Science Buddies: "Ask an Expert"

Dear Experts,

My project was on the "Absorption of radiant energy by different colors."
I as wondering if my data seems reasonable.

Average temp.(°C) before being heated up by an incandescent light bulb:
White: 20.5
Red: 20.6
Yellow: 20.0
Blue: 20.48
Black: 20.86
Average temp.(°C) after being heated up by an incandescent light bulb:
White: 21.6
Red: 21.81
Yellow: 21.89
Blue: 22.51
Black: 22.92
Power after calculations (equation 2)with the calculator given in bibliography
(http://hyperphysics.phy-astr.gsu.edu/hb ... tefan.html)
Didn't get to blue or black because I was wondering if my data was valid.
White: 0.044595008
Red: 0.054912368
Yellow: 0.081643247

Now you can clearly see there is a big difference between those numbers and the ones in the example given for equation 2 (Black = ~0.92), and I know that my 1-2°C changes are nowhere near the 15°C change in the example, but I was wondering if the data was correct/valid.

Also I was wondering if you could explain a bit more about how to find the emissivity of objects such as paper. It doesnt include emissivities in the equation but the calculator does and the calculator says emissivity is for objects that are not blackbodies. Anyways, I found an emissivity table from Thermoworks.com but it doesn't include blue in it and I want to know in general if the emissivities they have given are right.

Thank you very much in advance.
Happy 9 days before Christmas.

Is anyone going to respond?

Hi WillMcL,

Given the temperatures you posted, the energy values you found are in the correct range. So there's no apparent error with the energy calculations. The incandescent light that you used in the experiment was either too far away from your paper samples or simply emitted too little energy to heat the paper very much. As a result you found only small temperature changes. But you still have a clear trend in the data showing higher temperatures for the darker colored papers. You might consider running the experiment again using the sun as the light source. I'm sure you'd see higher temperatures and greater differences among the paper samples.

You are correct that emissivity appears in the radiated energy equation. The emissivity of the paper samples is assumed to be 1.0 for the purpose of this project. I'm not sure where you might find emissivity values for the type of paper you tested. An online search of “emissivity tables” turned up several references in addition to the one you mentioned in your post. Emissivity values may be listed in engineering or physics handbooks as well.

I hope this helps. Please post again if you have more questions.

A. Norman