Convection
One way that heat flows from one place to
another is by convection. This occurs when a
fluid that's in a gravitational field is heated from
underneath. There are familiar examples. At left
is a picture of one type of cloud called
a cumulus cloud.
These form because of convection. Sunlight
warms the ground and this warms the air just
above the ground. Air is a fluid, and when fluids
are heated, they usually become less dense.
When air near the ground becomes less dense
than air above it, it begins to float upward
because it now has bouyancy in the surrounding air (wood floats on water because it's
less dense than water). Each of the puffy cumulus clouds in the picture marks air that
is moving upward. Billions of water droplets condense from the gaseous water in the
air, and we see a cloud. As the air continues to rise, the water droplets eventually reevaporate, and the air cools and falls back down between the clouds.
(1) Make a drawing like the one above showing a few cumulus clouds and the surface
of the Earth beneath it, and draw lines with arrowheads to indicate the flow of air due
to convection.
Convection and Ocean Circulation
(2) Look at the Wikipedia entry for Thermohaline Circulation (the global circulation
of the oceans). Convection is part of what drives this circulation. Find out from the
article what factor other than heat helps drive this circulation.
(3) Look at the diagrams of the global ocean circulation and determine where on Earth
the water rises (it's rising where the blue line turns red)
(4) Where on Earth does the water sink (it's sinking where the red line turns blue)?
There are many other examples of convection in nature that are very important to how
our world moves. Much of the interior of the Earth is in very slow convective motion,
as seen in the figure above. The convective motion in the Outer Core is responsible
for generating the Earth's magnetic field.
(5) Find out here what the Earth's core is made of.
(6) At the temperatures at which we live, this material is (solid, liquid).
(7) Why is the material that makes up the core in a different physical phase than it is
up here?
(8) Find out the temperature of the Earth's center (if the answer you find has a K at the
end, this means that it's on the Kelvin scale. To convert into Fahrenheit, put the
Kelvin temperature in for x in this equation: y = 1.8x - 460). You can find the core's
temperature here.
Convection in the mantle brings heat up from the Earth's center and drives volcanism
and the motion of the tectonic plates, without which conditions on the Earth's surface
would not be stable enough for life.
(9) Look at the figure above. It indicates a bit
more clearly where the Earth's mantle is
thought to be rising up. The rising parts of
the mantle are mostly underneath the
(continents, oceans).
Another example of convection in nature is
in the interior of the Sun and other stars. The
picture at left shows the Sun's surface. Each
of the bright blobs is hot gas coming up from
the Sun's interior. Each blob is thousands of
kilometers across. Heat comes to the Sun's
surface from the superhot interior and is
released as sunlight. The convective motion not only brings up energy, but also
elements that are produced in nuclear reactions in the centers of stars (all the elements
that exist except for hydrogen and helium are produced in the nuclear processes in
stars). These elements are eventually released into space, and if it weren't for this
element enrichment of the galaxy by stars, we couldn't exist.
(10) Look at this animated gif file of convection on the Sun’s surface. It depicts a
section of a fluid convecting and makes it possible to see the hot fluid rise beneath the
surface. You can see the similarity to the appearance of the Sun's surface. When the
heat energy reaches the Sun's surface, in what form is it released into space?
When stars grow old, convection increases. Look at this animation of how an old star
might appear if you were close to it (stars are too far away to make such a film, so this
is a computer simulation). These old stars swell to hundreds of times their normal
size, so this object would be hundreds of millions of kilometers across. Look at the
time scale in the upper right hand corner.
(11) How many years does the animation represent?
So, convection is fundamental to the workings of the world, and very important to our
survival.
It's easy to create convection in various fluids. A candle is all that's needed to cause
air to convect. And placing a hot object in water will cause convection to occur in the
water. We'll do both of these and observe the convecting fluid.
To observe these fluids convecting, we'll shine a light through the fluid and the
convection pattern will appear in the silouette (figure below).
As indicated in the figure, the hot air flowing upward will be visible in the shadow.
This is because the hotter air refracts light differently than does the air around it, and
as a result, light from the light source doesn't fall evenly on the screen. To see the
convection clearly, it's best to place the candle more than one foot from the screen and
hold the light another two feet further from the screen.
(12) Hold the light and screen up off the table to see how far up you can trace the flow
of the hot air. Describe the motion of the air as it ascends away from the candle flame.
Note how it changes with height.
Convection in water can be seen in the same way. It's best to put the water closer to
the screen than the light, as seen in the above figure. Fill the square plastic container
about 3/4 full with tapwater and place it in front of the light so that the light shines
through the water and onto the screen. Now heat a metal cylinder in the boiling water
provided (lower it into the water using the wire). After 30 seconds, use the wire to
move the cylinder from the boiling water to the square container. Shine the light so
that the shadow of the square container appears on the screen.
(13) What happens? Explain why this is occurring?
(14) After a few moments, the flow seems to slow down. Think about why this
happens? Remember that the convection occurs because the hotter cylinder heats the
water around it.
Now put the cylinder back into the hot water. There is a container with vegatable oil
in it at the front desk. Bring this back to your table and put the hot cylinder into the
oil, observing the screen.
(15) Does convection occur?
(16) Describe how it looks different from convection in the water.
Remove the cylinder from the oil and wipe it with a paper towel. Now dump enough
water out of the square container so that it's about half full and ask the teacher to add
hot water. After the addition of the hot water, replace the container in front of the
screen and again shine the light through it.
(17) At first there seems to be movement all over the water. Then it settles down and
you see movement in one particular region. Where?
(18) Why do you think you see convection in this region and not others? Think about
the fact that heat escapes more easily from the water where it is directly exposed to
the air.
(19) Now dip a cool cylinder into the hot water and suspend it near the surface.
Describe what you see on the screen.
(20) Why do you see movement going downward from the cylinder?