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Helium, Temperature, and Buoyancy
Name: Kristine H.
Status: student
Age: 13
Location: N/A
Country: N/A
Date: 3/19/2003
Question:
Is there a specific temperature (between 100 degrees
F and 0 degrees F) that helium looses it's ability lift or it's ability
to lift drastically changes? When I carry a balloon outside in the cold,
they shrink. Do they loose their lifting ability when they shrink?
Replies:
Kristine,
The lifting ability of a helium (a low density gas) filled balloon is
related to the volume of (higher density) air displaced by the balloon --
the larger the amount displaced, the greater the lift. This is true for
any gas whose density is less than that of air. That is why balloons
filled with hot air can float in air. A balloon you blow up with your lung
power contains much cooler air. Thus, it cannot float in air.
Yes, the helium-filled balloon does lose lifting capacity when it shrinks.
Here is why: Other things equal, the volume of an ideal gas is inversely
proportional to its pressure. If the pressure on the gas is increased,
the volume of the gas decreases. Also, the volume of the gas is directly
proportional to the absolute temperature. If you heat the gas, it expands;
if you cool it, its volume decreases.
Heating the balloon makes the helium atoms move faster. The force of their
collisions with the wall of the balloon increases. The increased force, is
an increase in pressure which, in turn, stretches the rubber and makes the
balloon expand. Cooling does the opposite. When the balloon cools, the
pressure inside drops, then the elasticity of the rubber shrinks the
internal volume, and the overall volume of the balloon gets smaller. As a
result, it displaces less air and therefore loses some of its lifting capacity.
Regards,
ProfHoff 604
Kristine: I do not believe the lifting ability of helium changes
appreciably if at all as the temperature changes. Surely the volume of a
helium filled balloon decreases when it is taken out in the cold. After
all V=nRT/p and, for constant amount of gas (n) and constant pressure (p),
the volume (V) will decrease as the absolute temperature (T)
decreases. But remember that the density of the air also increases in
exactly the same proportion (assuming both act as ideal gases) as the
temperature decreases since
n/V = p/RT. So the shrunk balloon will have exactly the same lifting
ability as in cold air as the unshrunk balloon had in warm air.
This makes sense to me, but as I reread it, it may not to you. Please let
me know if it does not and give me another chance.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
You get an "A" for observation!!! What you are observing is the relation
between the volume of a gas (call it 'V') and its dependence on: 1. The
amount of gas (which in your case is constant gm/M.W.). 2. The pressure of
the gas (which in your case is about P = 1 atmosphere), and 3. The
temperature of the gas (which in your case is changing). The mathematical
expression of that behavior is called the "ideal gas law" because all
gases obey the same law at pressures and temperatures you are talking about.
Specifically, P x V = gm/M.W x R x T. You do have to be careful about the units you use
for each of these quantities.
the pressure "P" is expressed in atmospheres (1 atm = 760 mm Hg); the volume
"V" is in liters; the amount of gas is the mass (or weight) in grams "gm";
the molecular weight is M.W. (for helium M.W. = 4); and the temperature "T"
is expressed in Kelvins ( T (Kelvins) = t (Celsius) + 273 ). Because the
volume shrinks as the temperature cools it does lose buoyancy as the
temperature decreases (keeping everything else the same). The buoyancy is
proportional to the volume of the balloon (again, assuming you do not
change some of the other quantities).
In addition, you may have noticed that a helium filled aluminum /
Mylar balloon lasts longer than a regular helium filled rubber balloon.
This is something different. Because the atoms of helium are so small
compared to the rubber, they diffuse right through the rubber balloon and
those type of balloon seldom last more than a day or two before they are
deflated. The
aluminum / Mylar balloons are much less permeable to helium and they can
remain inflated for weeks.
Vince Calder
All gases shrink when they get colder, at about the same rate. Four grams
of helium takes up the same amount of space as fourteen grams of air at
all temperatures in the range between 0 and 100 F. The volume of this
amount of air or helium decreases about one-fifth as the temperature drops
from 100 F to 0 F. (The volume at 0 F is about 82% of the volume at 100
F.) The mass
the helium can lift stays the same, however: ten grams of load for every
fourteen grams of air displaced.
At higher temperatures, the greater volumes mean that larger, and thus
heavier, balloons are required to hold the same amount of gas. So it is
actually harder to make a balloon to lift a heavy load at high
temperatures than at low temperatures!
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
Dear Kristine,
The balloons simply shrink because it is cold outside. Their lifting
ability should not be compromised. Helium balloons float because they are
less dense than air. When you take any balloon to the cold outside, that
was filled while being in a warm inside, it will shrink. The molecules
slow down and stop pushing so hard on the balloon walls. Try blowing up a
balloon and then putting it in the freezer, it will shrink. Almost
everything shrinks in the cold, even solids. It is just more noticeable
in a gaseous substance.
Bridges have expansion joints because of this fact. The bridge gets a
little longer in hot weather than it does in cold weather. So they put
expansion joints in so the bridge will not buckle in hot weather. Look
for one next time you go over a bridge.
Martha Croll
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