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A Cup of Cold

This experiment comes from some research I am doing on "Science With Your Refrigerator," but it has its roots in my childhood. You may recognize some of your childhood too.

OK, picture this scene. You are a hungry teenager. There is a refrigerator full of food in front of you. You open the door and then start looking around for something that looks good to eat. As you stand there, trying to decide what to devour, a voice from behind you mentions that your family cannot afford to refrigerate the entire neighborhood and asks you to close the door until you know what you want. What really happens when you open the refrigerator door?

You will need:

  • a refrigerator or freezer
  • a pitcher or other large container

Lets start by thinking about what happens in the refrigerator when the door is closed. The "works" of the refrigerator are busy pulling heat out of the air inside and sending this heat into the room. That is right, the refrigerator is actually heating your room as it cools your food. You may be able to feel this warm air coming from under the refrigerator. Once the air inside is cooled to the proper temperature, then the chiller does not have to work so hard, just keeping up with the flow of heat that seeps slowly in through the refrigerator.

When you open the door, something dramatic happens. Try it by opening the door to your refrigerator just a tiny bit. Hold one hand at the top of the door and the other at the bottom. Where do you notice the biggest change in temperature?

At the top of the door, you probably did not notice much difference at all. At the bottom, you felt a stream of cold air pouring out. Cold air is denser than warm air. When you open the door, the cold air rushes out, pouring onto the floor. If you have trouble imagining this, imagine a refrigerator full of marbles. What would happen if you opened the door? They would all spill out the bottom, just as the cold air does. The cold air does not pour out as quickly as the marbles would, but it does not hang around for long. After a few seconds with the door open, most of the cold air has left and the air inside the refrigerator is almost the same temperature as the rest of the room.

You can easily catch some of this cold air. Place an empty pitcher or other large container under the door of your refrigerator or freezer. If you have a refrigerator with the freezer at the top, hold the pitcher under the freezer door. If the bottom of the door is near the floor, try using a cake pan instead. It needs to be short enough to go under the door, but it should still catch enough cold air for you to feel. Once the container is in place, open the door slightly. The cold air will stream out and quickly fill the container. After a few seconds, close the door and put your hand into the container. Don't touch the sides. Just hold your hand in the air inside. It should feel cooler. The denser, cold air has filled the container.

Even after the cold air pours out, you will still feel more cold air streaming out of the bottom. Warm air flows into the refrigerator to replace the cold air. It loses some of its heat to that jar of pickles or the container of mustard. This cools the air, making it denser. The denser air then flows downwards, out the door. This constant stream of cold lets you know that you are adding more and more heat to the food inside. This means more work for the refrigerator, using more electricity. It can also reduce the shelf life of some foods. As items like milk and meat warm up, bacteria can grow much faster, causing them to spoil quicker. While the first few seconds do the most "damage" by letting the cold air out, the longer you stand there with the door open, the more you let the food inside heat up. You would think that someone would invent a refrigerator with a window in the door, so you could look in and decide before you opened the door. For serious snackers like me, it would make a big difference.