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Cooling Curve Experiment
Name: Russell
Status: student
Grade: 9-12
Location: LA
Country: N/A
Date: 1/3/2006
Question:
I am doing a Science project on the classic, which color
absorbs more heat. I am using 2 sets of white and black jars, each white
jar with a corresponding black jar of the same size, and vice versa, but
to spice things up and stray a little from what everyone else is doing, I
also want to find which color --cools--
faster. I have attempted to fill the jars with hot water and then record
the temperature every minute but they appear to cool at almost exactly
the same rate. I think this is because there are far too many other
factors affecting the cooling rate and the color is not the main
variable. Do you have any ideas on how to isolate the color factor so I
can test my hypothesis and determine which one will cool faster?
Replies:
To do this experiment you would want to perform the experiment in the dark
so no radiation would be adding to the heat content. It should also be
performed in vacuum so that convective cooling cannot occur.
Having said that, let me at that at the temperatures you can achieve in this
experiment I would not expect to see much difference. The energy that is
emitted is mostly in the infrared and the visible colors you see will not be
indicative of what is happening in the infrared. You could try to
characterize your materials for heating in the infrared by using an infrared
light source instead of the sun. It would be interesting to see if you are
able to detect any difference in the heating with an infrared light source.
Greg Bradburn
Hi Russel
Be sure your jars are in the same environment; that is, the same
ambient temperature, the same light, same elevation, same air
movement, etc. Also be sure they are far enough away or insulated
from each other so jars will not interact with each other.
Color alone may not be a factor here. Emissivity in the visible light
range is different than in the IR range and is dependent on the
coating all white coatings are not created equally. You may be
seeing a dT/dt that could take some very sensitive instrumentation to
resolve. You may want to try flat black and a metallic paint it
might give you more of a spread you can measure.
Hope this helps.
Robert Froehlich
The rate of cooling is dominated by two factors that are independent of
color. The first factor is the temperature difference between the
temperature of the jar and the surroundings, which depends only on the
temperature difference and not the other factors in the experiment such as
the "color" of the bottle etc. The second is convection of air around the
bottles over which you have little control without more carefully designed
conditions. In addition, heat input depends upon the infrared radiation
impinging on the bottle samples -- again something that is difficult to
control simply. I fear that the color factor is going to be swamped by
these other larger variables. One point that somehow gets "missed" in the
teaching of absorption of heat/radiation is that "color", except under
very carefully controlled conditions plays a minor roll in the overall
effect.
"Color" is such a minor factor in the forest of factors affecting cooling
rate, it is futile to attempt to observe differences and reliably ascribe
the differences to the "color" of the object. Your instinct that "there
are far too many other factors affecting the cooling rate..." is right on
target and very succinctly stated.
Vince Calder
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