Melting Ice, Energy Transfer, Buoyancy, and Surface Area ```Name: Elizabeth Status: Student Grade: 6-8 Location: OH Country: United States Date: November 2008 ``` Question: We made five ice shapes with the same amount of water in each to see which shape would keep the water in which it was placed the coldest for the longest. The shapes were pyramid with a surface area of 175.31cm squared, a cylinder whose surface area was 150.72cm squared, a rectangular prism whose surface area was 127cm squared, a sphere with a surface area of 122.66cm squared, and a cone with a surface area of 114.55cm squared. The amount of water and the temperature of the water in which the shapes were placed were the same for each trial. Room temperature was also the same each time. The results were all over the place. The pyramid (largest surface area) did cool the most and stay the coolest. However, the cone (smallest surface area) stayed cooler than the other shapes with larger surface area. Also, the sphere and the rectangular prism whose surface area were very close to each other had very different outcomes. Where did I go wrong? What should the results have been? Replies: Elizabeth, First, kudos to you for performing experiments of your own. Experiments are a critical (and fun!) part of science. This experiment is a great way for you to get a better understanding of how heat transfer works. There are lots of possible reasons for your odd results. If you can learn a few basic concepts related to heat transfer, it would help you understand 'what results would be expected'. Explaining them here would make for a very long reply (and this reply is already way too long!), so I am going to get you started with a few concepts that you can read about on your own or with the help of your teacher: 1. Conduction -- this is one of the most important ways heat is transferred from one material to another. Keep in mind that heat flows from hotter objects to colder ones. 2. Heat capacity -- this is how much heat it takes to change the temperature of an object. 3. heat of fusion -- this is how much heat it takes for a 'phase change' (e.g. 'melting' from solid to liquid) 4. Convection -- along with conduction, this is a second way heat is transferred. If you have any questions about these feel free to email me / AAS back. Now for some comments: First, your problem statement describes two different things -- the temperature of the water *and* the time it is at that temperature. It is better to have a more precise problem statement -- such as how long it takes for the water to reach a temperature of "X", or at what temperature is the water at time "X". In general, it is better to have a quantitative, measurable hypothesis. Second, you were right to realize surface area matters, but you have to keep in mind some of the ice is not submerged (it is touching air), and the volume that is submerged is changing as the ice melts. I am assuming the ice is freely floating in the water. Off the top of my head, there are lots of other factors that you might consider -- 1. water temperature is not the same everywhere in your container -- the water next to the ice will be colder than water far from it. Depending on where in the container you measure temperature, you could get very different results. If you mix the water well, you might get more reliable results -- but be careful about how much you do this because it will affect the rate of melting. 2. time -- in the very short term, things like the initial surface roughness of the ice are more important than how much ice there is... and in the long term, shape will not matter at all (you will really just be measuring how fast heat is moved from the water-ice combination to the surroundings). It might help to have a more defined problem -- for example, how long it takes for well-mixed water around the ice to reach a certain temperature. 3. Equipment -- what are you using to measure temperature? Scientific equipment is going to give a lot more reliable results; home thermometers are probably not the best choice for this. You need a thermometer that quickly and accurately measures temperature. 4. Details details details! You say that air temp and water temp are "the same", but you do not say what they were. Did you assume they were 'close' or did you measure them explicitly? If you did not, you should record these values in your experiment. Record humidity too, as this will affect results. Last, make sure there are no drafts/heating ducts/sunlight around the container as changes in these things will all affect results. 5. More details! Based on the values you provide, I was not able to reverse-calculate the dimensions of your ice shapes. I could not find values that fit the required volume and surface area. I want to make sure one of us has not made a mistake. I suggest you go back and check your math on the surface area and volume (feel free to email me back to compare calculations). It is possible I (or you) made a mistake in math, or perhaps your shapes are not the same volume after all -- which would obviously also impact the results. p.s. One more topic you might want to read about: "significant figures" (or significant digits). This is something most students do not learn about until high school physics, but it is something that is important for scientists to understand. Especially for #5 above, it matters in this experiment as well! Hope this helps, Burr Zimmerman Click here to return to the Engineering Archives

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