# energy absorbtion

I have a question for all the Dopers here. Well first let me explain. My younger Sis is a six grader, and is doing one of her first science fairs. Well, being out of High School for 2 years I haven’t studied up on my General Science for at least that long. So, when she asked me for assistance, I had no idea what I was getting myself in for. The main idea of the project is to see which warms up faster, Dirt or Water. With the land representing the land masses and the water representing the ocean and a heat lamp representing the sun. Now from what I can remember, the sun puts off Solar energy, and the two constants absorb the energy. The thing I’m trying to figure out is why does one warm up faster than the other, and why does it do it. I’m not going to actually do the project for her, it’s just that I want to be able to answer her questions and atleast point her in the correct direction. Any help would be greatly appreciated. Thanks

Interesting idea for a project. There are a few different things happening here. First there is the absorptivity of the ground and water. Absorptivity varies with the wavelength of the radiation, so make sure the lamp is similar to sunlight, or better yet use sunlight. The other huge variable here is the dirt. Gravel has an absorptivity of 0.29. Brick is about 0.75. The quality of the “earth” you use will greatly affect the results. Then there is the water. If you put the water in a trough painted black it will absorb more solar radiation than water in a trough painted white, depending on the depth of the trough. IIRC, at 80 feet or so you have pretty much every color absorbed other than green, so it might be fair if you painted the trough an ocean green color.

Next there is thermal mass. This is a substances resistance to change in temperature. Water (fresh) is the baseline for a substance’s specific heat, so it is close to 1.0. I don’t know what it is for soil, but for concrete it’s 0.21. This means that even if the concrete is absorbing more heat energy it’s temperature won’t increase as fast as the water’s.

Next there is evaporation. The water is constantly cooling itself off by evaporating. If you weigh it before and after you could correct for it.

I think that’s the basics. It would be a more complete project if she measured both fresh and salt water, and several types of “earth”, like soil, sand and gravel.

Just to rephrase things in physics terms, there are several important effects involved in the project you outlines in the OP (all of these were essentially already discussed by Engineer Don in engineering lingo.)

reflectivity: the fraction of incident light that is reflected vs. absorbed (absorbed light is converted into heat). In sixth-grade science terms, dark things absorb most of the light that hits them, while white things reflect most of it. (Which is why it’s a bad idea to wear black leather in the summertime.) There are also other factors, including the light wavelength. As Don mentioned, reflectivity varies in different materials, so maybe try a couple of different soil types, as well as different colors of water troughs.

heat capacity: all things have an energy cost to raise their temperature. Things with a low heat capacity require very little energy to heat up. Things with a high heat capacity take a lot of energy to heat up. Water has a very high heat capacity, so it takes lots of energy to warm it up. One way to test this – put equal amounts of dirt and water in two pots and heat them on the stove. Make sure the dirt is packed down, so there is good contact with the bottom of the pot.

cooling: water can lose heat by evaporation, which means that the most energetic water molecules break free of the liquid, lowering the overall average energy of the water. Air moving across a hot surface can also cool it, so a surface with more surface area (craggy rocks) will cool faster than a flat, hard-packed surface (flat dirt). I don’t know if this is a big effect, though. (You could maybe test the effect with a fan.)

My guess is that of these three effects, the heat capacity is by far the most important.

Hope this helps!

If this is a 6th Grade Science Project you proably don’t need a whole bunch of numbers or equations so you don’t need a college level heat transfer book. Tell your Sister to look in a Earth science book that should give her the reason why but won’t confuse her or you.

Hints on the experiment:

Use several dirt and water samples
Submerge thermometers just below the surface of the samples
sunlight is better but lamp light is ok

Oh yeah the Dirt will heat up and cool down faster

Well if she is trying to predict which one of the bowls would heat faster, go with the water. If she trying to do a project on how the oceans and the earth itself contains the heat from the sun, then thats another matter. To be simple about it (mainly cause I forget the details), the earth absorbs a certain amount of heat from the sun. It also releases some aswell. Absorption and re-emission of radiation at the earth’s surface is only one part of an intricate web of heat transfer in the earth’s planetary domain. Equally important are selective absorption and emission of radiation from molecules in the atmosphere. If the earth did not have an atmosphere, surface temperatures would be too cold to sustain life. If too many gases which absorb and emit infrared radiation were present in the atmosphere, surface temperatures would be too hot to sustain life. The earth actually only absorbs about 51% of the sun’s heat, the rest is either taken in by the atmosphere or reflected off. As for the ocean I know it reflect its own certian amount of light, and the rest filters through. The deeper you go in the ocean the less light, and there for less heat. Hope that helps.