2.11 Probability, Thermal Equilibrium, and Entropy

Entropy

1. Reversible - these processes can be done forward or backwards. These processes do not increase the entropy of the universe: entropy stays constant. A famous example of this is the carnot cycle. Carnot cycles are the most efficient. They consist of 2 adiabatic processes and 2 isothermal processes. The gas is returned to its original state and no increase in entropy occurs.
2. Irreversible - these processes can only be done in one direction. The entropy of the universe does increase due to irreversible processes. All real life engines perform irreversible processes. Heat pumps and refrigerators are some of the most common examples.

Heat Engines and Refrigerators

1. Heat Engines - heat is imputed from a high temperature then some is converted to work and the rest in released as low temperature 🔥

1. Refrigerators/Heat Pumps - work is done in conjunction with low temperature to kick out higher temperature heat 🧊

• A heat engine is a device that converts heat energy into mechanical work.
• Heat engines operate on a cycle, which involves the transfer of heat from a hot reservoir to a cold reservoir through the engine.
• The hot reservoir is typically a source of high-temperature heat, such as burning fuel or steam from a boiler. The cold reservoir is typically the environment, which is at a lower temperature.
• Heat is used to do work by expanding a gas or fluid, such as steam in a steam engine.
• The efficiency of a heat engine is the ratio of the work it does to the heat it receives. The efficiency is limited by the temperature difference between the hot and cold reservoirs.
• Examples of heat engines include steam engines, internal combustion engines, and gas turbines.

• A heat pump is a device that moves heat from one location to another. It can transfer heat from a cold location to a warm location (such as a heating system), or from a warm location to a cold location (such as an air conditioner).
• A heat pump uses a small amount of mechanical work to transfer a large amount of heat. It is more efficient than a heat engine because it can operate at a higher temperature difference between the hot and cold reservoirs.
• A refrigeration system is a type of heat pump that is used to cool a space or to preserve perishable goods. It works by removing heat from the cooled space and transferring it to the environment.
• A refrigeration system typically consists of a compressor, a condenser, an expansion valve, and an evaporator. The refrigerant (a fluid that can easily change between a gas and a liquid) absorbs heat in the evaporator and releases it in the condenser. The compressor raises the pressure of the refrigerant, which increases its temperature. The expansion valve lowers the pressure of the refrigerant, which lowers its temperature.
• The efficiency of a refrigeration system is limited by the temperature difference between the cold and hot reservoirs and by the properties of the refrigerant.

To answer this question, you could describe the second law of thermodynamics, which states that the total entropy of a closed system will always increase over time. Define entropy as a measure of the disorder or randomness of a system, and explain how it is a state function, meaning that it depends only on the state of the system and not on the path taken to reach that state.

An example of a reversible process, such as a gas expanding and contracting in a cylinder, and explain how the entropy changes in a reversible process. Give an example of an irreversible process, such as a gas expanding into a vacuum, and explain how the entropy changes in an irreversible process. Explain how the increase in entropy in irreversible processes is what drives the arrow of time and why the past and future are fundamentally different.