Wednesday, January 19, 2011

Why do metals feel colder? An infrared view

Metals feel colder because they conduct heat faster, not because they are really "colder." This is often a misconception from students. A very simple IR experiment may dispel this misconception by visualizing what is going on when you touch a piece of metal and a piece of paper.

Lay a piece of aluminum on a foamcore board. Then cover it up with a piece of paper. Put one hand on top of the part of paper above the metal and the other on top of a part of paper that is not above the metal. Have your partner look at the hands on the plate through an IR camera. The reason that we want to cover the metal up with a piece of paper is because we want to make sure that the difference of temperature we observe has nothing to do with the difference of emissivity--the ability of a substance to emit infrared light--between metal and the base material.

The first IR image shows the initial temperature distribution when the hands were on. The second one shows the temperature distribution after two minutes. It clearly shows that the hand above the metal strip is losing more thermal energy than the hand above paper.

This simple experiment, once again, demonstrates the transformative power of IR imaging. IR imaging experiments such as this are much easier to do than conventional experiments. They provide more intuitive, richer results in a snap. Imagine how many other experiments out there that can be transformed by this new instrument!