Hot Air Balloon

I pretty much knew how hot air balloons work before this experiment but it's something I've always wanted to try (a miniature version and a ride in a real one!) so I decided to do it any way!

You need a thin plastic bag, some cardboard and a toaster. My parents insisted on supervising on the grounds that 'toasters are dangerous' or rather 'me and toasters together are dangerous' but secretly I think they just wanted to see it! You roll the cardboard into a tube that is longer than the bag. I think my tube could have done with being a bit wider so more air was funneled into the bag but I was limited by cardboard supplies and it still worked okay. Put the bag on the tube and that on the toaster. Then turn the toaster on!

It worked! After a painful wait (I had a crowd watching, well 3 members of my family, and I was so worried it wouldn't work!) the bag started to fill up with air and then rise up off the tube. But once the whole bag was off the tube the bag fell which was slightly disappointing but I'm still counting it as a success and to improve I would suggest a wider tube.

And then I tried again which on reflection was pushing my luck a little but in my defense I thought a warmer bag might go higher. What actually happens is the bag starts to melt!! So my Physics experiment became a Chemistry lesson because of course I was using a thermosoftening plastic which means it doesn't retain its shape on heating...

Anyway, back to the Physics - the bag lifts up because the toaster heats the air inside the bag (as my tube wasn't wide enough it also heated the air outside the tube a little so this supports my theory that this had an affect on the experiment) causing the air to expand slightly. This means that less air can fit inside the bag and some will come out making the bag lighter and less dense so the bag floats above the heavier air.