I like trifle, especially extra triple layer sherry trifle from Marks and Spencers. But how does the fridge keep my trifle cold. The cold space in the fridge needs to be kept at about 8 degrees Centigrade. To achieve this heat has to be dumped out of the cold space otherwise it would warm up and it wouldn't be cold anymore!
If we open the fridge door heat would flow into the cold space as we know from the second law of thermodynamics that heat flows from hot to cold. That is the direction of things!!. So how do we get the heat flowing in the opposite direction?
The big trick of the fridge is that it doesn't have to make heat flow "in the wrong direction". Our lovely fridge sticks to the rules, or at least doesn't have to exert hard work pushing against the tide.
My fridge and most fridges work as vapour-compression systems. See diagram below. A very very cold fluid (the refrigerant), colder than the cold space in the fridge, passes the cold space, dragging out the heat from the "hotter" cold space, into the colder fluid. This satisfies heat flowing from hot to cold. Somehow this heat stored in the refrigerant must now be dumped somewhere. To achieve this the fluid is compressed which heats it up to temperatures above that of the surroundings in my kitchen. This heat can then be dumped out the back of the fridge , which is why the back of the fridge is nice and warm.
The fluid is then cooled back down by expanding it through a valve, and then the cycle continues indefinitely, dragging heat out of the cold space, dumping it behind the fridge, and cooling the fluid through the valve.
If you don't believe me take a really sharp knife and poke around in the ice box of your freezer. If you poke hard enough you can perforate the plastic and release the fluid. [Warning, this will break your fridge, only suitable for adults who have enough money to buy a new fridge]