Q: Does opening a refrigerator cool down the room?

Physicist: Briefly yes, or no, not at all.  If you think of the room as including the inside of the refrigerator, then opening the door does nothing.  Otherwise, it does almost nothing.  But ultimately, if you leave the door open the only end result will be spoiled food.

Refrigerators, and the opening of same.

It seems like it makes sense to say that things like refrigerators and air conditioners “make cold”, but like every other machine ever created (or ever to be created) they ultimately just make heat.  A refrigerator “creates cold” in very much the same way that a drain “creates lack-of-water”; it moves heat from its inside to its outside.  Specifically, it pulls heat out of the freezer, and drops it into the coils on the back side. (It’s worth noting that about the worst possible place to put cooling coils is in a tiny gap next to a wall).

It’s a general thermodynamic fact (a law even!) that generating cold is impossible.  You can generate heat, and you can move it around by taking advantage of the fact that heat always tries to “even out” (this is the idea behind all “cooling devices”), but that’s pretty much it.  So just like any other machine, refrigerators generate heat.  When you first open the door you’ll get a burst of cold air, but that’s about it.  An open fridge is like a water pump in the middle of the ocean, pointlessly moving stuff around.  It’ll cool the room a little, but also heat it up a lot more.

A clever thing to do would be to put the heating coils outside of the room.  The room would get cooler, the outside would get warmer (slightly), and you’d have re-invented air-conditioning.

This entry was posted in -- By the Physicist, Engineering, Entropy/Information, Physics. Bookmark the permalink.

21 Responses to Q: Does opening a refrigerator cool down the room?

  1. michael jacobs says:

    Can you explain why you can generate heat but not cold? Is there no possibility that in the future someone could invent the opposite of a microwave oven, a device in which you put a warm can of beer, push a button and it comes out cold?

  2. Will says:

    Because cold is just the absence of heat. The way you ‘produce’ cold is by pumping heat from A to B and not letting any more heat get into A: The same way you ‘produce’ a vacuum.

    Basically, cold is to heat as a vacuum is to matter. Cold is basically ‘nothing’ and you can’t create nothing.

  3. Andy says:

    @ michael jacobs
    In principle a fridge is such an “anti oven”, except we leave it on all the time, and you could probably use a really powerful fridge to achieve the quick cooling of beer. The problem is that when we extract heat energy from the food in the fridge it has to go somewhere else and ends up being dumped in the coils. Also because the fridge is not perfectly efficient, it also requires electrical energy to run, and that energy gets dumped as well. Therefore, while we use the fridge to cool down food, it heats up the room much more than it cools down the food.
    This is called the increase of entropy (more or less the randomness of the universe) over time. As heat is a particularly random form of energy (difficult to control and very inclined to disperse into the air) it ends up that most processes convert usable energy into heat.

  4. michael jacobs says:

    I understand that cold is the absence of heat energy, and is to heat as a vacuum is to matter. But we can create a vacuum in a chamber. And I can quickly drain the energy out of a battery by shorting it. And I’m not saying I want a machine that destroys the heat energy, just one that transfers it into some other receptacle object, or that converts it to another form, like light or radio waves, that could safely be sent into a room. My “invention” is an object approximately the size of a microwave oven that only has to be turned on as needed (unlike a refrigerator) and in a minute or so can turn a glass of water into ice by quickly draining its heat energy. As a green bonus, I would add a converter to the back end that would take the removed heat and convert it into something useful, like electricity or light. (I know some energy would be lost in the process, that it can’t be 100% efficient.) Is this science fiction or just something we don’t know how to do yet?

  5. Locutus says:

    You do realize that the second law of the thermodynamics has been violated, right?
    So maybe, one day, we will be able to “create cold”. Of course, that might require the same technology to create universes, too.

  6. Will says:

    michael: What you want would be entirely possible if you had access to a small nuclear reactor to power the system. The reason we don’t have instant flash-freezing microwaves is not because it’s impossible but because the power requirement is obscene.

    There have been a few examples of people creating ‘super freezers’, one guy built himself a fully functional turbojet engine in his garage and then used it to power a beer cooler and even that took several minutes to cool to beer down.

