Topic 2 Ch 4 Energy

Topic 2: Energy and Radiation Balances

Chapter 4: Energy

Introduction

• Energy is the capacity to do work.

  • Forms of Energy:

    • Seen as light,

    • Felt as heat,

    • Experienced as movement.

• Law of Conservation of Energy: Energy cannot be created or destroyed but can be transformed and transferred.

Energy Transfer Mechanisms

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

   • Conduction: Transfer between molecules in contact.

   • Convection: Transfer by vertical motions in fluids.

   • Radiation: Transfer via electromagnetic waves.

2. As Work:

   • Adiabatic heating and cooling occur when the atmosphere expands or contracts.

Heat

Definition and Characteristics

• Heat is energy transferred due to temperature differences.

  • Addition of heat increases molecular movement.

  • Kinetic Energy: Energy of moving molecules increases with temperature.

Internal Energy

• Total Energy: Consists of thermal and potential energy:

  • Thermal Energy: Increases kinetic energy, resulting in temperature changes (sensible heat transfers).

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

Sensible Heat

• Refers to heat transfers affecting temperature changes.

  • Temperature changes affect sensations of warmth or coolness.

• Heat flow equation:

  • Q (heat flow in joules) = m (mass in kg) × c (specific heat) × ΔT (temperature change in K or °C).

Latent Heat

Components

• Definition: Heat absorbed or released during a phase change with no temperature change.

• Processes that Absorb Latent Heat:

  • Melting,

  • Evaporation,

  • Sublimation.

• Processes that Release Latent Heat:

  • Freezing,

  • Condensation,

  • Deposition.

Gases and Work

Work and Energy Transfer

• Work: Energy transfer through mechanical means (e.g., pushing a cart).

• Gas Expansion: Expanding gas does work on its surroundings, decreasing its internal energy.

• Gas Compression: Requires work input, increasing internal energy.

  • Work equations: W = F Δx, W = PA Δx, W = P ΔV.

The First Law of Thermodynamics

Temperature Changes in Gases

1. Add Heat: Increases temperature.

2. Add Work: Also increases temperature.

• Internal Energy Equation:

  • Q = U + W,

  • Includes heat transfer (Q), change in internal energy (U), and work (W).

Heat and Work Components

• m c ΔT: Amount of heat used for internal energy change.

• P ΔV: Heat involved in work.

Constant Volume vs. Constant Pressure

• Constant Volume:

  • All added heat raises temperature.

  • Specific heat, cv = 717 J ∙ kg–1 ∙ K–1.

• Constant Pressure:

  • Added heat causes temperature and volume increase.

  • Specific heat, cp = 1004 J ∙ kg–1 ∙ K–1.

Adiabatic Processes

Characteristics

• Adiabatic Process: Temperature change without heat transfer; results from work.

• Driven by pressure changes in rising or falling air parcels:

  • Expansion causes cooling (work done by parcel).

  • Compression causes heating (work done on parcel).

Atmosphere and Adiabatic Processes

• Pressure decreases with height, leading to adiabatic heating or cooling as the air rises or descends.

  • Distinction between adiabatic and diabatic (heat transfer-based) processes.

Heat Transfers

Conduction

• Definition: Transfers heat molecule to molecule.

• Conductivity: Highest in solids.

• Laminar Boundary Layer: Thin air layer in contact with the ground;

  • Warms through conduction, transferring heat upwards via convection.

Convection

• Definition: Transfers heat through fluid movement.

• Types:

  1. Thermal Convection: Driven by density differences (warm air rises).

  2. Mechanical Convection: Driven by external mechanical forces (winds and turbulence).

Radiation

• Energy travels as electromagnetic waves (e.g., light, microwaves).

• All objects emit and absorb radiation:

  • Emission: Transfers energy out,

  • Absorption: Transfers energy in.

  • Hotter substances emit more radiation.

Heat Transfers at Earth’s Surface

Daytime and Night-time

• Daytime: Solar radiation warms the surface; some heat transfers upward to the atmosphere and downward to the ground.

• Night-time: Radiation emission cools the surface; some heat transfers downward from the atmosphere to the surface and upward from the ground.