Helium balloons hold a magical quality
for young and old. When you buy one, it floats above
your head, gently moving in the breeze. If you pull
the string tightly, it bobs up and down. If you let
go, it flies away so quickly, your reflexes can't catch
it. In a split second it has gone as you stand on the
ground watching it floating away towards heaven, powerless
to stop its progress. But why does it float?
Why does anything
float?
We all know that things will float in water - somebody
has even invented soap that floats in the bathtub.
In a swimming pool or in the ocean, we can float so
we know what it feels like to float. The reason why
objects will float in water applies to air as well.
Let's have a look at water flotation:
If you take an empty 2 litre soft drink bottle with
the cap still on it is sealed full of air. Tie a string
to the neck of the bottle and take it into a deep
bath with you.
Pull the bottle down to the bottom of the bath and
you will feel resistance from the bottle as it tries
to float back to the surface. This is the same resistance
you feel from a helium bottle.
Losen your grip on the bottle and it will start to
rise, let go of the string and it will shoot straight
to the top of the water.
The reason for this is because water is a fluid and
the 2 litre bottle is displacing two litres of water.
The bottle and the air in it weight very little, just
a few grams, whereas the water that it is displacing
weighs about 2 kg. The weight of the bottle and its
contents is far less than the weight of the water
it is displacing it will float. This is called the
law of buoyancy.
Why does a helium
balloon float?
The same principle applies to a helium balloon. Instead
of floating in a sea of water, your balloon is floating
in a sea of air. The helium balloon is displacing
an amount of air because the gas 'helium' is much
lighter than air.. While the balloon and it's helium
contents are lighter than the air it is displacing
it will float.
Why does it stop?
Balloons are made of a biodegradable substance called
Latex, which comes from the sap of a rubber tree.
The size of a latex molecule is much larger than a
helium molecule. Once the balloon has been filled
with helium, the gas starts to leach through the latex.
In Australia, the flying time of a helium balloon
will vary according to climate. You can generally
expect to get at least 12 hours float time from a
helium balloon. Balloon gas is a mixture of pure helium
and air - as the helium leaches out of the balloon,
the ratio of latex to air changes and the balloon
becomes heavier and falls to the ground. All the helium
will eventually leach out, leaving a balloon that
is partially inflated with air.
