# I open the fridge - do I let the heat in or the cold out?

Conventionally, heat flows from hot to cold. How legitimate is it to redefine things in terms of "cold" rather than "heat" eg Cold flows from cold to hot?

• Both. Cold air flows out, and is replaced by warm air from the room. – mmesser314 Jan 26 '15 at 13:19
• And radiation ? – user56903 Jan 26 '15 at 13:30
• How can you do one without the other? – Carl Witthoft Jan 26 '15 at 14:14

It isn't really useful to replace the idea of heat with the idea of cold. Both the warm room air and the cold refrigerator air contain heat. Temperature is "heat density." Warm air contains more heat per gram than cold air. When you open the door and air gets mixed up, you increase the heat density in the refrigerator, and decrease it in the room.

Radiation is a small effect at room temperature, but it exists. Neither the warm nor cold air are red hot. But they do glow in the infrared. When warm and cold air are side by side, they are bathed in each other's radiation. Warm air has more heat, and therefore glows brighter. It loses more energy from radiation and cools. Cold air gains energy and warms up.

The transfer of heat can happen by radiation from a hot body to a colder one or by convection, air flow . Radiation plays a small role when opening the door of a fridge .

Please note that heat and cold are properties of matter, measured by temperature. Temperature is an intensive property of matter:

An intensive property is a bulk property, meaning that it is a physical property of a system that does not depend on the system size or the amount of material in the system. Examples of intensive properties include temperature, refractive index, density, and hardness of an object. When a diamond is cut, the pieces maintain their intrinsic hardness (until their size reaches a few atoms thick).

In the case at hand it it is convection at play that changes temperatures: The force of opening generates an air current, an under pressure at the inner surface of the door will lead the colder air to be pulled out while the warm air fills its place. This is what happens when a door is opened, so the cold air is let out, the air current starting from the motion induced by the door.

• I am familiar with conventional thermodynamics. For example, in electronics (in metal) electricity flows from positive to negative. Except, it doesn't. It is the other way around given that that the transport is electrons. Now, is it equally valid to claim (for example) that cold radiation flows from cold to hot? – user56903 Jan 26 '15 at 13:46
• There does not exist cold radiation. Radiation has a spectrum characterized by the temperature of the body emitting it and is always positive. Only in comparison one body has a higher or lower temperature. – anna v Jan 26 '15 at 13:48
• The laws of physics are time reversible on a microscopic scale (except perhaps QM decoherence). Cold radiation is time reversed hot radiation. – user56903 Jan 26 '15 at 14:02
• This last has no meaning for physics. – anna v Jan 26 '15 at 14:31
• My 2 cents: You could change $E=mgh$ to $E=-mgd$ where d is depth instead of height. In the same sense, you could talk about cold instead of heat, but it would not add anything useful to the discussion. It would be more confusing to talk about blackbody radiation at a given "coldness" even though it means the same thing. – mmesser314 Jan 26 '15 at 18:40

The refrigerator works by removing heat. When you open the door heat goes in.

• Hello, and welcome to Stack Exchange. If this answer is important to you then you might want to spend some time improving it; it's pretty skeletal at the moment. – Daniel Griscom Jan 4 '16 at 12:25

## protected by Qmechanic♦Jan 4 '16 at 8:03

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