3.3 Work in thermodynamic processes
3 min read•july 23, 2024
is all about energy transfer between a system and its surroundings. It happens when forces act through displacements, like a piston moving in a cylinder. Understanding work is key to grasping how energy flows in various processes.
Calculating work depends on the specific process - isothermal, isobaric, isochoric, or adiabatic. P-V diagrams help visualize work done in cycles. The difference between reversible and highlights real-world inefficiencies in thermodynamic processes.
Work in Thermodynamic Processes
Definition of thermodynamic work
Top images from around the web for Definition of thermodynamic work
Work (thermodynamics) - Wikipedia View original
Is this image relevant?
The First Law of Thermodynamics and Some Simple Processes | Physics View original
Is this image relevant?
Thermodynamic process path - Wikipedia View original
Is this image relevant?
Work (thermodynamics) - Wikipedia View original
Is this image relevant?
The First Law of Thermodynamics and Some Simple Processes | Physics View original
Is this image relevant?
1 of 3
Top images from around the web for Definition of thermodynamic work
Work (thermodynamics) - Wikipedia View original
Is this image relevant?
The First Law of Thermodynamics and Some Simple Processes | Physics View original
Is this image relevant?
Thermodynamic process path - Wikipedia View original
Is this image relevant?
Work (thermodynamics) - Wikipedia View original
Is this image relevant?
The First Law of Thermodynamics and Some Simple Processes | Physics View original
Is this image relevant?
1 of 3
- Energy transfer between a system and its surroundings due to a force acting through a displacement (piston moving in a cylinder)
- Occurs when a system changes its external parameters (volume, position)
- (PdV work) is due to a change in its volume against an external pressure
- Expressed as , where is the external pressure and is the change in volume
- is work done by a system on its surroundings through the rotation of a shaft (turbine, pump)
- Expressed as , where is the torque and is the angular displacement
Calculation of work in processes
- maintains constant temperature
- Work done is , where is the number of moles, is the universal gas constant, is the temperature, and and are the initial and final volumes
- maintains constant pressure
- Work done is , where is the constant pressure
- Isochoric process maintains constant volume
- No work is done, as
- has no heat transfer between the system and its surroundings
- Work done is , where is the ratio of specific heats
Work analysis in P-V diagrams
- Work done in a thermodynamic cycle is represented by the area enclosed by the
- (work done by the system) is a counterclockwise cycle
- (work done on the system) is a clockwise cycle
- Calculate the work done by integrating the pressure with respect to volume over the entire cycle
- , where the closed integral represents the integration over the complete cycle
Reversible vs irreversible work
- is the maximum amount of work that can be obtained from a system undergoing a process between two equilibrium states
- Occurs when the process is carried out infinitely slowly, with the system always in equilibrium with its surroundings
- Irreversible work is the actual amount of work obtained from a system undergoing a process between two equilibrium states
- Occurs when the process is carried out at a finite rate, with the system not always in equilibrium with its surroundings
- The difference between reversible and irreversible work represents the lost work due to irreversibilities in the process
- Irreversibilities (friction, heat transfer across finite temperature differences, unrestrained expansions) reduce the amount of useful work that can be obtained
- is the ratio of the actual work output to the maximum possible (reversible) work output
- Efficiency is always less than 100% for real (irreversible) processes
Key Terms to Review (20)
Adiabatic process: An adiabatic process is a thermodynamic process in which no heat is exchanged between the system and its surroundings. This means that any change in the internal energy of the system is entirely due to work done on or by the system, making it a critical concept in understanding various thermodynamic cycles and processes.
Boundary work: Boundary work refers to the energy transfer associated with the movement of a system's boundary during a thermodynamic process. This concept is crucial in understanding how work is done by or on a system when its volume changes, as it directly relates to the pressure and volume of gases and liquids. The type of process (like expansion or compression) influences the amount of work done, making it a key factor in analyzing thermodynamic cycles and efficiency.
Irreversible work: Irreversible work refers to the work done by a system during a thermodynamic process that cannot be completely converted back into usable energy, usually due to dissipative factors like friction or turbulence. This concept is essential for understanding real-world thermodynamic processes, as they often involve irreversible transformations that limit efficiency and performance compared to ideal processes. The recognition of irreversible work helps in evaluating the limitations and potential of energy systems in practical applications.
Isobaric Process: An isobaric process is a thermodynamic process that occurs at constant pressure. In such a process, any heat transfer into or out of the system results in a change in volume, while the pressure remains unchanged. This constancy of pressure plays a significant role in various energy exchanges and mechanical work done by or on the system.
Isothermal process: An isothermal process is a thermodynamic process in which the temperature of the system remains constant while heat is exchanged with the surroundings. This constant temperature implies that any internal energy changes in the system are fully compensated by heat transfer, making it an essential concept in understanding how systems behave under thermal equilibrium and the laws governing energy conservation.
Negative Work: Negative work refers to the situation in thermodynamics where the work done by a system on its surroundings is in the opposite direction of the displacement, resulting in a loss of energy from the system. This typically occurs when a force is applied against the motion of a system, such as when a gas expands against an external pressure or when a piston is pushed down, causing compression. Understanding negative work is crucial for analyzing energy transfers and efficiency in various thermodynamic processes.
