Science Buddies: "Ask an Expert"

I have a son who is a freshman in High School. He has been assigned a project to build a a greenhouse in which they will measure the initial and final heat. The kid tries very hard. The guidelines are as follows

They are to use a container of one gallon or less
(3.8 liters). They may paint, color or add anything to the greenhouse
they feel will enable it to absorb light, and radiat energy from that
light. Their greenhouse will be placed under a light for thirty minutes.
Inside the greenhouse we will place a thermometer that is about 30 cm
long. This thermometer may be completely inside the bottle or it may
stick out the top.

The minimum goal is to raise the temperature of the thermometer 2 degrees
celsius. There is a prize for the students who raises the temperature the
most. There is also descriptions and a few things like that due with the
project that Scott should know about. This is all due next friday March
19.

I'm not sure where to start. He thinks he should paint the bottle black to absorb light from the heat lamp but I'm not sure. I thought possibly hanging some black paper from the cap so the heat would be absorbed into the air inside the bottle, which is the overall point. Unfortunately neither him nor his father is much of a scientist. Does anyone have a suggestion as to how to make this successful? It makes me feel good to see his reaction when he does well. Thank you for your help

Hi,

I've moved your topic to the physical sciences forum, because I think the physical science experts might have some better advice about this than the life science experts.

Good luck!

MelissaB - Hopefully you will get some responses from some true 'thermodynamics' experts on the page, but as starters I thought I'd contribute my two cents for you to consider. You are on the right track in selecting a dark material that will more readily absorb the radiant energy from the lamp(s). I agree with your idea of not painting the exterior of the bottle, as the heat will develop on the outside surface of the plastic and will have to be conducted through the bottle material (I assume you are using a standard soda bottle made of PET or Polyethylene terephthalate). This material has some thermal conductance (insulation) factor and you want to develop all the heat you can inside the bottle. The material you choose to put in the bottle is also critical, because of thermal mass, or the capacity of the material to store heat. If your material has a high thermal mass, you will get very little measurable heat rise compared to a material that has low thermal mass. I don't know if your experiment has to be a true working greenhouse and has to be filled with some organic material that will support plant growth. Maybe the best Idea is to get as much dark absorbing surface area in the bottle as possible and let the majority of the bottle remain as air space. Wadded up balls of black construction paper come to mind. The loose wadding might give you an expanded surface area and the air medium is a low thermal mass environment. You need the air to be able to move around by convection so you get an accurate temperature reading. I hope this helps and that I have not violated all three laws of thermodynamics.

Rick Marz

Mark,
Thank you for your response. I realize this is a little beneath your expertise so I appreciate your help. I guess basically I'm just looking for a design, using a 2 liter bottle, that when placed under a heat lamp at the end of 30 minutes will show significant internal temperature increase. Would painting the inside of the bottle wall (after cutting off the top and then duct taping back on)be more beneficial than the black construction paper? It is a contest to see whose project increases internal temperature the most. My son said some students were putting tinfoil on the inside for light/heat reflection but neither one of us understands why. If the heat is already on the inside why reflect it? Any suggestions would be greatly appreciated, as always.

Yes, it is a tandard 2 liter bottle and no, there will be no organic material in the bottle.

Mainefather - Cutting the bottle in half, painting the inside black and taping it back together might be the simplest approach. Another suggestion is to use some fine sandpaper to take the sheen off the outside surface to minimize reflected radiation loss. I agree with you that I don't understand the use of aluminum foil inside the bottle. It would tend to reflect back to the outside much of the energy coming from the heat lamp.

Rick Marz

Rick,
Many thanks. He put two strips of 3/4" painters tape vertically on the inside 90 deg apart and didn't sand the same areas on the outside. He then painted the inside and removed the tape after (semi)dry. The not sanding the outside above the tape lines was so not to distort the view of the thermometer inside. I even had black duct tape. And even if it doesn't significantly increase the internal temp he believes just including the explanation of "lightly sanding the outside sheen to minimize reflected radiation loss" should earn him a few points. He may change the terminology so he doesn't have to cite you however. He's nothing if not rule abiding. Again Rick, thank you.