Archive for February 2012

Visualizing the latent heat of fusion of ice

February 27th, 2012 by Charles Xie

Latent heat is the heat released or absorbed by an object during a change of state without the change of temperature. The latent heat of fusion is the energy absorbed when a substance melts or released when it freezes.

The two videos in this post present a visualization of the latent heat of fusion of ice. Three containers, one filled with brine, the other two with freshwater, were stored in a freezer for two days. The temperature of the freezer was enough to freeze the freshwater in the two containers but not the brine. They were then taken out and put on a foam board. The left one was the brine.

The first video shows the temperatures of the three containers shortly after salt was added to the middle one. The second video shows their temperatures after about an hour. Both videos show that the middle container was the coldest. Two factors contributed to the cooling of the middle container. One is the latent heat of the melting of ice due to the contact of salt. The other is the negative heat of solution of salt--that the dissolving of salt absorbs heat.

Initially, the brine container on the left was colder than the third container on the right. After an hour, the brine container became significantly warmer than the third container. What was cooling it? It is the latent heat of fusion due to the melting of ice in the container. Since the brine was in liquid state all the time, there was no latent heat involved and all the heat it absorbed from the room was used to increase its temperature.

Sketch up a village using Energy3D

February 15th, 2012 by Charles Xie
 Energy3D is our signature software program for designing and making buildings. It actually can be used to sketch up a street or a village. The three images in this post show this capacity.

With the built-in heliodon, students can study if a new construction will cast a large shadow on a neighboring building and affect its daylighting (be nice to your neighbors :-)). Students can also take a virtual walk of the street they design to feel about it. The villages can be uploaded to the web and shared with others.

They can even "print out" the entire street to assemble it. The example shown in this post has 68 pieces to assemble, probably not too much for a class of 20 students. Collaborating on designing and building a village may offer some great opportunities of learning to everyone.



25 IR imaging experiments added

February 12th, 2012 by Charles Xie
I have added 25 experiments and their videos to the world's first website for IR imaging experiments launched last month. These experiments are all easy to do.

For example, to the right is an experiment that involves only a paper strip and a cup of tap water. Hang the paper strip above the water and slightly lower it into the water. Guess what you will see? Wait for a while and look at the IR view again. What will you see?

The first video shows what an IR camera recorded when the paper strip was just lowered to touch the water. You may expect that the paper would cool down because it has touched the water that appeared to be cooler than the ambient temperature. But, on the contrary, the paper warmed up! How is that possible? We all know that heat flows from hot to cold. How could heat flow from the cool water to the warmer paper strip and warm it up even more?

The second one shows what an IR camera recorded 20 minutes later. It shows that, on the paper, the thin strip just above the surface of water appeared to be the coolest.

Can you explain these temperature distributions and their time variations? I will leave these questions to you.