The entropy change of the universe refers to the overall change in the level of disorder or randomness in the universe. It is a measure of the energy that is no longer available to do work.
The entropy change of the universe is calculated using the laws of thermodynamics, which take into account the energy and entropy changes within a system as well as the exchange of energy and matter with its surroundings.
The 6kg block and lake system will cause an increase in the entropy of the universe due to the transfer of heat energy from the hotter block to the cooler lake. This transfer of energy results in an increase in the overall disorder of the universe.
According to the second law of thermodynamics, the entropy of the universe will always increase over time. While individual systems may experience a temporary decrease in entropy, the overall change of the universe will always result in an increase in entropy.
The entropy change of the universe is closely related to the principle of energy conservation. As the universe becomes more disordered, the amount of energy available to do work decreases, leading to a decrease in the overall level of energy in the universe. This is why the second law of thermodynamics is often referred to as the "arrow of time" or the idea that the universe is constantly moving towards a state of maximum disorder.