PFE010: Refrigerators

I will admit that when this came up with my former roommate, I completely dropped the ball.

The question is an age old one:

If you leave the refrigerator door open, will it cool your house?

First up, answer: No. Leaving your refrigerator door open will heat your house, and probably ruin your chicken casserole from last week.

Unlike the airplane on a treadmill situation (which is always unnecessarily confusing) this one is more straightforward, but somehow less obvious.

First, for the sake of completeness and clarity, I should point out that "refrigerator" includes freezer too. Anything that is supposed to keep stuff cold really.

And this brings me to the first caveat or asterisk:

I am assuming that your method of keeping food cold is not more than sixty years ago - it was around the 50s when modern refrigeration became commonplace. I should note right here that an ice box is exactly the opposite of a modern refrigerator in this regard.

Next, I have never seen a refrigerator with a direct exhaust, so I will assume that yours doesn't have one (mine doesn't, unless it goes to the neighboring apartment, I haven't really checked though).

To consider this problem from a physics point of view is a perfect demonstration of both where I previously failed in explaining this and to what Feynman discusses about context. Talking about PV (pressure - volume) graphs and vapor-compression cycles to someone who hasn't studied them, will never be successful even if he/she is involved in science.

While any such explanation is accurate, that doesn't mean it is useful. In this case, the useful explanation relies on a few well known facts. The first thing to keep in mind, is that as the heat gets "pulled out" of your leftover-saver, it has to go somewhere. That heat is energy and doesn't just disappear. Of course, how that heat is "pulled out" has to do with PV graphs and energy cycles, but isn't really relevant to the discussion here because the refrigerator ends up the same as it begins. That is it doesn't consume anything (except for electricity, we'll get to that in a minute) so the net result is that the heat that got in when you opened the door or put your taco leftovers in, has to be kicked out somewhere, typically out the back (and still in your house).

But if this was everything, then leaving the door open basically means that air goes in the front and out the back, or something (while your beer gets warm). So admittedly not cooling the house, but not warming it either, and sending waves of cool freshness over you, standing in front of it, after your long run (or tough walk up the stairs). Unfortunately, we're not this lucky, because  the second thing to remember is that anything that people build is going to be inefficient. There is going to be energy leaked somewhere, and this will turn into heat. Your refrigerator is running and fans are blowing and some crazy freon thing is going on in the back, all of which take energy. Some of this is going to be leaked as heat, somewhere. So, sadly, your house heats up.

Hopefully this explanation sufficed, but if it isn't there is another even simpler one. This has to do with the premise of a closed system. If we first consider the refrigerator as a closed system, all it can do is move heat around from one place to another (best case scenario). In which case no net change to temperature. But a refrigerator is not a closed system, it is plugged in

and uses electricity. Since there is energy being used up by the refrigerator/freezer somewhere in its process of making tasty ice cubes, that energy is going to always eventually end up as heat. Extra heat in your house.

So what about different systems? What if we are allowed to violate our asterisks? An ice box has one external input. Instead of adding electricity to the system, ice boxes merely add ice. While electricity is always going to add heat, if the ambient temperature is warmer than the ice (let's hope so) then the ice box will actually cool the house in addition to the food since heat from the air in the house will eventually result in the melting of the ice.

The other condition was a direct exhaust. If the refrigerator has a ventilation system to toss the hot excess air straight outside then, well, I guess I've just described air conditioning.

That's refrigerators.