Glossary
Arrow of Time
The tendency of an isolated system to move toward a state of higher disorder, explaining why time appears to only move forward.
Example:
The continuous aging of a building, where it naturally degrades and becomes more disordered over time, illustrates the arrow of time.
Carnot Cycle
The most efficient theoretical thermodynamic cycle, consisting of two adiabatic and two isothermal processes, where the system returns to its original state with no increase in entropy.
Example:
Engineers use the theoretical efficiency of the Carnot cycle as a benchmark to evaluate how well real-world heat engines perform.
Cold Reservoir
The environment or a sink at a lower temperature to which a heat engine expels waste heat or from which a refrigerator absorbs heat.
Example:
The surrounding air outside a refrigerator acts as the cold reservoir where the absorbed heat is released.
Entropy (S)
A measure of disorder, molecular freedom, randomness, or lack of predictability within a system.
Example:
A perfectly organized bookshelf has low entropy, but after a toddler plays with it, the books are scattered, representing high entropy.
Heat Engines
Devices that convert heat energy into mechanical work by transferring heat from a hot reservoir to a cold reservoir in a cyclical process.
Example:
The internal combustion engine in a car is a heat engine that uses the heat from burning fuel to move the vehicle.
Heat Pumps
Devices that move heat from one location to another, either from a cold location to a warm one (heating) or vice versa (cooling), using mechanical work.
Example:
In winter, a heat pump can extract heat from the cold outside air and transfer it indoors to warm a house.
Hot Reservoir
A source of high-temperature heat from which a heat engine or refrigerator draws energy.
Example:
In a power plant, the steam generated by burning fuel acts as the hot reservoir for the turbines.
Irreversible Processes
Real-world thermodynamic processes that can only proceed in one direction, always resulting in an increase in the total entropy of the universe.
Example:
A dropped glass shattering on the floor is an irreversible process; it won't spontaneously reassemble itself.
Molecular Freedom
Refers to the extent of movement and arrangement possibilities available to molecules within a system.
Example:
Water vapor has greater molecular freedom than ice, as its molecules can move and arrange themselves much more freely.
Randomness
The lack of predictability in a system, often associated with a higher number of possible arrangements for its components.
Example:
Shuffling a deck of cards increases its randomness, making it highly improbable to return to its original ordered state.
Refrigerators
Devices that use mechanical work to transfer heat from a colder location to a warmer location, effectively cooling the cold space.
Example:
Your kitchen refrigerator uses a refrigerant and work to move heat out of its insulated compartment, keeping food cold.
Reversible Processes
Idealized thermodynamic processes that can proceed in both forward and backward directions without any net change in the entropy of the universe.
Example:
A theoretical frictionless pendulum swinging back and forth perfectly without losing energy would be an example of a reversible process.
Second Law of Thermodynamics
States that the total entropy of a system and its surroundings can never decrease; it either stays constant in ideal, reversible processes or increases in real-world, irreversible processes.
Example:
When a hot cup of coffee cools down in a room, the Second Law of Thermodynamics dictates that the overall disorder of the coffee-room system increases.
State Function
A property of a system that depends only on its current state, not on the path taken to reach that state.
Example:
The altitude of a mountain climber is a state function; it only depends on their current height, not the specific trail they took to get there.
Thermodynamics
The branch of physics that deals with heat and its relation to other forms of energy and work.
Example:
Studying how a car engine converts fuel into motion involves understanding the principles of thermodynamics.