Flying with solar power

Solar power moves ... explore

...explore: Up into the air with solar heat

	Materials: black tubular plastic film, 10 m (the black, high density polyethylene film (HD-PE) is 0.01 mm thick and weighs 10 g per square meter) a piece of string, 10 m two rubber bands
	You can only fly the solar balloon in sunlight and calm. Carefully unfold the dark tubular film. Hold the tube on one end and close it with a rubber band.
	Now, hold the other end wide open and move a bit forward to make air flow into the interior of the tube that will inflate the entire tube. This works better in pairs. In the end, the tube should not be filled with too much air. Close the other end with a rubber band as well, and fasten the 10 m string. Put the tube in the sun.
	From this moment on, you will need to keep holding the solar balloon, or even better tie it somewhere - otherwise it can fly away suddenly!
	Does the solar balloon also fly if its lower end is open? Undo the string and the rubber band on the one end of your solar balloon
	Knot a piece of wood to the string, and tape the piece of wood to the solar balloon. Hold the balloon vertically, with the open end pointed down, and see if your "hotair balloon" soars like this as well.
If the sun shines on the black solar balloon, the air in its interior heats. Because the air particles can move much more freely than the atoms in the water, they clear more space for themselves and push the close-by atoms aside. And here's what you can observe: The solar balloon becomes fully inflated without new air being added! The hot air in the balloon's interior expands until the film doesn't allow it anymore. You will find an animation showing this process under Explanations. The solar balloon on a string is not allowed to fly higher than 60 metres, and you have to be at least 3 kilometres away form any aerodrome. (Act: http://www.admin.ch/ch/d/sr/c748_941.html Art 15 and 16; in German, French and Italian only).

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