refrigerator
A refrigerator is a heat engine run in reverse: instead of letting heat flow downhill from hot to cold and skimming off work, it uses work to force heat uphill, from a cold inside to a warmer outside. Left alone, heat never flows from cold to hot on its own — that would violate the second law — so a fridge must pay for the wrong-way flow by consuming electrical energy to drive a compressor. That is why your refrigerator is plugged in and why it feels warm at the back.
In one cycle a refrigerator absorbs heat Q_c from the cold space (the food compartment), takes in work W from the electricity supply, and dumps a larger amount of heat Q_h = Q_c + W into the room. Its quality is rated not by efficiency but by the coefficient of performance, COP = Q_c / W: how much heat you pull out of the cold space for each joule of work you pay. A good fridge has a COP well above 1, meaning it moves several joules of heat for every joule of electricity used.
The second law again sets the ceiling: the best possible COP between temperatures T_c and T_h is the Carnot value COP = T_c / (T_h - T_c), which shrinks as the temperature gap widens. This is why a freezer struggles more on a hot day, and why cooling something to very low temperatures gets rapidly harder. The honest point: a refrigerator does not destroy heat, it relocates it — the kitchen as a whole gets warmer, not cooler, when the fridge runs.
A fridge removes Q_c = 300 J of heat from its interior using W = 100 J of electrical work. It expels Q_h = Q_c + W = 400 J into the kitchen, and its coefficient of performance is COP = Q_c / W = 300 / 100 = 3.
Work pumps heat from cold to hot; the room receives Q_c plus W.
Leaving the fridge door open does not cool the kitchen — it warms it, because the fridge dumps more heat into the room (Q_c + W) than it removes back through the open door (Q_c).