P-v diagram: A p-v diagram, or pressure-volume diagram, is a graphical representation that illustrates the relationship between the pressure and volume of a thermodynamic system during various processes. This diagram is crucial for visualizing work done by or on the system, as well as understanding different thermodynamic cycles, including how heat engines operate and the efficiency of these processes. The area under the curve in a p-v diagram corresponds to the work performed during the process, making it an essential tool for analyzing both practical applications and theoretical concepts in thermodynamics.
Piston-Cylinder System: A piston-cylinder system is a common mechanical assembly used to contain and control the movement of fluids or gases through a cylinder with a movable piston. This setup is crucial for understanding how work is done in thermodynamic processes, as the piston can compress or expand the gas inside the cylinder, resulting in energy transfer in the form of work.
Positive Work: Positive work refers to the energy transfer that occurs when a system expands against an external pressure, resulting in an increase in the system's volume. This concept is crucial in understanding how energy is transformed and conserved during thermodynamic processes, and it highlights the relationship between work and changes in a system's state. Positive work indicates that energy is being supplied to the surroundings, playing a key role in various physical and engineering applications.
Reversible Work: Reversible work is the maximum amount of work that can be extracted from a thermodynamic process when it occurs in such a way that the system and surroundings can be returned to their original states without any net changes. This concept highlights ideal processes where the system undergoes infinitesimal changes, ensuring that the process can be reversed with no losses due to friction or other dissipative effects, making it a crucial consideration in understanding efficiency in thermodynamic processes.
Shaft Work: Shaft work refers to the mechanical energy transferred to or from a system through the rotation of a shaft, typically as a result of forces acting on the shaft. This type of work is essential in many thermodynamic processes, as it directly relates to the energy conversion and performance of engines and turbines. Understanding shaft work helps to analyze how systems convert thermal energy into mechanical energy, allowing for efficient operation in various applications like power generation and propulsion.
Thermodynamic Efficiency: Thermodynamic efficiency is a measure of how well a system converts energy from one form to another, specifically the ratio of useful work output to the total energy input. It is important because it helps evaluate the performance of various processes and systems, indicating how much energy is conserved and how much is wasted. Understanding this concept is crucial for optimizing systems in various applications, including mechanical engines, biological processes, and assessing the irreversibility in thermodynamic cycles.
W = -pδv: The equation w = -pδv represents the work done during a thermodynamic process, where 'w' is the work, 'p' is the pressure, and 'δv' is the change in volume. This formula indicates that work can be calculated as the negative product of pressure and the change in volume of a system. The negative sign signifies that when a system expands, it does work on its surroundings, which decreases its internal energy. Understanding this equation helps in analyzing energy transfers in thermodynamic processes.
W = ∫ p dv: The expression $$w = \int p \, dv$$ represents the work done by or on a system during a thermodynamic process, where 'w' denotes work, 'p' is the pressure of the system, and 'dv' signifies a change in volume. This integral essentially sums up the incremental work done as the system undergoes a change in volume, reflecting how pressure impacts work. Understanding this equation is crucial as it connects mechanical and thermodynamic principles, illustrating how energy is transferred during various processes.
W = ∮ p dv: The expression $$w = \oint p \ dv$$ represents the work done by or on a system during a thermodynamic process, calculated as the integral of pressure ($$p$$) with respect to volume ($$v$$) over a closed path. This formula emphasizes the importance of pressure-volume relationships in understanding how energy is transferred in thermodynamic systems, especially during cyclic processes. It highlights that work can be path-dependent, influenced by the specific thermodynamic cycle the system undergoes.
W = p(v2 - v1): The equation w = p(v2 - v1) represents the work done by a gas during a thermodynamic process, where 'w' is the work, 'p' is the pressure, and 'v2' and 'v1' are the final and initial volumes, respectively. This equation highlights the relationship between pressure and volume changes in a system, showing how work can be quantified when a gas expands or contracts. Understanding this term is crucial for analyzing energy transfers in various thermodynamic processes, as it connects mechanical work to changes in state variables like volume and pressure.
Work Done by a System: Work done by a system refers to the energy transfer that occurs when a system exerts a force over a distance during a thermodynamic process. This concept is crucial for understanding how systems interact with their surroundings and the energy exchanges that happen during processes like expansion and compression. It highlights the relationship between mechanical work and thermodynamic principles, which helps to clarify energy conservation in different physical situations.
Work done on a system: Work done on a system refers to the energy transferred to that system when an external force is applied to it, causing displacement. This concept is crucial in thermodynamics as it describes how energy changes occur when a system interacts with its surroundings, influencing properties such as pressure, volume, and temperature. Understanding this process is essential for analyzing various thermodynamic processes, including expansion and compression, where energy transfer plays a pivotal role.
Work in Thermodynamic Processes: The equation $$w = \frac{p_1 v_1 - p_2 v_2}{\gamma - 1}$$ represents the work done during a thermodynamic process where pressure and volume change. In this context, 'w' stands for work, 'p' signifies pressure at different states, 'v' denotes volume, and '\gamma' is the heat capacity ratio. This equation is crucial as it links thermodynamic principles to practical calculations of work output or input during processes involving gases.
Work in thermodynamics: Work in thermodynamics is the energy transfer that occurs when a force is applied to an object, causing it to move. In this context, work can be done by the system or on the system during various processes, such as expansion or compression, and it plays a crucial role in the first law of thermodynamics by linking energy transformations.