As part of the Ask a ScienceBlogger series, reader Jim Swanson asks:
When I open the dishwasher after washing and the contents are still hot, why do the glass and ceramic items dry off more quickly than the plastic items?
This is a great question. Great because it is something most everyone can relate to and great because there is some good science. Really, this shows the difference between temperature and thermal energy. I think the common idea is that temperature is a measure of the energy something has – but this isn’t quite true.
Ok, let me first start with a sample case. Suppose you are outside with your grill cooking up some hot dogs or something. The lid is closed and it is quite hot. Now, it begins to rain. As rain hits the top, it forms drops of water that quickly disappear. The hot grill lid has energy. Some of this energy is transferred to the water. When the water molecules have enough energy, they can break out of the drop going from water to gas. (evaporation is another story)
The point is that water on a hot thing will evaporate.
Back to the dishwasher. After the dishes are all clean and still wet, there is a heater in there to help them dry. Let’s assume that this heater brings everything in the washer to the same temperature (maybe it is like 180 degrees F). Also, let me assume there are two cups in there. A ceramic coffee cup and a plastic cup that is the same mass as the coffee cup. After a while, they are at the same temperature (180 F).
Although the two cups are at the same temperature, they do not have the same amount of thermal energy. You can think of thermal energy as a measure of how much the particles in the cups are moving around. It depends on how many particles make up the cup and also the types of particles. So, these two materials are different and thus have different amounts of thermal energy.
The ceramic cup has more thermal energy per mass at that temperature, so it can give more energy to the water sitting on it to make it evaporate. Of course, there is not an infinite amount of thermal energy in the coffee cup, but it is enough to evaporate all of the water sitting on it. You are left with a dry cup. For the plastic, there is not as much thermal energy. Not enough to evaporate all of the water. Oh – also, the surface of the plastic could be different enough that it can also hold more water on the surface.