This video will focus on a solar tube a
60 foot long balloon available from
educational innovations for about $14.
Solar tubes are useful for exploring the
ideal gas law and Archimedes principle.
We will fill the solar tube with a leaf
blower and then tie it off making sure
there are no leaks, so that the amount of
air in the balloon remains constant over
time.
Note that the balloon sinks and lies on
the ground due to the weight of the
balloons plastic. This situation involves
Archimedes principle, which says that
there is an upward force of buoyancy on
an object equal to the weight of displaced fluid,
which can be either a liquid or gas. 
Adding up all forces in the vertical
direction upward forces being positive
the upward force of buoyancy is not
sufficient to support the downward
weight of the air inside the balloon and
the weight of the plastic.  Thus the
balloon falls to the ground where it can
be supported by an additional force from
the ground. We can use an infrared camera
to measure the temperature of the
balloon immediately after it is filled.
Now note that the sun is shining on the
balloon.  What do you think will happen
over time? Let's check our understanding
with a peer instruction question.
Classroom students should follow normal
procedures or instructor guidelines.
Viewers not in a classroom should record
your vote and explain your reasoning on
a piece of paper. Please pause this video
and answer the question. Realize that a
black object will absorb more light and
heat up faster,  than lighter colored
objects around it, which are reflecting
much of the incident light. Thus the
temperature of the balloon and the air
inside it will rise.  We will use
time-lapse video to speed up the next
few minutes for the balloon,  and we
quickly see that the balloon rises up
off of the ground.
Once again we will use an infrared
camera.
Note that the temperature has risen.  How
can we use basic physics to explain why
the balloon rose?  Gases expand when
heated as described by the ideal gas law.
The ideal gas law in this circumstance
can be written as pressure times volume
is proportional to temperature.  So as
temperature increases the air pressure
inside the balloon will increase, 
exerting greater pressure on the walls
of the balloon, and it will expand
increasing its volume.  So the balloon is
now larger than it was when it was lying
on the ground.  As the temperature has
risen, both the pressure pushing on the
inside of the balloon and the volume of
the balloon have increased. Note that an
up close view of the balloon allows us
to see how smooth the outer surface of
the balloon is now that the gas inside
has expanded. Thus the force of buoyancy is now larger because the balloon is
larger and is displacing more air. In
fact, the force of buoyancy is larger
than the combined weight of the balloon
and the air inside, until an additional
force of restraint is provided by the
demonstrator. Note that Archimedes
principle is indirectly responsible for
many important aspects of astronomy. More
teaching materials are available on the
web at astro.unl.edu.
