🫴Physical Science Unit 10 – Thermodynamics and Heat Transfer

Key Concepts and Definitions

• Thermodynamics studies the relationships between heat, work, temperature, and energy
• Heat is the transfer of thermal energy from a hotter object to a cooler object
• Temperature measures the average kinetic energy of particles in a substance
  • Measured using scales such as Celsius (°C), Fahrenheit (°F), and Kelvin (K)
• Thermal equilibrium occurs when two objects in contact have the same temperature and no net heat transfer
• Specific heat capacity is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C
• Latent heat is the energy absorbed or released during a phase change without a change in temperature
  • Latent heat of fusion is the energy required to change a substance from solid to liquid (melting)
  • Latent heat of vaporization is the energy required to change a substance from liquid to gas (boiling)

Laws of Thermodynamics

• The zeroth law states that if two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other
• The first law states that energy cannot be created or destroyed, only converted from one form to another
  • Mathematically expressed as $\Delta U = Q - W$, where $\Delta U$ is the change in internal energy, $Q$ is the heat added, and $W$ is the work done by the system
• The second law states that the total entropy of an isolated system always increases over time
  • Entropy is a measure of the disorder or randomness of a system
• The third law states that the entropy of a perfect crystal at absolute zero is zero
• These laws govern the behavior of energy in thermodynamic systems and processes

Heat Transfer Mechanisms

• Conduction is the transfer of heat through direct contact between particles of a substance
  • Occurs in solids, liquids, and gases
  • Rate of conduction depends on the temperature gradient, cross-sectional area, and thermal conductivity of the material
• Convection is the transfer of heat by the movement of fluids (liquids or gases)
  • Occurs due to differences in density caused by temperature variations
  • Examples include hot air rising and cold air sinking, or the circulation of water in a pot on a stove
• Radiation is the transfer of heat through electromagnetic waves
  • Does not require a medium and can occur in a vacuum
  • Emitted by all objects with a temperature above absolute zero
  • Examples include the sun's energy reaching Earth and the heat from a fire

Thermodynamic Systems and Processes

• A thermodynamic system is a region of the universe under study, separated from its surroundings by a boundary
  • Open systems can exchange both matter and energy with their surroundings
  • Closed systems can exchange energy but not matter with their surroundings
  • Isolated systems cannot exchange either matter or energy with their surroundings
• Isothermal processes occur at constant temperature
  • Example: a gas expanding or contracting in a piston while in contact with a heat reservoir
• Adiabatic processes occur without heat transfer to or from the surroundings
  • Example: rapid compression or expansion of a gas in an insulated piston
• Isobaric processes occur at constant pressure
• Isochoric (or isovolumetric) processes occur at constant volume

Energy and Work

• Energy is the capacity to do work or cause change
  • Kinetic energy is the energy of motion, calculated as $KE = \frac{1}{2}mv^2$, where $m$ is mass and $v$ is velocity
  • Potential energy is the energy stored in an object due to its position or configuration
    • Examples include gravitational potential energy and elastic potential energy
• Work is the energy transferred when a force acts through a distance
  • Calculated as $W = F \cdot d$, where $F$ is the force and $d$ is the displacement
• Power is the rate at which work is done or energy is transferred
  • Calculated as $P = \frac{W}{t}$, where $W$ is work and $t$ is time
• The relationship between energy, work, and heat is described by the first law of thermodynamics

Entropy and the Second Law

• Entropy is a measure of the disorder or randomness of a system
  • A system's entropy increases as it becomes more disordered or random
• The second law of thermodynamics states that the total entropy of an isolated system always increases over time
  • This means that heat flows naturally from hot objects to cold objects, not the other way around
  • It also implies that no process can be 100% efficient, as some energy is always lost as waste heat
• The arrow of time is a consequence of the second law, as entropy increases in the direction of the future
• The concept of entropy helps explain why certain processes are irreversible and why perpetual motion machines are impossible

Applications in Engineering and Technology

• Heat engines convert thermal energy into mechanical work
  • Examples include internal combustion engines (gasoline, diesel) and steam turbines
  • Efficiency is limited by the second law of thermodynamics and depends on the temperature difference between the hot and cold reservoirs
• Refrigerators and heat pumps move heat from a cold reservoir to a hot reservoir, using work input
  • Used for cooling (refrigerators, air conditioners) and heating (heat pumps)
• Thermal insulation reduces heat transfer between a system and its surroundings
  • Used in buildings, clothing, and industrial processes to maintain desired temperatures and save energy
• Thermodynamic principles are applied in the design of power plants, HVAC systems, and energy-efficient devices

Problem-Solving Techniques

• Identify the thermodynamic system and its boundaries
• Determine the type of process (isothermal, adiabatic, isobaric, or isochoric)
• Apply the relevant laws of thermodynamics and equations
  • Use the first law ($\Delta U = Q - W$) to analyze energy conservation
  • Use the second law to determine the direction of heat flow and entropy changes
• Consider the initial and final states of the system, and calculate changes in properties such as temperature, pressure, volume, and entropy
• Use tables and charts for properties of specific substances (e.g., steam tables, ideal gas tables)
• Break down complex problems into smaller, manageable steps
• Double-check units and ensure consistency throughout the problem-solving process