First law of thermodynamics

The first law of thermodynamics states that energy is conserved: a system’s internal energy changes through heat transfer and work, not through creation or destruction of energy.

Internal energy: The energy associated with the microscopic states and interactions of particles within a system. Its change is the quantity the first law accounts for.
Thermodynamic state function: A property whose value depends only on a system’s state, not on the path taken to reach it. Internal energy is a state function, so its change depends only on endpoints.
Heat engine: A device that converts heat from a hot source partly into work while rejecting heat to a colder sink. Its energy balance shows why heat input divides between work and rejected heat.
Second law of thermodynamics: The law that constrains the direction of thermodynamic processes through entropy and limits the conversion of heat into work. Energy conservation alone does not determine which processes can occur spontaneously.
Heat: Energy transferred between systems because of a temperature difference. Heat transfer can increase or decrease a system’s internal energy.
State variable: A measurable property that specifies part of a system’s thermodynamic state. State variables characterize the initial and final conditions in an energy balance.
Refrigerator: A device that uses work to transfer heat from a colder region to a warmer one. The first law accounts for the work input and heat removed and delivered.
Perpetual motion machine of the first kind: A hypothetical device that produces more energy than it receives, violating energy conservation. The first law rules out this class of proposed machines.
Thermodynamic work: Energy transfer between a system and its surroundings through macroscopic forces acting across the boundary. Work is the other principal way energy crosses a system boundary.
Quasistatic process: An idealized process carried out slowly enough that a system remains near equilibrium throughout. It makes work calculable from successive equilibrium states.

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