmly against the sides of the two glasses – one on each, at about the same height.
If you've got a heatproof table mat, then put the glasses on this to help avoid heat loss through the table.
Safety: Be careful of hot tap water
Now fill your mixing jug with hot water from the tap. You might want to let the tap run a little to make sure it's nice and hot (but it doesn't need to be dangerously hot).
Fill both glasses with exactly the same amount of the water from the mixing jug. This will ensure that they both start at the same temperature.
Pop one of the 1 litre bottle bottoms over each of the two glasses. Over just one of them, also put the bottom of the 2 litre bottle. It should fit over quite comfortably, leaving a small air gap between the inner and outer layers of plastic. This creates your 'double glazing' on one glass, whilst your other is 'single glazed'.
If you've got a heatproof table mat, then put the glasses on these to help avoid heat loss through the table.
If you're using a thermometer which you need to dip in the water, use this to measure the temperature of each glass at the start, and then every minute or two for the next 10 minutes.
If you're using the strip thermometers and your water is really hot, you may need to wait a few minutes for the temperature to display if your water is really hot. But when it starts to cool down, the temperatures should start to appear, and will change as the two glasses cool.
The temperature of the water in each glass should drop slowly as they cool down, but the water in the glass under the two plastic bottles ('double glazing') should cool more slowly than that under just one ('single glazing').
If you're using strip thermometers, make sure that they are tightly stuck against the side of the glasses, so that they are accurately recording the temperature.
If you're using a dip thermometer, make sure that you don’t leave the plastic bottles off the top of the glasses for too long when you take the temperature, or you might be losing a lot of heat.
Make sure that the plastic bottles covering your glasses are tall enough to touch the table top, and if there are gaps between the plastic and the table top, bung them up with something, or you might be losing heat near the bottom of the glass.
Try doing the experiment in a colder place, like outside.
You might get a bigger difference if you start with hotter water – but be careful: boiling water can crack glasses as well as scald your skin.
Windows used to be made from a single sheet of glass, but now we are all being asked to replace these with double or even triple glazing.
Double glazing consists of two sheets of glass with a gap between the two, and they can prevent much more heat from escaping from the house.
The reason that double glazing is much better at insulating is partly due to the gap between the sheets. Air, along with other gases often used in the space between the sheets, is a good insulator – heat doesn’t travel through it very easily.
The air gap between the two plastic bottles in this experiment works the same way, helping insulate the hot water and slow down the cooling process.
You can use this set-up to experiment with other ways of keeping heat in your house.
Try replacing the second plastic bottle with a winter hat. This is like adding insulation to your roof space – does it slow down the cooling process?
Try cutting a hole in one of the plastic bottles – this is a bit like leaving a window open in your house. Does it make the water cool down faster?
What else can you think of trying out?
Why does the note made by a spoon against a cup change as a hot drink is stirred?
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