Question

The spontaneous flow of heat is always:
A. From low to high pressure
B. From high to low pressure
C. Unidirectional from lower temperature to higher temperature
D. Unidirectional from higher temperature to lower temperature

Answer 1

Hint: The second law of thermodynamics is "the entropy of the universe increases for an irreversible process".
Entropy stays constant for a reversible process.

Complete step-by-step answer: Entropy is the measure of the extent of disorderness in a state of a thermodynamic system.
It demonstrates the anomaly or absence of uniformness in the structure of a thermodynamic system.
This is signified by ‘S’.
It is an extensive property as the value of entropy or entropy change relies on the quantity of matter present in a thermodynamic system.
Entropy is a state function as its value does not depend on the path by which the system has attained a specific state.
It depends on only the initial and final state of the system.
According to the second law of thermodynamics, "the entropy of the universe increases for an irreversible process".
It implies that a thermodynamic system tends to progress from lower entropy to higher entropy.
Entropy rises with a rise in the temperature.
A temperature rise implies that the atoms or molecules of the substance have greater kinetic energy.
The rapidly moving atoms or molecules possess more disorder than the atoms that are changing positions at a slow pace at a lower temperature.
So, the system tends to move from lower temperature to higher temperature.
Another form of the second law of thermodynamics states that "it is impossible to convert heat energy into work without compensation".
It implies that during the flow of energy there is some loss in the form of heat.
For example, let us consider a rod with the left end being hotter and the right end being colder.
After a considerable amount of time, the heat energy will transfer from the hotter part to the colder part. Then the rod will maintain a uniform temperature.
In the transfer process, energy is lost in the form of heat.
So, the energy flow in a spontaneous process is only in the direction in which there is a loss of heat. It cannot be reversed.
Hence, the spontaneous flow of heat is always unidirectional from lower temperature to higher temperature.

So, option C is correct.

Note: A thermodynamic system tends to move from an unstable state to a stable one. An unstable state has a higher temperature. So, the system tends to lose some energy in the form of heat and move towards a stable state which is the state having a lower temperature.