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Jar Color and Heat Transfer
Name: Alexa T.
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
In my science experiment I painted identical jars
different colors and left one clear. They were filled with equal
amounts of water, then placed two at a time in front of a
sunlamp. The temperature of the water after an hour was cooler in
the white jar and warmer in the black as I expected, but why was
the colorless jar just as warm as the black?
Replies:
Hi Alexa-
I think I know what is happening in your lamp experiment.
Your sunlamp was an incandescent floodlight bulb, right?
(The kind of bulb with a bright tungsten-metal wire inside making the
light...
check with your teacher if not sure.)
It is too bad, but so far man-made light-bulbs
just are not the same as sunlight
for these very reasonable experiments.
In addition to making visible light,
Incandescent light-bulbs make lots of invisible "infra-red" light rays,
which you cannot see but they carry heat energy just like visible light.
in fact, incandescent bulbs are pretty weak (inefficient):
They use 100 Watts of electric power to:
project maybe 3 watts of visible light rays (not much!),
project 10-20 watts of InfraRed light rays,
and about 80 watts of heat which just makes glass of the bulb hot.
(No wonder there is publicity that we should use fluorescent bulbs,
which make 3-4 times more visible light for a given 100 Watts.)
So there is maybe 5 times as much InfraRed energy as visible!
(for this kind of bulb).
I think many of the infra-red rays bounce off the white paint,
much like visible light does.
All the infra-red gets absorbed in the black paint,
exactly like visible.
As for clear water and glass:
- all the visible rays go through, making no heat (also no problem)
- at least half of the InfraRed rays are absorbed in the water. (!)
It turns out that water, though very clear for visible light,
is pretty dark for InfraRed light.
Some scientists actually use thick tanks of clear water,
just like your jar, as a special color-filter
to take IR rays out of any bright white light.
Suppose you put lots of dark food-coloring in the water.
For InfraRed light, that is what water looks like
even when you do not add any coloring.
I think if you make a jar of darkened water
it will always get warm, just like the jar with black paint.
You see, just as there are many radio channels
and your radio cannot hear them all,
there are many frequencies of light,
and our eyes only see a certain range we call "visible" light.
This range is from 400 to 700 nanometers.
400 is the blue end of the range. 700 is red end. Past 700 is
'infra-red"
Some common video cameras see IR (InfraRed) out to 1000 nanometers,
called "near-InfraRed". I think water is still clear there.
But the incandescent bulb makes radiation all the way out past
10,000 nanometers ("far infra-red"), very long slow waves.
It makes them quite strongly,
and the glass bulb lets some of them out.
Past 1500 nanometers , called "mid-Infra-Red",
the water in the jar is dark and absorbs these rays and gets warm.
My advice is to find a good south window with direct sun
and repeat your experiment there, using sunlight, not bulbs.
The sun makes mostly visible light and much less infra-red,
so your light-absorption experiment will work much more
like you expect and like what you see, if you use pure sunlight.
If you make a jar covered in front with aluminum foil,
that will stay cooler than the white-painted jar,
no matter sunlight or light-bulb.
Metal foil reflects all InfraRed just as well as it reflects visible light.
White paint usually absorbs a some of the IR,
and you are never sure how much.
If your jars are on a broad counter top that gets hot in the sunlight,
it might make sense to cover the counter-top with foil
to stop that heating.
This way temperature rise in your jar is due to absorption,
and not to warm air rising up from the counter top.
It may also nearly double the amount of sunlight shining on your jar,
which should help make more obvious results.
Sorry if I made this letter too long.
Please show it to your teacher, too.
Jim Swenson
Alexa T.,
Black paint absorbs visible light, gets warm, and then heats the water a
little bit. White paint reflects visible light. No heating occurs from
visible light. For the clear jar, the light actually reaches the water.
Some of the light passes through the water, but the water directly
absorbs some as well. Energy can get to from the sunlamp to the water
for the black jar and for the clear jar. Energy cannot reach the water
for the white jar.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
If the black jar heats more quickly than the white, that is an
indication that the black paint is absorbing more radiation than the
white paint. The black paint absorbs the radiation, warms up, and
then transfers heat by conduction to the glass and water. However,
the water itself can absorb radiation too. In the unpainted jar, the
water and jar are absorbing heat directly (no paint), and therefore
their temperature rises.
Even more complicated, just because something is black in the
visible range of the spectrum does not mean it always/must absorb
more infrared radiation. All objects have their own characteristic
absorption spectrum for radiation, and you could find coatings that
look black yet reflect infrared radiation. The same thing is true
for 'clear' materials like water and glass. Even though they do not
absorb visible light readily, it turns out water is very active in
the infrared spectrum. So water is likely doing most of the infrared
absorption.
If both the painted black jar and the unpainted jar are absorbing
nearly the same amount of radiation, then that explains why the
temperatures are nearly equal.
I would guess that if you performed the experiment more carefully,
you would (or could) find a temperature difference between the
unpainted and painted jars. You could also find different black
paints that absorb radiation differently. The key here is
'absorbance' -- the amount of radiation that a material absorbs.
Just like objects can have different colors (they absorb visible
light differently), so can the absorb other radiation as well.
Burr Zimmerman
The white jar reflected light in the infrared, which is the
major wavelength that is converted to heat, even though you can't
"see" it with your eye. Where the surprise comes is that the water
in the colorless jar still absorbs the infrared radiation -- even
though you can't "see" it. Your thermometer is not sensitive enough
to distinguish between the difference between a "black" jar and a
"clear" jar because the radiation involved is not visible radiation
but infrared radiation.
You did not mention whether the jars were open or had a lid.
They all should have a lid to prevent evaporation, which could
decrease the temperature rise because of evaporation, which reduces
the increase in temperature.
Vince Calder
because the light was not reflected off the glass as the white
bottle afforded.....here we have a case of the light passing through
the glass......and absorbtion ....great scince project to compare
those two and graph data.
prof przekop
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Update: June 2012
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