Entropy is a fundamental concept in thermodynamics that relates to the degree of disorder or randomness in a system. It is often described as a measure of the system's microscopic arrangements or the number of possible ways in which its particles can be arranged.
More formally, entropy can be defined as a state function that quantifies the distribution of energy in a system and its availability to do work. It is denoted by the symbol S and is typically measured in units of joules per kelvin (J/K) or entropy units (J/K/mol).
The second law of thermodynamics states that the entropy of an isolated system tends to increase over time. This principle is often referred to as the "arrow of time" and is associated with the concept of irreversibility. In practical terms, it means that processes in nature tend to move towards a state of higher disorder or randomness.
Regarding your second question, when heat moves from the surroundings to the system, it does not always result in an increase in entropy. The change in entropy of a system depends on the specific conditions and the nature of the process.
If the heat transfer occurs at a constant temperature (isothermal process), such as in a reversible heat transfer, the change in entropy of the system will be given by ΔS = Q/T, where Q is the heat transferred, and T is the temperature. In this case, if heat is transferred from the surroundings to the system (Q > 0), the change in entropy will be positive (ΔS > 0) as long as the temperature is positive.
However, if the heat transfer is not isothermal or occurs under non-equilibrium conditions, the change in entropy can be more complex. It is possible for the entropy of the system to decrease (ΔS < 0) even if heat is transferred from the surroundings to the system. This occurs when the increase in the entropy of the surroundings compensates for the decrease in the system's entropy, resulting in an overall increase in the entropy of the combined system.
Overall, the change in entropy depends on the specific thermodynamic process and the overall system considered, and it is not solely determined by the direction of heat transfer.