Topic 2_Ch 4_"Energy"

Topic 2: Energy and Radiation Balances

Chapter 4: Energy

• Introduction

  • Energy: The capacity to do work

    • Perceived as light

    • Felt as heat

    • Experienced through movement

  • Law of Conservation of Energy: Energy can neither be created nor destroyed; it can transform from one form to another or transfer between objects.

Energy Transfer Methods

• Energy Transfer Types:

  1. As Heat: Flow of energy from warmer to colder objects.

     • Conduction: Transfer of energy between molecules in contact.

     • Convection: Transfer of energy via vertical fluid motion.

     • Radiation: Transfer via electromagnetic waves.

  2. As Work: Involves mechanical energy transfer, such as adiabatic heating/cooling in the atmosphere through expansion or contraction.

Page 2: Heat

• Definition of Heat: Energy transferred due to a temperature difference.

• Effects of Heat:

  • Increases molecular movement leading to higher kinetic energy.

• Temperature: Measures the average speed of molecules.

Internal Energy

• Total Energy: Comprises both thermal and potential energy.

  • Thermal Energy: Increases kinetic energy leading to temperature changes (sensible heat transfers).

  • Potential Energy: Alters molecular attractive forces, resulting in phase changes (latent heat transfers).

Sensible Heat

• Definition: Heat transfers resulting in temperature changes.

  • Objects feel warmer or colder.

  • Different substances warm differently under the same heating conditions.

• Heat Flow Calculation:

  • Q = heat flow in joules, m = mass in kg, ΔT = temperature change in Kelvin or Celsius, c = specific heat.

Page 3: Sensible Heat and Latent Heat

Specific Heat

• Defined as the amount of energy needed to raise the temperature of a unit mass by one degree K.

Latent Heat

• Definition: Heat absorbed or released during phase changes without temperature change (energy is “hidden”).

• Processes that Absorb Latent Heat:

  • Melting, Evaporation, Sublimation.

• Processes that Release Latent Heat:

  • Freezing, Condensation, Deposition.

Phase Changes of Water

• Visual Aid: Figure 4.2 shows phase changes of water.

Page 4: Gases and Work

• Work Definition: Energy transfer through mechanical means.

  • Example: A person pushing a shopping cart.

• Gas Expansion:

  • Expanding gas does work on surroundings leading to decreased internal energy.

• Gas Compression:

  • Requires work on the gas, increasing internal energy.

• Work Calculation:

  • W = F Δx, W = PA Δx, W = P ΔV.

Page 5: The First Law of Thermodynamics

• Temperature Increase Methods:

  1. Adding heat.

  2. Doing work on the gas.

• Internal Energy Relation:

  • Q = heat transferred, U = change in internal energy, W = work done.

  • Energy relation in terms of internal energy: m c ΔT (heat) and P ΔV (work).

• Understanding internal energy: All heat transferred must result in a temperature change, work, or both.

Constant Volume Processes

• Air confined at constant volume: All added heat leads to temperature rise.

• Specific Heat Constant for Volume Process (cv): 717 J ∙ kg–1 ∙ K–1.

Page 6: Constant Pressure & Adiabatic Processes

Constant Pressure Processes

• Unconfined air (constant pressure): Some heat contributes to expansion, increasing both temperature and volume.

• Specific Heat Constant for Pressure Process (cp): 1004 J ∙ kg–1 ∙ K–1.

Adiabatic Processes

• Definition: Temperature change without heat transfer, driven by work.

• Examples: Air rising (cooling due to expansion) or descending (heating due to compression).

• Adiabatic Process Equation:

  • cp ΔT represents internal energy change, ΔP/ρ represents work from gas expansion/compression.

Page 7: Atmospheric Adiabatic Processes

• Behavior of Air:

  • Pressure decreases with height; air expands while rising or contracts when descending.

  • Changes in pressure cause adiabatic heating/cooling.

• Diabatic Processes:

  • Heat transfer processes include conduction, convection, radiation, and phase changes.

  • Real-world processes often involve a combination of both adiabatic and diabatic transfers.

Page 8: Heat Transfers

Conduction

• Definition: Heat transfers from one molecule to another directly.

  • Faster molecules share energy with slower ones.

  • Conductivity generally highest in solids.

• Boundary Layer:

  • Laminar boundary layer of air in contact with the ground warms by conduction and transfers heat upward through convection.

  • Surface heats ground by conduction during the day, reversing at night.

Convection

• Definition: Heat transfer through liquid and gas movement (fluids).

• Types of Convection:

  1. Thermal Convection (free convection): Driven by density differences; warm air rises due to lower density.

  2. Mechanical Convection (forced convection): Driven by mechanical forces such as wind.

Radiation

• Definition: Energy travels as electromagnetic waves (light, microwaves, radio).

• Emission and Absorption:

  • Hotter substances emit more radiation than cooler substances.

Page 9: Heat Transfers at Earth's Surface

• Daytime Heat Transfer: Solar radiation warms the surface, heat is transferred upwards into the atmosphere and downwards into the ground.

• Night-time Heat Transfer: Emission of radiation cools the surface. Heat is transferred down from the atmosphere to the surface and upwards from the ground.