Question

Why second law is called directional law of nature?

Answer 1

In nature heat only moves naturally from warmer systems to cooler systems. One direction only. Never naturally from something cold into something hot.

We can pump heat out of a system by doing work on it, such as a refrigerator where the refrigerant is compressed - making it much hotter than the surroundings - then letting it give off heat to the surroundings, then expanding it across a valve where the evaporation and expansion causes it to get colder than the inside of the fridge - then allowing it to absorb heat from the inside of the fridge, then sending it back to the compressor to start all over again.

Answer 2

2nd law includes 2 statements(Roughly)

1)Kelvin plank- its about a cyclic engine can not covert all heat supplied into work with 1le reservoir.

- it implies low grade energy(heat) can not fully convert to high grade(mechanical) energy. (Shortly it implies flow of energy)

2) Clausius = its about flow of energy form cold to hot body with help of input work.

It also implies to flow of energy from high grade form to low grade.

So 2nd law is basically called directional /degradation of energy/qualitative law.