Thermodynamics I

🔥Thermodynamics I Unit 4 – Energy Analysis of Closed Systems

Energy analysis of closed systems is a fundamental concept in thermodynamics. It focuses on how energy transfers and transforms within systems that don't exchange mass with their surroundings. This unit covers key principles like the first law of thermodynamics, work, heat transfer, and internal energy. Students learn to apply these concepts to real-world scenarios, using problem-solving strategies and thermodynamic property tables. Understanding closed system analysis is crucial for grasping more complex thermodynamic processes and applications in engineering and science.

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Key Concepts and Definitions

  • Thermodynamics studies the relationships between heat, work, and energy in systems
  • A system is a region in space or quantity of matter under study
  • The surroundings include everything external to the system
  • The system boundary separates the system from its surroundings
  • A closed system allows energy transfer across its boundaries but no mass transfer
  • An open system allows both mass and energy transfer across its boundaries
  • State refers to the condition of a system, typically described by properties like temperature, pressure, and volume
  • A process is a change in the state of a system from an initial state to a final state

Energy Forms and Transfer

  • Energy is the capacity to do work or transfer heat
  • Kinetic energy is associated with the motion of an object
  • Potential energy is associated with the position or configuration of an object
  • Internal energy is the sum of the microscopic kinetic and potential energies of a system
  • Heat is the transfer of energy due to a temperature difference between a system and its surroundings
    • Occurs from high temperature to low temperature regions
    • Represented by the symbol QQ
  • Work is the transfer of energy due to a force acting through a distance
    • Occurs when a system interacts with its surroundings, causing a displacement
    • Represented by the symbol WW

First Law of Thermodynamics

  • The first law of thermodynamics is an expression of the conservation of energy principle
  • It states that energy cannot be created or destroyed, only converted from one form to another
  • For a closed system, the change in the system's energy equals the net energy transfer to the system
  • Mathematically expressed as ΔE=QW\Delta E = Q - W
    • ΔE\Delta E is the change in the system's total energy
    • QQ is the net heat transfer to the system (positive if heat is added, negative if heat is removed)
    • WW is the net work done by the system (positive if work is done by the system, negative if work is done on the system)
  • The first law provides a framework for analyzing energy changes in thermodynamic processes

Closed System Analysis

  • In a closed system, no mass crosses the system boundary, but energy can be exchanged as heat or work
  • The first law of thermodynamics is applied to analyze energy changes in closed systems
  • The change in the system's energy is determined by the net heat transfer and net work done
  • Examples of closed systems include:
    • A piston-cylinder device with a fixed mass of gas
    • A sealed, insulated container with a fixed quantity of liquid
  • Analyzing closed systems involves identifying the system boundary, initial and final states, and energy interactions

Work in Thermodynamic Systems

  • Work is an important mode of energy transfer in thermodynamic systems
  • Mechanical work occurs when a force acts through a distance, such as a piston moving in a cylinder
  • Electrical work involves the flow of electric current through a potential difference
  • Shaft work is associated with the rotation of a shaft, such as in a turbine or compressor
  • Boundary work (or PVPV work) is the work done by a system due to changes in its volume against an external pressure
    • Calculated using W=PdVW = \int P dV, where PP is the external pressure and dVdV is the change in volume
  • The sign convention for work is positive when done by the system and negative when done on the system

Internal Energy and Enthalpy

  • Internal energy (UU) is the sum of the microscopic kinetic and potential energies of a system
  • It is a state function, meaning its change depends only on the initial and final states, not the path taken
  • Enthalpy (HH) is another thermodynamic property defined as H=U+PVH = U + PV
    • UU is the internal energy, PP is the pressure, and VV is the volume
  • Enthalpy is useful when analyzing processes at constant pressure, such as in open systems
  • Changes in internal energy and enthalpy are related to heat transfer and work in a process
  • For a closed system, ΔU=QW\Delta U = Q - W and ΔH=QP\Delta H = Q_P, where QPQ_P is the heat transfer at constant pressure

Specific Heats and Energy Tables

  • Specific heat is the amount of energy required to raise the temperature of a unit mass of a substance by one degree
  • Specific heat at constant volume (cvc_v) is the specific heat when the volume is held constant
  • Specific heat at constant pressure (cpc_p) is the specific heat when the pressure is held constant
  • The specific heats are related by cpcv=Rc_p - c_v = R, where RR is the specific gas constant
  • Thermodynamic properties, including specific heats, are often tabulated in energy tables for various substances
    • Tables provide data at different temperatures and pressures
    • Interpolation or extrapolation may be necessary for values not directly listed
  • Using specific heats and energy tables simplifies the calculation of energy changes in thermodynamic processes

Problem-Solving Strategies

  • Identify the system and its boundaries, specifying whether it is open or closed
  • Determine the initial and final states of the system
  • Apply the first law of thermodynamics, considering heat transfer and work interactions
  • Use appropriate sign conventions for heat and work (positive for heat added and work done by the system, negative for heat removed and work done on the system)
  • Employ specific heat and energy table data when necessary to calculate energy changes
  • Consider assumptions and simplifications, such as constant pressure or temperature processes
  • Analyze the results and check for consistency with physical laws and principles
  • Perform unit conversions and dimensional analysis to ensure proper units in the solution


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.