Heat, Work, and Energy in Thermodynamics

Objectives

• Define Thermodynamics and Internal Energy
• Explain the 1st law of thermodynamics.
• Demonstrate that heat can be turned into work.
• Predict the convention signs of heat and work in a system.
• Calculate the change in internal energy within a system.

Heat (Q)

• Definition: Heat is the thermal energy that flows from a substance of higher temperature to one of lower temperature.
• Formula: $Q = m imes c imes riangle T$ Where:
  • $Q$ = Heat in Joules or Calories
  • $m$ = mass in grams
  • $c$ = specific heat capacity ($J/g/°C$)
  • $riangle T$ = Change in temperature

Work (W)

• Definition: Work is the energy transferred when an object is moved against a force.
• Formula: $W = F imes D$ Where:
  • $W$ = Work in Joules
  • $F$ = Force in Newtons
  • $D$ = Distance in Meters

Thermodynamics

• Definition: A branch of physics that deals with physical laws relating heat and mechanical work; the science of energy transfer between systems and surroundings.
• Internals of Thermodynamics: A system is any region enclosed within a boundary; everything outside it constitutes its surroundings.

Internal Energy (U)

• Definition: The total kinetic energy from the motion of molecules and potential energy related to the arrangement of molecules and their atomic components.
• Change Mechanisms:
  • Heating the system.
  • Doing work on the system.
  • Adding or removing matter.

Energy Changes in a System

• Energy Inputs:

  • +Q: Heat added to the system
  • +W: Work done on the system

• Energy Outputs:

  • -Q: Heat released by the system
  • -W: Work done by the system

System Types

• Open System: Exchanges both energy and matter.
• Closed System: Exchanges only energy, not matter.
• Isolated System: Exchanges neither matter nor energy.

First Law of Thermodynamics

• Conservation of Energy: Total energy of a system is constant; it can be transformed but not created or destroyed.
• Implications:
  • Change in internal energy $riangle U$ is equal to the heat added to the system (Q) minus the work done by the system (W).
  • Formula:
    $riangle U = Q - W$

Units for Heat and Work

• Units include Joules (J), Kilojoules (kJ), Calories, Kilocalories, and BTU.
• Understanding Signs:
  • Q > 0 when heat is absorbed; Q < 0 when heat is released.
  • W > 0 when work is done by the system; W < 0 when work is done on the system.

Applications of First Law of Thermodynamics

• Examine processes in systems like heating water or operating engines.

Second Law of Thermodynamics

• States heat naturally flows from hot to cold without external energy.
• Examples of spontaneous processes include melting ice and rust formation.

Heat Pumps

• Devices that transfer heat from a cold area to a warm area, which requires work.
• Common examples: refrigerators and air conditioners.

Human Metabolism and Thermodynamics

• Metabolism as energy conversion in the body.
• Internal energy can be influenced by heat transfer during metabolic processes.
• Importance of maintaining balance in energy intake (food) and expenditure (work and heat transfer).

Summary of Key Concepts

• Heat transfer, work done, and internal energy are critical in understanding thermodynamic systems.
• The 1st Law describes how these elements interplay in various processes, including biological and mechanical systems.