  7. Stefan says:

    But then again, in supermarkets there are a lot of open freezers. And the air is noticeably colder in their vicinity. Why is that?

  8. The Physicist The Physicist says:

    The radiators for those refrigerators dump their heat outside of the building. If you walk around the back, usually around the loading docks, you can often see/hear them.

  9. vignesh says:

    how is cold climate created in universe?where is the heat dumped??

  10. The Physicist The Physicist says:

    Anywhere else. In the case of refrigerators the heat is dumped into the coils on the back.

  11. Darrell says:

    Why don’t we connect the back of the fridge to a vent system to exit the house like a dryer or oven? Wouldn’t that make sense if you live in a hot climate? Even better would be a vent that could either be directed inside or outside the house, so that you could keep the heat inside in the winter and have it exit the house in the summer.

  12. The Physicist The Physicist says:

    I have no idea. That’s something that really, really needs to be made standard.

  13. Danny says:

    If a fridge is only extracting hot air, then how is it that there is a vent inside blowing in cold air?

  14. The Physicist The Physicist says:

    They do generate cold air, but they generate more hot air.

  15. Danny says:

    So, how are they able to blow cold air?

  16. so in refrigrator heat is taken
    negative work is done
    Am i right?

  17. Emanual Reeves says:

    if we use heat exchanger for extracting cool air into the room and providing the condenser outside then we can cool the air or not….. and if we do so does there any adverse effect on compressor

  18. santanu says:

    can’t we use the heat dumped by refrigerator to heat things in a microwave,by connecting the coils or by any method?

  19. The Physicist The Physicist says:

    Absolutely! The system we use today is remarkably wasteful.

  20. BillyHW says:

    They do have very quick freezers. I saw them on the Iron Chef. Still not as quick as microwaves though.

  21. Mark Johnson says:

    Does opening a refrigerator cool down the room?

    [email protected]

    Let’s answer the question. Does opening a refrigerator cool down the room? This is a typical high-school hypothetical question to get students to think. My answer is No. In fact, the room heats up; seems counter-intuitive. Before I elaborate, assumptions are needed: the refrigerator is a typical kitchen appliance with cooling coils on the backside and is located in a perfectly insulated room with no additional heat entering or escaping relative to outside the room. Assume the fridge door can be opened remotely and the mechanism that opens the door produces no heat. The room is otherwise empty. Outside the room, a display is remotely monitoring the room’s internal temperature which is recorded periodically. Beginning with a stable (steady-state) room temperature, the first temperature measurement is recorded. The door is then remotely opened. As time elapses, the observer notices the temperature rising. Why? Because it takes work to remove heat from the contents of the fridge. The internals of the fridge are now exposed to the room which is at a higher temperature. Granted, some of the cold air escapes toward the floor (cool air is more dense) and is replaced by warmer air (lower density) entering the top section of the fridge’s open cavity. Assume the thermometer is not near the fridge door and is not influenced by the air currents when the door is opened. The fridge, sensing a lower cavity temperature, turns on the electric motor-driven compressor in an attempt to lower cavity temperature. The compressor runs continuously and the cavity absorbs heat from its immediate surroundings (via liquefied, lower-than-ambient temperature coolant) and delivers this heat to the rear-mounted discharge coils (where the coolant changes phase from liquid to gas, thereby raising its temperature to above ambient where heat can be radiated into the “cooler” room). This is the action of a heat pump. The hotter air at the back of the fridge ultimately flows back around to the exposed cavity and the cooling cycle continues. During this time, as the motor is working, it too is discharging heat into the room because it cannot convert electrical energy to mechanical energy to drive the compressor with 100% efficiency. Some electrical power is lost due to motor heat and frictional heat of the compressor and it bearings. Since the room represents a closed system (it is after all perfectly insulated), this added heat is injected into the room and, consequently, average room temperature rises. Question answered. If the fridge’s rear-mounted heating coils and motor/compressor could be located outside the room, then cavity heat would ultimately be ejected from the room and the room would eventually cool down to the fridge’s internal setpoint temperature. Such cooling systems exist and are called split-systems. Central air conditioners and large walk-in freezers use this principle.

Leave a Reply

Your email address will not be published. Required fields are marked *