❄️ Refrigerator — pump heat from cold to hot by doing work

Heat will not flow from cold to hot on its own (second law). A refrigerator pays for it: work W is done to pull heat Q_C out of the cold interior and dump Q_H = Q_C + W to the warm room. Its coefficient of performance is COP = Q_C/W = T_C/(T_H − T_C).

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Heat Q_C is drawn OUT of the cold interior (blue arrow) — the opposite of its natural direction.

Cold interior T_C 270 K

Room temperature T_H 310 K

COP = T_C/(T_H − T_C) = — · extracting Q_C = 200 J needs W = — J · dumps Q_H = — J
Q_H = Q_C + W · a smaller temperature gap → a better COP

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