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You must have seen water flowing naturally from high low to low level, if not please go and see the Niagara Falls. Now, what if I had to move water from lower level to the higher level? Suppose, I have to fill a water tank located on third floor of our building and the water point is available at the ground floor. I will have to fill buckets of water, pick them up, climb the staircase to the third floor and empty the buckets in the water tank. This requires lots of energy, and hard work. If you still cannot understand this, try it yourself! In these modern times we have small and high capacity water pumps to lift water of various quantities to various high levels. But you need electricity to run these water pumps, they will not operate on their own.
Then, have you ever wondered how the things are kept cool in refrigerator at about 4 degree Celsius or even lower, though the temperature of the surroundings is much higher? Or how the air-conditioner keeps inside room temperature within the comfort zone of around 26 degree Celsius even though the external temperature is 35 degrees and even higher? If you don’t know answer to these questions, look at the back of refrigerator and air-conditioner. You will find a compressor, condenser coil, expansion valve or capillary, and evaporator or cooling coil, which collectively keep the cooled space at low temperature by continuously removing heat from it. Just observe if they do it naturally or effortlessly on their own. You will notice that refrigerator and air-conditioner consumes lots of electricity and they are major cause of high electricity bills of our house and office. And why do they consume electricity? Just to keep the space or room cool by removing the heat from low temperature space and throwing it to high temperature space.
Just as the natural tendency of water is to flow from high level to low level, the natural tendency of heat is to flow from body of high temperature to the body of low temperature. Just as you need work or power to lift water from low level to high level, similarly, work or power is required to transfer heat from low temperature body to high temperature body.
This is what the second law of thermodynamics says. It says that heat flows from body at high temperature to the body at low temperature. If you want to transfer heat from low temperature to high temperature body, external work has to be done.
The major application of second law of thermodynamics is observed in the field of HVAC where the flow of heat is against its natural tendency. You will also find its applications in refrigerators, cold storages, water chilling plants, ice making plants, etc.
The field of thermodynamics, invented by Sadi Carnot, involve first, second, and third law as well as Zeroth’s law. It is an important and major subject area of the mechanical engineering, where you will find various applications of thermodynamics laws.