The solar bag is a fun toy to use. If you have access to a large open area on a sunny day, with little or no wind, you can try it out there. Since the bag is black, the solar energy heats it, and the air inside, to a temperature greater than that of the surrounding air. As a result, the density of the air inside the bag becomes lower than the density of the air outside the bag. This causes the air inside the bag to become lighter than the air outside the bag, and due to the resulting upward force created by buoyancy, it floats. It works just like a hot air balloon, except that the heat source for a solar bag is the sun, and not a gas burner.
Check out a video of a solar bag:
Helium balloons float because the gas inside them (helium) is lighter than the surrounding air. A solar bag floats because the heated air inside is lighter than the surrounding air. Hence, they work based on the same principle.
If you're a teacher, you can use this to impress your class of kids. Take them to the nearby park or the school yard, and try it out. They can learn about solar energy, and buoyancy.
It is best if the outside air temperature is not too warm. The solar bag will still heat up, as long as its sunny. But if the outdoor air temperature is on the cooler side, the outdoor air density will more easily be higher relative to the (heated) air density inside the bag. As a result, the bag will float better.
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