Hands-on science: Toaster Balloons
Toaster Balloons
Make more than toast fly
Use an ordinary toaster to turn a bin bag into a hot air balloon. How high will yours fly?
More than one pair of hands makes this more fun and safer. An adult should supervise children trying this out.
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Step by step
Yan's video guide
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Dr Yan shows you how to try Toaster Balloons safely
Checklist
| Difficulty: none | Just have a try |
| Time/effort: quick | A few minutes to set up |
| Hazard level: care needed | Follow the safety precautions and supervise children |
SAFETY: Using an electric toaster to heat a plastic bag means adult supervision of children is essential.
Having two people is helpful. One can look after the bin bag, the other can switch the toaster on and off.
Don't touch the hot toaster except by its switch or handle. Never let plastic or cardboard come into contact with the toaster.
Do not run the toaster over and over for more than 5 minutes at a time. Let it cool down or you could damage it.
You need
An electric toaster
Either: a cardboard box, at least 30cm tall and big enough to go over the toaster
or alternative: sticky tape and some flat cardboard, large enough to go around the toaster's four sides and stand 50cm high
A small bin bag. The best type are the cheapest (made of very thin, crinkly plastic) and are pedal bin sized with no holes
A heatproof surface, such as a metal tray
What you do
Stand the toaster on something heatproof
Put your toaster and the cardboard on a heatproof surface, as near the floor as you can. A metal chair could work well.
Pop the top and bottom of the cardboard box out so that it becomes a tube. Check you can easily lift it over the toaster.
Alternative: if you can't get a box big enough for your toaster, make a tube that fits over instead, from sticky tape and cardboard. The top should be at least 20cm above the top of the toaster. Don't let any sticky tape touch the toaster, put it on the outside of the home-made tube.
Open up a bin bag and wave it around so it has some air inside. Check the mouth of the bag will fit across the top of the cardboard. Then remove the bag.
Take the cardboard box or tube off the toaster.
Set the toaster to run for about 1 minute.
Keep your hands away from the heat as you hold the bag around the cardboard
Safety: Once the toaster is running, don't touch the toaster. Don't let the bag droop near it either.
Place the box over the toaster.
Fit the bag over the cardboard and lower it 5–10cm down the outside. Lightly hold the bag on the cardboard with your fingers.
Wait 15–20 seconds and the bag will start to lift.
Safety: Once the bag is clear of the cardboard, stop the toaster or switch it off at the wall socket.
What should happen
If you get the right size of bag (so it fits snugly over the cardboard) you should get your bag to reach the ceiling. You may find the bag turns over as it rises.
If it doesn't work for you
The power of your toaster and the size and weight of your bin bags could affect how well the balloons work for you.
Try to find bags that are as thin as possible. Expensive bags tend to be of thicker plastic and don't fly as well as cheaper ones. Make sure they're big enough to slip over the cardboard tube easily.
The kit can take a while to warm up to 'full steam'. Have a few tries but bear in mind you must let the toaster cool down every 5 minutes or so.
What's going on?
Why the bags rise
The toaster warms up the air that's in the bag. Most things expand when they get warmer, including the air inside the bag. The trapped air wants to fill a larger volume and the bag puffs up.
Once the bag can't puff up any more, expanding air is instead forced out of the bag's open end.
The bag now has the same volume of air in it, but that air weighs less than when it was colder. (Its density is reduced.)
So a litre of warm air inside the bag weighs less than a litre of room air outside the bag.
A pocket of air surrounded by something more dense has buoyancy and will rise, just like an air bubble underwater. The buoyancy of the warm air is enough to lift a very light plastic bag.
If air has weight, why can't we feel it? If this activity leaves you with questions, ask Bang Goes The Theory's online experts.
Why they often turn over
Although the plastic weighs very little, the bags are still slightly top heavy. The seam holding the closed end of the bag together is the thickest part.
You can attach something very light to the bottom to rebalance the bag. Try experimenting with short pieces of sticky tape.
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