Science Buddies: "Ask an Expert"

in my science fair experiment i built solar batch collectors using plastic bags. i used white plastic bags and black plastic bags. each bag had the same amount of water in it. i left two bags outside in the sun for 15 minutues then 15 more minutes for a total of thirty minutes. each time it only seemed like the temperature went up a few degrees and the temperature of the water of the two bags was around the same temperature. i'm not positve but it seems like to me that the black bag should absorb more of te sun's energy. am i right? or should it be the white bag that absorbs more energy? or should they both absorb about the same energy? i dont have much time before my project is due. so can someone please reply quickly.

Hi there,

You are correct in thinking that the black bag should absorb more of the sun's light energy, and should thus warm the water more. The white bag (not transparent, correct?) should reflect a lot of the light, and thus not heat up as much as the black bag.

One question I have is: How big are your bags, and how much water did you put in each one? If you are trying to heat up a lot of water, 30 minutes may not be enough time to see a big difference.

Also, are the bags made of the same material? Some materials will absorb energy differently from others.

Finally, one thing you can do is measure the surface temperature of the bags. Even if the water inside does not seem so different, you may at least see a difference on the surface of the bags.

Please provide a few more details about your experiment, and let me know if you have more questions.

Heather

In addition to the great advice already given, here are a couple more pointers:
You might want to try to put aluminum foil on the ground and then put the plastic bags on top of it to collect/absorb more sunlight...
Also try to use smaller plastic bags with smaller amounts of water (though still equal in all the bags) so that way it is easier for temperature change to occur and perhaps you will get more accurate data...
And again try to place it outside in the sunlight for a longer period of time and try to keep all other variables (factors) the same...
Good luck and hopefully this time you will see the black bag heating up more than the white bag (which is what should happen )
Feel free to ask any more questions/clarifications! :]

One small suggestion in addition to helpful advice already given. Since I'm not certain how much water you're placing in each bag, nor the size of the bags, the only suggestion I have is for you to reduce the volume placed in each bag enough so that there is less than an inch (water depth) throughout the bag. This will allow the water inside to heat quicker if the volume is minimized enough.

Actually, I performed an experiment similar to this when I was in elementary school. I looked at the heat absorptions of different colors of felt after they were exposed to a lamp for a period of time. I found that at short exposures, the cloth did not have time to absorb enough light energy to create measureable differences in the temperatures, but as I exposed the cloth to the light source for longer periods of time, I was able to notice a much greater difference in the temperatures of the different colors. So perhaps you might want to experiment with different exposure times to the sun.

Nithin