Learning Forum

[Karthik R]

We know that the first law states that the energy must always be conserved. And therefore when a cup of hot coffee is kept in a room and it cools down, the heat given out by the coffee is equal to the heat absorbed by the surroundings. However, have you every wondered why the coffee cup always cools down and never heats up? Think about it - we can still have the energy conserved where the heat absorbed by the cup would equal that rejected by the surroundings. So in effect the first law is still valid, but this never happens. Its as if something is setting a direction to the way heat can flow. ?

[prajput]

You are correct, there is a thermodynamic property called Entropy which governs the directionality of thermodynamic processes. What you pointed out is the limitation of the first law - i.e. it is only concerned about the conservation of energy. The restriction for the direction a process can take is enforced by the second law of thermodynamics, which states that:

"There exists a variable of state, the entropy ?. For a closed system (without exchanging heat and work with surroundings), ? increases in any spontaneous process, and ? is maximum once the systems reaches equilibrium."

Simply put, the process moves in a direction in which the entropy of the system + surroundings either remains constant or increases. In the second scenario where the cup heats up by absorbing the energy form surroundings that are at a lower temperature, the entropy actually would decrease and thus it would violate the second law. 

[Karthik R]

Thanks for the detailed discussion!