To demonstrate the principle of buoyancy of warm air.
The density variation between warm and cool air can be used to explain why warm air possesses higher buoyancy. As hot air contain atoms and molecules with higher kinetic energy than cold air, it has a lower density and rises up while the colder air goes downwards. In other words, cool air ‘sinks down’ displacing the warm air.
- A pair of scissors
Estimated Experiment Time
Less than 5 minutes
- 1. Inflate a balloon with warm air and secure tightly with the string
- 2. Tie a small piece of cardboard to the end of the balloon’s string
- 3. Adjust the size of the cardboard so that it is heavy enough to keep the balloon from rising
- 4. Use a pair of scissors and start trimming off small pieces of the cardboard so that the balloon rises above the surface but does not float away and touch the roof
- Use a closed room where air currents will not alter the course of the experiment.
- Ensure the pieces of cardboard you cut are extremely small in order to save time redoing the exercise!
When the right amount of cardboard is cut away, the balloon remains suspended in midair.
The principle behind the experiment is that cool air is heavier and denser than warm air which has a greater degree of buoyancy. The balloon thus remains suspended in the lower part of the room where the air is denser as compared to the areas around the roof which will have comparatively warmer air. The piece of cardboard balances the weight of air contained in the balloon making it too light to sink into denser air (near the surface / floor of the room) and is too heavy to rise into the warmer air (towards the roof of the room).
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