Thursday, July 8, 2010

Hot and Cold

Recently, at work I've been working on a heat transfer problem. The basic problem is that I have a cement room that is built into a hill so only one wall is facing atmosphere. Everything else is under the ground of the hill. There are no windows, there is one regular door, and one garage-esque door. The question is: can twenty-four people survive in this room for 2 hours, with no air conditioning, if the temperature outside is 100 degrees Fahrenheit?

This problem seems straightforward, but its actually fairly complex. I've been working on it for a while now, so its much on a mind. I am not going to spend this post boring you with the details of my heat transfer analysis. I am, however, going to bring up a point that a lot of people don't realize.

Hot and cold are like light and dark.

You might think I've gone insane at that statement. You might think "duh". Or you might just be confused. So let me explain.

As I'm sure most of you are aware, darkness is just the absence of light. Darkness cannot overwhelm light. It cannot fight light. If you shine light into darkness, the darkness goes away. End of story.

Granted, this is not talking about metaphorical darkness. I'm talking about the real thing. As in a room with no lights on versus a room with lights on. I cannot create darkness in a brilliantly lit room except by shading an area from the light.

Cold is the same thing. Cold is the absence of heat. If I have a piece of ice and I place it on your arm, I am not giving you coldness. I cannot give you coldness. Rather, the ice is taking heat from you.

I can create heat. I can run around, and my body will produce heat. I can hook up a computer, and my computer will create heat. You know how a light bulb might be 60 W? Well some of that wattage is converted into power that the light uses to work, and some of that wattage is converted into heat (because the light is inefficient).

I cannot create cold like this. You might laugh and say "That's ridiculous! My freezer creates cold air." Yes, it does, but it does this by taking the heat from the freezer and dumping it somewhere else. "Creating cold" is really just moving heat.

So what's the lesson in this post for writers? I'm not really sure. I guess it's to be aware. You cannot put some sort of machine in your story that creates cold unless that cold is going somewhere. Granted the device could be a heat sink, something that is so cold that it basically sucks all the heat around it into it, taking the heat from the air (or anything else) around it. Cold will suck away heat until the cold object and the hot object are at the same temperature. So if your crazy machine is at 10 Kelvin and the surrounding air is at 260 K, the machine will pull down the air's temperature until they're both at the same temperature - which will not be 10 K. It will be somewhere between 10 K and 260 K.

It's like putting ice in your water. The ice sucks in the heat from the water, making the water colder. However, by taking in the heat, the ice is raising its temperature, which makes it melt. Eventually its all one substance and at the same temperature.

Oh, and a note in case you don't know. For math problems you always use Kelvin or Rankin, never Celsius or Fahrenheit. Kelvin and Rankin are absolute. As in absolute zero. You can't have a negative Kelvin or Rankin temperature.

So that's it. Be aware, and please don't "create cold". Just transfer heat.

No comments:

Post a Comment