So I think I'm gonna have to agree with your mom. Empty space in a refrigerator is inefficient.
Imagine a giant refrigerator full of nothing but cold air and one
small hunk of cheese. Let's say that the fridge is holding the inside
steady at 35 degrees. When you come along and open the door to get some
cheese, the cold and warm air begin mixing together in the
now that some air from the room has mixed with the cold air, the
fridge is going to have to run again to pump that extra heat out of the
refrigerator and back into the room. On the other hand, if all that
empty space had been filled with jugs of water then there'd be no room
for warm air to get inside and replace some of the cold air. So it'll
be the same temperature when you close it again.
The real key here has to do with how heat moves in air. Since air
isn't very dense, it's a poor conductor of heat. So, heat in air moves
primarily by masses of warm and cold air mixing and trading places. In
an empty fridge, lots of warm and cold air can change places when you
open the door. In a full refrigerator heat can only enter by conduction
of heat directly to some object that's inside (i.e. the cold broccoli
isn't going anywhere, and it isn't being replaced with warm broccoli so
it'll have to be warmed up by conduction only). And... air is a poor
conductor of heat.
I do agree that it'll take more energy to cool down the
fridge when you first fill it with water jugs. But that
only has to happen the first time. Basically, by filling
the empty space, you're make a smaller refrigerator.
(republished on 07/27/06)