Hot air rising

Description:

A bin bag made of thin plastic is held over a toaster to collect the warm air from the toaster when switched on. Once “full” of hot air, the bag is released and rises to the ceiling.

Requires

  • Bin bag made of thin plastic
  • Sticky tape
  • Toaster
  • Two pieces of wire 50 cm in length or cardboard to build a box with out a lid around the toaster

Method:

Place the bag over the toaster and switch the toaster on. Hold the bag until it is “full” of hot air and then release it.

Principle:

This is another example of Archimedes’ Principle in action (see “Hot Water Rising”) and is also how a hot air balloon works. In this case, we are using a fixed volume of air (a bag full), but are heating it up at atmospheric pressure, so as the air in the bag gets warmer, the mass of air in the bag goes down. The upthrust that the bag feels is the difference in weight between the volume of air in the bag and the weight of the same volume of air outside the bag. The latter contains more mass and therefore weighs more, so the bag (and the hot air it contains) experiences a net upward force.

Tips:

Be aware that these bags can be melted by the toaster, so some means of holding the bag “out of harm’s way” is very useful. I use wire as it is easy to transport. Another way to do this, is to build an open topped cardboard box around the toaster.
If this is used as a demonstration, warm the toaster first, as the effect will be quicker. If the bag is held down long enough, it will float to the ceiling in a school hall. Be aware that the effect will always be better in winter than summer!

Safety:

The surfaces of the toaster may become hot and melted plastic can burn if it touches skin. The open topped box around the toaster is to prevent the plastic touching the sides of the hot toaster and to prevent it falling into the toaster and catching fire. If you are doing other experiments at the same time be aware that toasters and water do not mix.

Tags: Archimedes principle, toaster, density, hot air, balloon