|MadSci Network: Physics|
Because I do not know your age and grade level I will keep my response to your question simple. A long time ago a man named Archimedes studied why some things float and developed some rules regarding buoyancy (why ships float on water and why hot-air balloons float). Today these rules are called Archimedes' Principles. These rules, if applied to a balloon, state that the buoyancy force is equal to the density of the air surrounding the balloon times the volume of the balloon times the pull(force) of gravity. For a helium balloon this means that the more helium you put into the balloon the faster it wants to rise.
To understand how high the balloon will rise, you have to understand what happens to the air outside the balloon as the balloon rises. As the balloon rises, the air outside the balloon changes in three ways. First the air becomes thinner (less air pressure), secondly the air becomes colder, and thirdly the air becomes less dense. Because of these changes to the air, a balloon will begin to get larger as the balloon rises. Eventually the balloon will get so big that it will burst. If a special helium balloon could be made to expand enough and not burst, it could float all the way up into outer space.
For a typical birthday-style balloon, the height it would rise to before it would burst will depend upon how much helium was put into the balloon. If you filled a balloon all the way up, it could easily burst before it reached 2000 meters in height. If you filled the same size balloon with only half as much helium (to make it rise more slowly) it might reach 6000 meters or more before it burst. Although such a balloon could easily reach the altitudes used by small single-engine type airplanes, they would not be a concern to the pilot. I am a licensed private pilot, and do not believe a balloon could get past the spinning propeller to clog up the air intake for the engine. I personally see no danger nor would I expect any danger to the larger jets should a balloon be sucked into a jet engine.
If you are interested you might want to read about weather balloons and such, which are specially designed to float up to specific altitudes and stay at that altitude. How they are designed to do this I will leave for you to discover on your own. I will also leave you with one further question to discover on your own. Why is it that when you go to bed at night, your helium filled balloon is still floating at the ceiling, but when you wake up in the morning it usually no longer floats, but rests on the floor. (Hint, the helium is not leaking through the tied opening.)
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