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?
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.
The refrigerator works by removing heat. When you open the door heat goes in.
protected by Qmechanic♦ Jan 4 '16 at 8:03
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